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Materials that enable weight reduction

Material solutions that enable weight reductions in the automotive industry

Abstract

The automotive industry is committed to lower carbon emissions and increasing fuel efficiency. This reduction has three different dimensions: increasing the efficiency of the drivetrain, the development of energy storage systems and the reduction of the overall weight of the car.

Vehicle weight reduction plays a strategic role because it enhances fuel reduction, lowers vehicle emissions and improves driving performance, although the tendency, in the last years, has been to decrease weight in the structural parts to allow for additional weight in other car components such as safety systems, driving pleasure systems and additional motorized options.
The trend to reduce the weight of the vehicles is made possible by materials substitution, new designs that provide equal solution with less components and the use of new manufacturing technologies that can provide new innovative leaner designs.
The objective of this work is to follow some of this trends regarding weight reduction in the automotive industry. This paper will cover the applicability of metals such as Aluminum and Magnesium, the increase of High Strength Steels and the possibility to use Composites in the structural parts of a vehicle. Also covers the use of new technologies such as tailor-welded blanks for closure panels, hydroforming and metal foaming applications, among others.

Contents
Abstract 1
Introduction 3
Evolution on weight reduction in the automotive industry 4
Vehicle weight reduction 9
Materials and technology applications 11
Materials 11
Steel 12
Aluminum 13
Composite materials 14
Plastics 16
Magnesium 17
Nano-materials 17
Shape memory alloys 18
Technologies 19
Hot stamping 19
Tailor welded blanks 20
Tubular hydroforming 21
Sheet metal hydroforming 22
Superplasticity & forming of advanced materials 23
Reaction injection molding 24
Resin transfer molding 25
Techno-economic evaluation of materials and technologies 26
Conclusions 26
Bibliography 27



Introduction
Vehicles in the future must be designed and engineered to be in harmony with people and nature. The environmental and safety issues today call rapid technological improvements in the near future. The most important issues identified are the reduction of CO2 emissions with improvement of fuel economy, together with crashworthiness improvements. This issues can only be solved with strong enhancements in material and technologies, [1].
The automotive industry is committed to lower current carbon emissions and to increase fuel efficiency. Reducing CO2 emissions is possible by three general ways: reducing the total kilometers driven, improvement in the fuel efficiency of new vehicles in the market and a shift to alternative propulsion systems that use non-petroleum energy sources, (natural gas and hydrogen for example), [2]. The reduction of total transportation is a somewhat unrealistic scenario. The number of km will increase, according to Turton, [3], and the demand for passenger cars will also increase [4]. The rapid development countries (China, India, Brazil, Russia, Mexico, Indonesia…) will be responsible for most of the increase, but the USA, EU and Japan will also increase the demand in those predefined scenarios.
It is a fact that improving fuel efficiency is directly linked to reductions in the CO2 emissions responsible for climate change. According to the European Commission, the regulation of the fuel consumption and CO2 emissions of new cars is considered the single most effective policy measure the EU can take to simultaneously tackle climate change, reduce dependence on oil, and spur investment in low-carbon car technologies in Europe and elsewhere, [5]. Other work must be done in the areas of mobility and public transportation in order to achieve the ultimate goal of CO2 emissions.
Current opinion state that the shift to alternatives to internal combustion engine such as hydrogen or natural gas will only occur when the restrictions CO2 emissions increase to a level where manufacturers actually have something to gain; otherwise no shift should be expected.
To achieve an effective CO2 emissions reduction from vehicle transportation there are three different dimensions that can be pursuit: increasing the efficiency of the drivetrain, development of energy storage systems for different propulsion and the reduction of the overall weight of the car. Vehicle weight reduction plays a strategic role because it enhances fuel reduction, lowers vehicle emissions and improves driving performance. Albeit this advantages, the tendency in the last years has been to decrease weight in the structural parts to allow for additional weight in other car components such as safety systems (airbags, shock absorbers, driving assisting cameras), driving pleasure systems (DVD’s, GPS, HVAC) and additional motorized options (electric windows, electric rear view mirrors, electric and warm seat modules).
This paper will address the current trends on vehicle weight; will try to decompose vehicles in order to reveal where weight reduction can be more effective, followed by a section where a brief overview of materials and technology trends related with vehicle weight reduction are illustrated. In the end, the need for techno-economic evaluations is proven and some conclusions are drawn.

Evolution on weight reduction in the automotive industry
In Europe, the target to reduce average new vehicles CO2 emissions to 120 g/km was first proposed in October 1994. The objective was to lower fuel consumption of diesel vehicles to 4.5 liters per 100 km and gasoline vehicles to 5 liters per 100 km. This target was announced in 1995 and represents a 35% reduction over 1995 levels. The target date was set for 2005, but has been postponed three times since. In the last postponement, in December 2007, the European Commission proposed to move the target for 2012 from 120 to 130 g/km. No postponement is expected from the European commission, although European manufactures are challenging for another postponement, mainly due to the current economical and financial crisis, [5].
Figure 1 shows that the evolution of fuel consumption has been rather slow in Europe and in the Unites States. Although slow, the main drivers for that reduction have been the development of technologies that have reduced vehicle mass relative to power output, enhanced thermodynamics engine efficiency as well of rolling and aerodynamic resistance. Another driver, in Europe, is the dieselization.

Figure 1 – Evolution of fuel consumption of new cars in the European Union and the United States, 1975-2002. For more detail on data sources consult Zacharidis, [2].

Nevertheless the reduction of fuel consumption, vehicles are becoming heavier and faster. Vehicles today are also superior regarding safety and comfort. Figure 2 shows the evolution of the weight of European compact from the seventies until 2002. The same evolution is presented in Figure 3 and Figure 4 for the United States and Japanese vehicles. This evolution is explained by an increase of car functions: safety, entertainment, driving assistance, comfort and, of course, the overall increase in vehicle dimensions.

Figure 2 – Weight of European compact cars at date of model introduction, source:[6].

Figure 3 – Weight of Japanese cars 1980 to 2004, [7].

Figure 4 – Fuel economy and performance: 1975 to 2006, [8].

Another angle of vehicle evolution is related with specific power output of the engines. Cousins worked with horsepower information in the UK, from 1938 to our days, to conclude that technical improvements have enabled much more power to be obtained from the same engine size. Figure 5 shows that in sixty years all engine sizes more than doubled the power output within each category, [9].

Figure 5 – Evolution of power output by engine size, [9].

Observe that the increase of the car weight is strongly related with consumers and fashion issues. Consumers are demanding for new products with more value (better safety, performance, comfort, power output) and, in most cases, are unavailable to pay the weight reduction cost. On the other end, vehicle manufactures can only get premium profits with the introduction of these new features in vehicles and will only change with strong regulation from government policy.
What are the options for reducing vehicle fuel use and with that, reducing CO2 emissions?
Bandivadekar et al., consider that the options available are: to augment the perception on reducing fuel consumption; new alternative propulsion systems and new fuels; augment the use of public transportation that enables reduction of km driven and weight; and size reduction by replacing steel and iron with alternative lightweight materials like aluminum and magnesium, and downsizing new vehicles, [10].
To further improve the fuel efficiency of conventional engines the technologic options are, [11]:
· High-powered ignition systems that ensure complete combustion of the fuel available
· Improved fuel injectors
· Computer controlled engine management
· Improved compression at low engine loads
· Engine friction reduction
· Variable valve timing
· Variable geometry turbocharger
· Cooled Exhaust Gas Recirculation (EGR)
· Six-speed manual transmission
· Electric-motor-driven power steering
· Stop-go systems
· Cylinder deactivation
· Continuously-variable transmission (CVT) to improve gearing efficiency
· Mild hybrid (means integrated starter-alternator with motor assist)
· Reduced vehicle weight
· Reduced mechanical friction
· Reduced air drag and rolling resistance
But not all changes have the same impact on fuel consumption. For example, the improvement of 10% of vehicle mass has a 5% impact on fuel consumption, (see Table 1). Manufacturers generally focus more on powertrain efficiency rather than reducing weight, air-drag and rolling resistance. This decision making framework is strongly related with the customer needs and wants and of course with product architecture and supply chain management. The change in the engine (a module of the vehicle) is much more contained in the supply chain than the body or the vehicle closures. Vehicle weight is the most important of these parameters, although not giving the most direct impact on fuel consumption, because it allows for secondary savings and smaller engines and is also the one offering the highest potential for improvement, [11].
Reducing vehicle weight is also essential for making fuel cells an economically viable option. Lower weight represents lower fuel cell size and lower fuel cell cost, or may represent more batteries in an full electric car increasing autonomy. This is an effect of mass being decisive for the power needed for accelerating the vehicle and for driving uphill. The power required at constant speed to overcome rolling resistance and air-drag falls well below the power required for acceleration, [11].

Table 1 – Fuel saving potential of various technologies, [12].

Improvement of 10% of: Impact on fuel consumption
Powertrain efficiency 10%
Vehicle weight 5%
Rolling resistance 2-3%
Power consumption of auxiliaries 1-2%

A study from the FKA considers the effects of primary and secondary weight savings on the fuel consumption, [13]. The simulations were conducted for vehicles with base weight, for vehicles with the reduced weight and for vehicles with reduced weight and re-sized powertrain. Figure 6 and Figure 7 present the results of the simulations with three different vehicle types; the three types were combined with five different propulsion systems and considering two common driving cycles: NEDC and NYZEM. Results of the simulations show that maintaining the same acceleration, the weight saving in the body in white and secondary savings associated with it and the resizing of the powertrains allows for savings from 4.9% to 8.2% in fuel consumption for the NEDC cycle and 3% to 6.8% in the HYZEM cycle, [13].

Figure 6 – Influence of 10% weight reduction on fuel consumption in NEDC cycle,[13].

Figure 7 – Influence of 10% weight reduction on fuel consumption in HYZEM cycle,[13].

Reducing vehicle weight can provide, along with other options, a major step in the reduction of CO2 emissions and individual vehicle fuel consumption. Deal with the climate change is very important, but the reduction of fuel consumption is also very important to reduce European Union, United States and Japan dependence on oil. In July 2008 the barrel oil price was 137 dollar putting the occidental world in a very delicate international dependence situation.
Another very important issue is the current research and development of electric vehicles. Lightweight solutions for the automotive industry will be an enabler for battery size and autonomy of future electric vehicles.
The question is how to reduce vehicle weight: where, when and with what materials and technologies.

Vehicle weight reduction
Vehicle weight reduction will be a massive effort for the industry. New materials available for substitution often imply technological changes, corresponding to high investments and strong interactions with the supply chain. New technological processes also have problems with cycle times and raw material market availability. Also, the strong correlation of the automotive industry with the steel industry granted, over the years, a strong knowledge of steel based architectures that will be very difficult to overcome.
In order to understand where to change vehicles to get significant weight reduction there’s a need to understand vehicle architecture and the product development phases. A functional subsystem breakdown and an average percentage of the curb weight made up by each functional subsystem are showed in Table 2. The study was done averaging the results of full vehicle teardown of 52 sedans (2007 and 2008 year models) of a major constructer, [14].
Table 2 – Functional decomposition of a vehicle, subsystems and components, [14].

Subsystem Components Fraction of curb weight
Structure Full frame; Mounts; Body structure 25%
Interior Instrument panel; Seats; Airbags 12%
Engine 11%
Suspensions Front suspension; Rear suspension 10%
Closures Fenders; Door; Hood; Deck lid 10%
Tires and Wheels Tires; Wheels; Wheel trim 6%
Fuel and Exhaust Air cleaner; Exhaust system; Fuel tank 6%
Transmission Manual transmission; Automatic transmission 6%
Exterior Windshield; Window; Fascia; Impact bar; Grille; Energy absorber 5%
Electrical Sensors; Generator; Battery; Power converter 3%
HVAC Heating; Ventilation; Air conditioning 3%
Steering and Brakes Steering shaft; Steering column; Park brake; Brake apply 2%
Info and Controls Switches; Speakers; Antenna; Entertainment system 1%

Bjelkengren suggests a methodology for considering secondary mass savings when analyzing the value of different lightweighting options and the interactions of this options with the product development phases, [14]. The reduction of weight in the structure of the vehicle can allow the reduction of weight of the chassis, brakes and gears, resulting in secondary reduction in the weight of the vehicle, [15].
In all the subsystems in Table 2, steel plays a major role in most of the subsystems. Steel has demonstrated versatility over many years in the automotive industry since Henry Ford’s introduction of mass production,[16]. Figure 8 illustrates the evolution of the use of steel and other materials in a vehicle in 1975, 2005 and a forecast for 2015, [17]. From 1975 to 2005 plastics and other non metallic materials grew from 18.6% to 24.2% of vehicle content. Also, the use of mild steel decreased from 55.9% to 44% and the use of medium and high strength steel increased from 3.6% to 10.9%. Aluminum increased 2.2% to 7.7% and plastics increased from 4.6% to 8.4%.

Figure 8 - North American Light Vehicle Material Content Segmented by Type of Material: 1975, 2005 and 2015 forecast, [17].

The importance of the supply chain system to the automotive industry regarding materials selection cannot be understated. Material properties are of basic engineering importance but unless the material can be accommodated comfortably within the manufacturing operations and the environmental restrictions it will have adverse effects on both productivity and facility costs. Again steel industry has a strong lead in the market. The total production of steel in the world is 1,170.000 million metric tons whilst carbon fiber production is only 31,700 metric tons.

Materials and technology applications
When considering materials and technologies that may enable weight reductions one must consider direct materials substitution, new designs that provide equal or more integrated solutions and new manufacturing technologies that can provide new designs.
Resistance to change, regarding materials and technologies, will only be mitigated with more awareness about materials solutions available, more simulation methods to assure the capability of this new materials and processes and techno-economic evaluation of this new materials and processes.
Global awareness can be accomplished with physical demonstrations on high scale and sports vehicles. Simulation and techno-economic evaluations can be done by the Academia and vehicle manufacturers. FEM design techniques are now proving invaluable in reducing the timeframe of product development cycles. Note that all the evaluations must be done considering the life cycle of the products and changes in the supply chain.
Next sections will try to point up materials and technologies already in use, although limited and new material and technologies that can enable strong vehicle weight reduction.
Materials
Steel has demonstrated its versatility over the last century. It has remained reasonable in cost, the life of components has been extended through the use of zinc coating technology, and the range of strength levels has increased to meet increasingly stringent engineering needs. Steel is also very easy to repair and can be easily recycled.
Aluminum has always been considered an alternative material to steel and can be recalled of being used in models in the early 1900s and was used in volume production in fifties, but until now cost (initial materials and processing costs), has discouraged widespread adoption.
Polymeric materials have massive advantages, particularly because it allows complex shapes obtained in one operation and also the ease of incorporation of many parts and functions in one assembly.
Magnesium is also finding limited application within the structure and although this has been an option for items such as gear box covers in the past, corrosion and ease of forming have constrained the scope of application. With improved alloys, proven in the aero industry, the time has come for reappraisal of magnesium solutions.
Another option is the use of secondary forms of steel, aluminum and plastics. Examples are tailored designs using steel and aluminum: hydroformed steel and aluminum sections and tailor welded blanks (TWBs). High performance and competition cars are also making extensive use of honeycomb materials which when consolidated with composite skin layers provide ultra-light high strength impact and structural sections. Because of their exceptional strength to weight ratio these may be the future first choice of body material for electric and alternatively fuelled cars.

Steel
Mild steels are the most common steels used in auto making today, remember Figure 8. The main reason is that mild steel still has forming and cost advantages compared to high strength steel. To reduce the weight of the vehicle and thus CO2 emissions, the proportion of high strength steel (HSS) used in vehicles is gradually increasing (Figure 8).
The availability and increase use of HSS represents the steel industry’s response to the increasing use of lower density materials such as aluminum and magnesium. The goal is to develop increasingly higher strength materials while maintaining, or even improving formability, making possible simultaneously to improve the strength characteristics of the parts and reduce the weight through reduction of the steel sheet thickness. The result is that the steel industry has recently produced a number of advanced high strength steels (AHSS) that are highly formable with an excellent combination of strength, durability, strain rate sensitivity and strain hardening. Other examples of these developing steels are ultrafine grain, low density, and high Young’s modulus steels, [18].
The UltraLight Steel Auto Body (ULSAB) is another initiative from the steel industry’s to push steel in the auto industry. Started in 1994, the ULSAB is a consortium of 35 sheet steel producers from 18 countries set out to demonstrate a lightweight steel auto body structure that would meet a wide range of safety and performance targets, [19]. The ULSAB concepts confirms steels main attributes and it explains why high-strength steel is the fastest growing light weighting material in automotive structures. Steel has a low cost, in comparison with other materials, and strong, it is easy to form into complex shapes and structures, and it is highly suited for mass production of vehicles. Its proven ability to absorb energy in a crash is well known.
Whilst conventional high strength steels (HSS) are hardened by solid solution, precipitation or grain refining, advanced high strength steels (AHSS) are hardened by phase transformation, and the microstructure may include martensite, bainite and retained austenite. AHSS, including dual phase steel, TRIP steel, complex phase steel and martensite steel, are superior in strength and ductility when compared with conventional HSS and thus facilitate the energy absorption during impact and ensure safety when reducing weight. AHSS for automotive industry include hot-rolled, cold-rolled and hot dip galvanized steel, which are all strengthened by phase transformation hardening. [20]
Figure 9 shows a graphic comparison of the strengths and percent elongation for various grades of automotive sheet steels.

Figure 9 - A comparison of lower (or initial) yield strength and % elongation for various grades of steels, [18].

The American Iron and Steel Institute prepared application guidelines of Advanced High-Strength Steel (AHSS). Those guidelines focus on press-forming, fabrication and joining processes for automotive underbody, structural, and body panels designed for higher strength steels, [21].

Aluminum
Aluminum has always been a strong contender of steel. The percentage of aluminum increased from 2.2% in 1975 to 7.7% in 2005, as shown in Figure 8, and is expected to continue to rise in the next years. The strategy of the Aluminum industry changed from a push for full aluminum vehicle structures to peaceful coexistence with other materials. This strategy enabled the industry to grow the contents of aluminum in vehicles, based on their strengths in performance, safety and enabling fuel savings, rather than a full supply chain change necessary for a full aluminum vehicle.
Wheels, powertrain and suspension components represent the biggest categories for aluminum. Other significant applications include exterior body panels, structural components and body-in-white, [22]. The greatest growth potential in the next few years is in the closure panels–doors, roof, hood, and deck lid. A 45% weight savings can be obtained for the hoods and deck lid. A 50% weight savings can be obtained for the fender(s) and 35% for the doors, [23-25].
Audi presented the first mass production aluminum car (Audi A2), from 1999 to 2005, after the introduction of an aluminum space frame structure for the A8 luxury model. The Audi A2 weighs 895 kg, about 150 kg less than comparable compact vehicles with a conventional steel body,[26].
One of the main advances of aluminum is its availability in a large variety of semi-finished forms, such as shape castings, extrusions and sheet. Such semis are very suitable for mass production and innovative solutions in the form of compact and highly integrated parts that meet the high demands for performance, quality and cost efficient manufacturability. The main challenges involved here are joining and surface treatment issues.
Miller et al., summarizes the recent developments covering aluminum use in castings, extrusions and sheet. The use of aluminum in BIW applications and the development of alloys for the inner and outer panels are described. The tradeoffs between strength, formability and surface quality of the 5xxx and 6xxx alloys are discussed for both inner and outer panels,[27].
Aluminum casting products
The highest volume of aluminum components in cars are castings, such as engine blocks, cylinder heads and chassis parts. The substitution of cast iron engine blocks continues. Even diesel engines, which continue to gain a substantial increase in market share in Europe, now are being cast in aluminum where, due to the high requirements on strength and durability, cast iron has generally been used. Table 3 shows typical applications for aluminum casting alloys.

Table 3 – Typical applications for aluminum casting alloys, [28]

Alloy Typical Applications
319.0 Manifolds, cylinder heads, blocks, internal engine parts
332.0 Pistons
356.0 Cylinder heads, manifolds
A356.0 Wheels
A380.0 Blocks, transmission housings/parts, fuel metering devices
383.0 Brackets, housings, internal engine parts, steering gears
B390.0 High-wear applications such as ring gears and internal transmission parts

Aluminum sheet applications
The main aluminum alloy classes for automotive sheet application are the non-heat treatable Al-Mg (EN 5xxx series) and the heat treatable Al-Mg-Si (EN 6xxx series) alloy system, some especially tailored by variations in chemical composition and processing, e.g. Al-Mg alloys optimized for strength and corrosion resistance for use in chassis or Al-Mg-Si alloys applied for autobody sheets have been improved for formability, surface appearance and age hardening response, [29].

Aluminum extrusion products
Another wide field of aluminum solutions and applications is opened by making use of the well established technology of aluminum extrusions. Here quite complex shapes of profiles can be achieved allowing innovative light weight design with integrated functions. Medium strength AA6xxx and high strength AA7xxx age hardening alloys are mostly used where quenching occurs during the extrusion processing. Formability and final strength is controlled by subsequent heating for age hardening. Extrusions have been applied for bumper beams and crash elements and boxes which is a major market for aluminum extrusions, [30].

Composite materials
Niche production vehicles have already demonstrated the feasibility of using composites and automotive designers are now looking to widen their application. All the major car manufacturers are conducting research programs involving composites, thus confirming their interest in using these materials. The COMPOSIT project was a networking initiative sponsored by the European Commission that examined the future use of composite materials in the transport sectors. COMPOSIT organized a series of workshops that discussed ten key aspects relating to composite usage – repair, design, crashworthiness, manufacturing, lightweighting, joining, recycling, modeling, fire safety and new material concepts, [31].
The Rocky Mountains Institute (RMI) developed the Hypercar® concept. A Hypercar® vehicle is designed to capture the synergies of: ultralight construction; low-drag design; hybrid-electric drive; and, efficient accessories to achieve 3 to 5-fold improvement in fuel economy, equal or better performance, safety, amenity and affordability, compared to today's vehicles. Lovins et al., [32], summarizes the ultralight, hybrid-electric Hypercar® concept, emphasizing why mass optimization is crucial to its design. The authors summarize potentially applicable high volume advanced composites technologies and recount two recent studies of the main barriers to structural composites use in the BIW. The authors also stated that a whole-system application, not the conventionally prescribed component-based incrementalism, is probably the best way to conquer these barriers and create new opportunities. Finally the authors explore in great detail a key barrier of advanced composites: their manufacturing cost. The authors believe that the composites must be show as a whole-system application to dodge the high costs, [32].
Note the difference between the aluminum strategy and the composites strategy for the market. Aluminum gave up the full change to aluminum and relies on small changes and composites rely on full materials substitution.
Costs associated with composite materials are considered a major disadvantage of these materials, but a study from Fuchs et al., considers composites to have a significant economic potential when considering emerging advances in polymer composite BIW design against the mild grade steel body on the road, [16]. The authors evaluate the relative competitiveness of a carbon fiber reinforcement polymer (CFRP), a glass fiber reinforcement polymer (GFRP) composited against the mild steel unibody. All the composite components are designed to be produced by the structural reaction injection molding (SRIM) process. Results show that composite technologies hold not only the potential to reduce vehicle weight, but also to do it a cost-effective manner. However this advantage is strongly dependent on production volumes.
Another issue in the composite industry is to reduce the cost of the raw materials. Currently, production of carbon fiber involves the heating and oxidation of a polyacrylonitrile (PAN) fibre under tension which yields a long chain carbon fiber. Carbon fibers are divided into strands and then rolled and packaged for delivery. PAN is an expensive precursor and since 50% of it is burned off during carbon fiber formation, costs are increased by a factor of 2. Oak Ridge Laboratory is testing different precursors, such as textile-grade PAN (similar to that used for making rugs), lignin (a by-product of hardwood or softwood processes), or even polyolefins. Another possibility is not splitting the carbon fiber into individual fibers but rather leaving it as a sheet and processing it directly. Some of these processes may be unsuitable for so called aeronautic grade carbon fiber but may be highly suitable in automotive uses, [33].
If the benefits of composite materials in terms of light weight, durability and ease of forming are to be exploited more widely by the automotive industry, then there is a need to solve the critical technical barriers that exist. Composite materials advantages are also its disadvantages. The capacity to enable tailored solutions, putting the strength where it’s needed, and the capacity to integrate several parts, enabling less parts and manufacturing costs, is a major engineering challenge to the automotive industry.

Plastics
Plastics invaded vehicles over the years. They are mostly used the interior of a vehicle and interact directly with the costumer. By volume, from bumper to bumper, cars today contain more plastics than traditional materials. Yet, thanks to their light weight, they account for on average only 8,4% of the total weight (remember Figure 8), [34].
Technological innovation by the plastics industry is a key feature in the continuing development and use of plastics in cars. Today, they provide multi-component, tailor-made solutions for many new requirements, replacing more traditional and heavier materials in the process.
One of the issues of plastics for the automotive industry is recycling. The current process of End of Life Vehicles recycling leaves 25% of the vehicle weight going to landfill. This is mainly made up of non-metallic material such as plastic, rubber and glass. Around 10% (100kg) of a vehicle's weight is made up of plastic which has the potential for recycling and hence the generation of income to the dismantler. However to achieve the maximum value for this material, a significant number of problems need to be overcome. There are around 25 types of plastics, each one of which could have a number of variations in filler content, additives and colorants. The majority of these plastics cannot be mixed together for the purpose of recovery. So a key issue is how to adequately segregate these different materials. In addition, a large proportion of recoverable components are contaminated with metal clips, screws, labels, foam etc. all of which have to be removed prior to processing, [35]. Table 4 show the main plastic types of plastics used in a vehicle and their weight.

Table 4 - Plastics’ use by type and weight in an average vehicle, [34]

Part Main plastic types Weight in average vehicle (kg)
Bumpers PP, ABS, PC 10
Seats PUR, PP, PVC, ABS, PA 13
Dashboard PP, ABS, PA, PC, PE 15
Fuel systems PE, POM, PA, PP 7
Body (including body panels) PP, PPE, UP 6
Under the bonnet components PA, PP, PBT 9
Interior trim PP, ABS, PET, POM, PVC 20
Electrical components PP, PE, PBT, PA, PVC 7
Exterior trim ABS, PA, PBT, ASA, PP 4
Lighting PP, PC, ABS, PMMA, UP 5
Upholstery PVC, PUR, PP, PE 8
Other reservoirs PP, PE, PA 1
Total 105

Magnesium
Automotive die castings make up 85% to 90%, of the total annual weight of die castings produced, and are expected to retain this share for the foreseeable future, [36].
In order to stimulate growth in the use of magnesium components, the two large competitive automotive organizations in the US and Europe, the United States Council for Automotive Research (USCAR) and EUCAR, are sponsoring R&D programs in their respective communities to address issues that are hampering the large-scale application of magnesium in chassis components, [37].
USCAR presented a document, called “Magnesium Vision 2020: A North American Automotive Strategic Vision for Magnesium”, where it benchmarks and identifies challenges, develops R&D proposals and suggests solutions that will advance the use of magnesium in automotive applications, [38]. The challenges of using more magnesium solutions in vehicles are the higher perceived cost for current high pressure die casting (HPDC) components, general and galvanic corrosion and finally fastening and joining of magnesium components with other metals, [38].
Magnesium content per vehicle is expected to rise from the present 0.2% to 0.6% in 2015, remember Figure 8. Aghion et al., state that due to standards and other environmental legislations, most car producers are going to use 40 to 100 kg of magnesium alloys in the near future, [39]. Recent growth has been fueled by the AM alloy series (Aluminum and Manganese alloy). These alloys possess excellent energy absorbing properties, permitting use in safety related applications such as steering wheels, instrument panels and beams, seat structures, brackets and inner door panels, [36].
The major process for the manufacture of vehicle body components from magnesium is die casting. The cost of producing a die cast mould is generally less than for press tooling, making the application of cast components particularly suitable for lower volume applications. In pressure die casting the molten magnesium is injected into the mould cavity under pressure. As soon as the part is solid the mould is opened and the part ejected. Cooling water and mould release agent are then applied to the mold before the cycle proceeds onto the next part. Similar technology is applicable for both magnesium and aluminum although shrinkage rates will be different. Sheet forming of magnesium at room temperature is limited due to the hexagonal crystal structure of the material, and twinning characteristics allow only low levels of deformation. It is possible to increase the elongation considerably at elevated temperatures and at 235 ºC it has been shown that elongations of over 35 per cent have been recorded. Forging has been proposed as an alternative means of component production but even near net shape techniques like precision forging are not really relevant to many body applications, [40, 41].

Nano-materials
Nano scale materials already play an important role in the automotive industry, namely in coatings and paints.
Under the 6th framework program, a roadmap for the automotive applications of nanomaterials was developed. The authors state that the design and manufacturing of light vehicles, trucks and buses can be affected by nanotechnology and related technologies up to 60% in 2015. Fields of study for the next years are: the frame and body parts, engines and powertrain, suspension and breaking systems, lubrification and fuel, tyres, exaust systems and catalytic converters and electric and electronic equipment, [42]. Figure 10 shows some of those potential applications.

Figure 10 – Potential applications for smart materials and multifunctional composites in automotive applications

Another application will be on hydrogen storage for fuel cell. QuantumSphere’s patented manufacturing process has enabled the production of nanomaterials that will make fuel cells smaller and more practical in automotive applications, [43].
From the steel industry research, new types of steel with nano scale particles are being developed. Nano steels are designed to avoid the low values of edge stretch (local elongation) experienced by DP and TRIP steels. Instead of islands of martensite, the ferrite matrix is strengthened with ultra-fine nano-sized

Shape memory alloys
Shape memory alloys are growing in importance in the aeronautical and medical products. For vehicle applications, one of the most promising applications are mechanical actuators, [44, 45]
Now the costs of materials and processes are prohibitive for the automotive industry. Cycle time is too high, incompatible with mass production. High costs and time of manufacturing required for the improvement in performance is justified only for “low volume” and “high added value” products (i.e. sport cars).
Shape memory actuators can enable weight reductions in vehicles because they can substitute discrete actuators and sensors that currently exist. Shape memory actuators act simultaneously as sensors and actuators, they don’t need external mechanisms and allow high level of miniaturization.
In another field of research, two teams have developed a new class of materials known as magnetic shape memory foams. The foam is made of nickel-manganese-gallium alloy and they allow for 10% length increase when a magnetic field is applied. The researchers believe that the porous alloy has great potential for uses that require light weight and large strain, such has space and automotive industry, [46].
Technologies
New technologies may enable better products, both in performance and weight reduction. They can also work for leaner designs with integration of functions and reduction of the overall number of parts. New technologies must also work their way in the supply chain, namely cycle times can be deterring, and also the cost. The best thing about new technologies is that they can provide with more than materials direct substitution, where steel has an enormous advantage. Another important aspect with new technologies is the capability of the products to interface with other systems in a car.

Hot stamping
Ultrahigh-strength steels, such as Manganese-Boron steels, pose a major challenge in processing because of their limited formability and pronounced spring-back at room temperature. By adding heat during the forming process and then cooling rapidly, the minimum tensile strength of 500 MPa displayed by the material in the as-delivered condition can be increased to a maximum of 1650 MPa, [47].
Hot stamping with die quenching of boron steels appeared at the end of 1990s for producing some rather simple automotive parts like door beams and bumper beams. This process can overcome some of the typical difficulties associated with cold stamping. For example, hot-forming of the quenchable boron-alloy steel 22MnB5 can produce complex, crash-resistant parts such as bumpers and pillars with ultrahigh strength, minimum spring-back, and reduced sheet thickness, [48].
In hot stamping, forming and hardening are combined in a single operation. Two different methods are used: direct and indirect. In the direct method, the blanks are austenitized at temperatures between 900ºC and 950ºC for four to 10 minutes inside a continuous feed furnace and subsequently transferred to an internally cooled die set via a transfer unit. At high temperature (650ºC to 850ºC), the material has excellent formability, so that complex shapes can be formed in a single stroke. The blanks are stamped and cooled down under pressure for a specific amount of time according to the sheet thickness after drawing depth is reached. During this period the formed part is cooled in the closed die set that is internally cooled by water circulation at a cooling speed of 50ºC/sec to 100ºC/sec, completing the quenching (martensitic transformation) process, [48].
The total cycle time for transferring, stamping, and cooling in the die is 15 to 25 seconds. Finally, the part leaves the hot-stamping line at about 150ºC and with high mechanical properties: an ultimate tensile strength of 1400 to 1600 MPa and a yield strength between 1000 and 1200 MPa. Unlike the direct process, indirect hot stamping provides for a part to be drawn, unheated, to about 90 percent to 95 percent of its final shape in a conventional die, followed by a partial trimming operation, depending on edge tolerance. Then the preforms are heated to austenitization temperature in a continuous furnace and hardened in the die. The reason for the additional step is to extend the forming limits for very complex shapes by heat-treating the cold formed parts, [48].
Manganese-boron steels for hot stamping are suitable for welding with steels of the same or different grade on the condition that the welding parameters are matched to the material. Resistant spot welding, shielded gas welding and laser beam welding are particularly suitable, [47]. Hot-stamped boron steel was successfully joined via friction stir spot welding using polycrystalline cubic boron nitride tooling, [49]. The Oak Ridge National Laboratory & Pacific Northwest National Laboratory have a project where the primary objective is to develop friction-stir spot welding as a superior method to join advanced high strength steels, [50].
In 2004 the estimated total consumption of flat boron steels for hot stamping and die quenching was about 60,000 to 80,000 tons per year in Europe. In 2008-2009 yearly consumption in Europe is expected to increase to about 300,000 tons. Japan and North America are following this trend. Expectations are for more than 20 new hot-stamping lines (heating furnace and press) to be built between 2004 and 2009 throughout the world, [48].

Tailor welded blanks
The concept of combining various steel options into a welded blank was developed to enable product and manufacturing engineers to “tailor” the blank so that steel’s best properties were located precisely within the part where they were needed. This process not only reduces the weight of the finished part, but also can be used for part integration, thereby eliminating many reinforcements and stiffeners.
The tailor welded blanks (TWB) designation is used in the broad sense to include conventional TWBs, two or more sheets of steel welded along adjacent edges as flat blanks prior to forming, tailor welded tubes (TWT) of multi-gage or grade side-walls, and patch-type TWBs, or a steel "patch" overlapping another steel blank. Although the conventional joining method has been either CO2 and YAG laser or resistance mash welding, other joining methods were also considered, including spot welding in the case of patch-type TWBs, [51].
Tailored blanking for vehicle body structures is a well known process with the first applications being done for mass production which started in 1985. Below listed are the main reasons for the decision to use tailor welded blanks, [19]:
· Mass reduction due to the possibility of placing optimum steel thicknesses and grades where needed.
· Elimination of reinforcements with appropriate material gage selection.
· Simplified logistics due to the reduction of parts.
· Investment cost reduction of dies, presses etc. due to fewer production steps.
· Better corrosion protection by the elimination of overlapped joints.
· Improved structural rigidity due to the smoother energy flow within the tailor welded blank parts.
· Better fatigue and crash behavior compared to a conventional overlapped spot welded design solution.
Laser welding and mash seam welding are the most common processes for the manufacturing of tailor welded blanks today. Induction and electron beam welding have a minor importance and are still under development. All these processes have their advantages and disadvantages, related to the process and the machine itself, [19]. Figure 11 shows a concept for a tailor welded panel body.

Figure 11 – Example of a Tailor welded panel body, [19].

Silva, et al., studied single point incremental forming (SPIF) of aluminum tailored welded blanks produced by friction stir welding (FSW). Results show that the combination of SPIF with tailored welded blanks produced by FSW seems promising in the manufacture of complex sheet metal parts with high depths, [52].
Aluminum TWBs offer additional weight savings and would address the concerns of increased cost and decreased crashworthiness, which normally accompany aluminum for steel material substitutions. While aluminum TWBs are currently being implemented for less structural components such as deck lids and hoods, door inners and floor pans are being investigated as possible Aluminum TWB applications for the future, [53].
The use of tailor welded panels has a disadvantage because they don’t allow for continuous stamping increasing cycle time of the process.

Tubular hydroforming
Today, tubular hydroforming is a well-established process in automotive manufacturing. With the focus on mass savings, hydroforming can reduce the number of parts while helping to optimize available package space. The hyroforming process is described very simply as: “put a tube between a lower and an upper die, close the die, fill the tube with water and increase the internal pressure in order to force the tube to expand into the shape of the die.” However, several things must be taken into consideration within this process technology. This method will work only for straight tubes. In all other cases the tube has to be prebent or preformed depending on the final shape, [19]. Figure 12 and Figure 13 show an example of tube hydroforming with preforming tool concept.

Figure 12 – Example of tube hydroforming with preforming tool concept, [19].

Figure 13 – Example of tube hydroforming with preforming tool concept: section A-A, [19].

Sheet metal hydroforming
Hoods, roofs and door panels (large body outer panels) produced by conventional forming methods often lack sufficient stiffness against buckling in the center area of the part. Due to the low degree of deformation in the center, there is only a little work hardening effect that could be achieved. Therefore, material thickness has to be increased to meet the dent resistance requirements on those parts. This of course leads to heavier parts and creates extra costs. The “active hydromechanical sheet metal forming process” is a forming technology that uses an active fluid medium. The die consists of three main components: a drawing ring, which is designed as a “water box,” the blankholder (binder) and the drawing punch itself, [19]. Figure 14 show the concept of sheet metal hydroforming.

Figure 14 – Example of sheet metal hydroforming. Sheet (location 3) is deformed by the movement of the punch (location 1) toward the pressure medium (location 4), [54].

The application of sheet hydroforming is expected to increase during the next decade, [54]. While the process has a disadvantage, namely the low cycle times, it offers many advantages for automotive applications. Some include:
· Obtaining greater draw depths with better strain distribution.
· Drawing complex shapes in one press cycle.
· Reducing die costs because one die is used.
· Obtaining excellent finish of the sheet surface exposed to the medium.
Figure 15 shows a hood and decklid outer panels made by sheet hydroforming.

Figure 15 – Pontiac's sheet hydroforming display at the 2006 Detroit International Auto Show. Shown are the Solstice hood outer and decklid outer.

Superplasticity & forming of advanced materials
The expanded forming limits associated with superplastic forming (SPF) makes it an attractive option for the manufacture of complex parts from aluminum sheet.
Superplasticity is the capability to deform crystalline solids in tension to unusually large plastic strains, often well in excess of 1000%. This phenomenon results from the ability of the material to resist localized deformation much the same as hot glass does. As high elongations are possible, complex contoured parts can be formed in a single press cycle often eliminating the need for multipart fabrications. This enables the designer to capture several detail parts into a one piece complex, formed structure. Thus materials with superplastic properties can be used to form complex components in shapes that are very near the final dimension. Superplastic forming also enhances design freedom, minimizes the amount of scrap produced, and reduces the need for machining. In addition, it reduces the amount of material used, thereby lowering overall material costs.
The biggest disadvantage is its slow forming rate. Cycle times vary from two minutes to two hour; therefore it is usually used on lower volume products, Luo et al., developed a novel superplastic forming process that utilizes mechanical pre-forming to enhance formability, reduce costs and improve production efficiency, [55].
Barnes discuss current and likely future trends including applications in the aerospace and automotive markets, faster-forming techniques to improve productivity, the increasing importance of computer modeling and simulation in tool design and process optimization and new alloy developments including superplastic magnesium alloys, [56].
The limited reforming properties of magnesium alloys can be associated with its hexagonal lattice structure which causes problems during processing. The utilization of the superplastic deformation behavior of magnesium materials can give remedy to this limitation because this way the productivity of the mechanical processing can be increased considerably in comparison to conventional forming processes. Superplastic forming of magnesium is a perfect and cost saving process for near-net-shape forming as an alternative for complicated machining or joining processes, [57]. Figure 16 show an example of super plastic forming of aluminum for the outer body panels.

Figure 16 – The Ford GT's aluminum body uses "Super plastic forming," of aluminum for the outer body panels.

Reaction injection molding
RIM is a process where reactive liquid components, usually thermoset polyurethane, are mixed inside a closed mold cavity under pressure. This process is widely used in the automotive industry to produce internal and external parts. Reinforced Reaction Injection Molding (R-RIM) is a close-molding process. Two resins are heated separately and poured into a mixing container with milled glass fiber. Once the resins and milled fiber are blended, the composite mixture is injected into a mold cavity and compressed. The resin quickly reacts and cures to form a composite such as a Class-A automotive body panel. Structural Reaction Injection Molding (S-RIM) uses glass fiber fabrics, mats and preforms to make structurally strong composites. In SRIM, a thermosetting urethane resin is mixed at high pressure just prior to injection into a hot mold containing the fiber reinforcement. The composite mixture solidifies into a finished part such as large pickup truck boxes, and can be foamed to make low density nonstructural parts such as interior trim panels for cars. Figure 17 show a RIM made bumper.

Figure 17 – Peugeot 807 R.I.M. bumper, [58]

Resin transfer molding
Out of many composite manufacturing processes available, resin transfer molding (RTM) is one of the most efficient and economical process for high volume automotive applications due to its capabilities such as: non-expensive process equipment, excellent control on mechanical properties, closed mold process, low filling pressures, incorporation of metal inserts and attachments, possibility of producing large and complex parts and low labor costs, [59, 60]. Figure 18 show a hood made by RTM technologies.

Figure 18 - SLP now offers an all-new C6 composite hood with functional hood scoop to fit 2005-08 Corvettes.


Techno-economic evaluation of materials and technologies
The choice of materials and related technologies involves complex decision-making and is done in the early stages of product development. This decision making would have to balance different intricate functional, technological and economical criteria reflecting the function of the part/product, its geometrical shape, the manufacturing process, the production volumes, the final cost and the implications to the environment through a life-cycle assessment.
Cost is an issue in every material decision made on a vehicle and can be viewed in several different ways. When making a material substitution, cost can be viewed as a simple part-by-part substitution, as a manufacturing and product system cost, and/or, increasingly, as a life-cycle cost (LCA). As OEMs apply aluminum as a technology to lightweight vehicles, system cost and LCA generally dictate decisions to improve driving performance or reduce fuel consumption and harmful tailpipe emissions, etc.
Whilst in some niche markets, very expensive cars can allow the use of new materials and technologies, mainstream markets will only change once the economic evaluations for those materials and technologies are positive. But decision making is not that simple. A balance must be achieved in a very complex cost/weight-saving/strength/recyclability/emissions/repair equation.

Conclusions
Rather than forced downsizing, lighter cars can actually be larger (offering better crash protection), yet still boost fuel economy and cut emissions.
New materials and technology products are available and are ready to use now, but changes made now will have to wait some four to five years to be available for the costumer.
The current paradigm will change with:
· New technologies and materials with techno-economic advantages
o Aluminum and composites will play a major role but problems with recycling, repair and materials availability must be solved.
o Magnesium will also play a major role in the next years.
o Hot technologies, although environmental costly, will enable new materials and new products.
· A Policy that increases pressure on the automobile industry
o CO2 effective reduction policy.
o Oil (in)dependence policy.
o Electric vehicles may become strong contenders, demanding for lightweighting solutions.
· Mobility solutions
o Private transportation versus Public transportation. Public transportation is far more concerned with use costs than with acquisition costs, and can be more acceptant to higher manufacturing costs.

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RPD 2008 - http://www.mouldsevent.com/364/rpd.html

Materials selection in Product design and development

Marco Leite, Arlindo Silva and Elsa Henriques

Abstract

Material selection is a live process inside the more global process of product development. The choice of a material and manufacturing technologies for a certain product is a fundamental task, often jumped over, either because the choice looks obvious, or because habits of convergence thinking lead to past proven solutions. In consequence, more often than wanted, there’s a need to do extensive rework, with obvious first order problems, man hours increase and renegotiation of contracts and other second order problems like higher time to market or pushing a bad product to market.

Concept development defines the product architecture and normally represents a point of no return to the new product. Decisions concerning materials at these stages should account for functionality, technological and economical criteria.

There´s a need to understand what methods are available, at each stage of the product development, to the decision making process of materials selection.

A review of some current methods for materials and process selection at the different stages of product design is presented and a new Materials Selection Engine that frames the decision making process at the several decision gates is proposed. The proposed Materials Selection Engine includes technical performance, predictive estimation of the cost drivers and environmental issues.

Introduction

A successful product development puts a lot of pressure in companies. They should be capable of developing more and more complex products in a shorter development time, with a high perceived quality and within a market compatible cost [1].

One of the fundamental aspects of successful product development is the choice of the right materials. Materials are responsible for function (to support loads, to assure pressure, etc.), structure (physical embodiment) and interaction with the customer/user. Materials also connect the computer calculations and drawings with a real or working design. New materials are responsible for the appearance of new products or radical changes in current products.

An effective material and process selection directly determines the perceived quality and the cost of the product and is indirectly connected to the time to market. In the phase of generating new concepts, different materials may support major changes in product architecture, thus increasing the probability of achieving a successful product. In opposition, a non effective materials selection increases the risk of downstream rework and disappointing market results. The failure rate of good technical products is alarming; well over 90% according to estimates don’t appeal to the unforgiving consumer, [2]. Although these failures are not all related to materials and processes selection, this issue certainly contributes for a significant percentage of disappointments.

On the other hand, the number of materials available to design teams is over 120 000 and this number is growing at an increasing rate, [3]. Moreover, the ability to change the materials properties in the manufacturing process increases even more the complexity of the selection process.

“Not too long ago, materials selection was considered a minor part of the design process. Today that is an unacceptable approach for all but routine and simple designs”, [4]. That vision in 1991 is more and more true nowadays: materials shortages resulting from high market demand, low cost and low weight pressures from market, the need for higher performance materials and the underlying need to do all this with “greener” materials pose a serious challenge to the design teams. In fact, design teams face a difficult challenge: to identify the “best material”, among an enormous set of alternatives, considering selection criteria ranging from technological and aesthetics to economical and environmental issues.

Fig. 1 summarizes the factors that influence product design. Material decisions must be made within this framework. The four main factors upon which the designer relies when making materials choices are the relation between material specification and technical performance, economic performance, environment performance and industrial design embedded in the product and its function, [5].

The word “selection” implies a “decision”. The selection of materials involves complex decision-making processes. This decision-making would have to balance different intricate functional, technological and economical criteria reflecting the function of the part/product, its architecture, the manufacturing process, the production volumes, the final cost and the implications to the environment through a life-cycle assessment.

There’s a myriad of methods for material selection. The next pages try to bring up some of the most relevant methods with their strengths and weaknesses and at what phase they can be used in the long product development cycle.

Materials selection engine

A methodology is proposed to address the individual components that must to be considered in an effective material selection process: technical performance, economical performance and environmental performance induced in a new product for the different available materials.

The objective of the proposed methodology is to come up with individual indicators for each of the components (technical, economic and environmental performances) and use a multi-criteria decision tool to aid the decision making process. Another objective of the methodology is to use different methods for each of the components at different stages of product development.

The top zone of Fig. 2 shows the product development flow (based on Pahl & Beitz, [6]), connected with the interactions at its successive stages with the materials selection procedures.

Concept generation phase

A new product generally arises from market needs. At this stage there is a need to identify the original system requirements (functions of the product).

After the establishment of the systems requirements, the product goes to a stage of concept generation. In this phase different concepts arise and are ranked. The original set of requirements is translated into workable concepts (engineering material properties) and a set of materials classes that adjusts to the requirements are identified.

The identification of a materials class able to fulfill the technical specifications of the new concept is the first step of the selection process. This preliminary materials identification in conceptual design is essentially function-oriented and functionalities-centered. In the concept generation stage the focus is on achieving the required systems functions [7], so the identification of viable materials supports the conception of feasible solutions. Material properties charts are probably the most common and visual way of screening material classes for a given application, [3]. This method allows for the selection of a set of candidate materials, by comparing two engineering properties at a time. A similar method can be used to perform process selection. For simple problems the method can also be used to rank the candidate materials by the use of the performance indexes.

Other authors developed software systems for materials identification. The creation and maintenance of databases to store and process data of materials can provide easy access to materials data [8]. But the access to the data is only part of the problem. In order to effectively use the data, the designer should have the right knowledge, which allows him to formulate an intelligent approach to the search and to retrieve useful data and get the better adequate materials class. A knowledge base system (KBS), as a computer system that attempts to represent human knowledge or expertise in order to provide quick and easily accessible knowledge in a practical and useful way, are the new computer aided tools capable of assisting the user in an interactive way to solve various problems in the field of materials screening, [8].

Embodiment design phase

The following phase in the Product Development is the embodiment design stage, where the level of detail increases and the concepts are translated into sketches with some level of detail. In this stage the materials selection takes a paramount importance, since the concepts details can be different depending on material selection.

At the embodiment design level, the “Materials Selection Engine 1^st loop” is proposed (Fig. 2). The data available is identified and for each concept a set of candidate materials is identified. An evaluation of the materials screening is necessary. For that a preliminary estimation of part features is needed using for example CAD and even simple CAE tools. One must mention that the objective is not to have an extensive and validated design, but only to have an idea of the effect of the material on the most relevant part features and especially on part dimensions.

With a preliminary geometry found it is possible to use methods for ranking the materials in the 3 components of evaluation: technical, economical and environmental.

Each of the components can be estimated, from a given set of material attributes. For example the technical performance component can have a contribution of some mechanical properties like corrosion resistance, weight of the part, stiffness, among others. Each of these attributes has a different importance for the technical component, so the ranking of the materials is also, by itself, a multi-criteria decision making problem.

The technical performance components can be assessed by several methods. Even the use of simple methods like the simple addictive weights is often complicated, in particular the estimation of the weights that each property takes. In fact, often the biggest concern is about the subjective character of the weights. For example: for a cantilever beam, what is more important: the stiffness, the strength or the weight? All of course are very important, so 33% importance to each factor can be used. But one can argue that stiffness in a beam is far more important than strength (given that all the alternatives can support the load), or otherwise in other applications. To assess the weights there are some techniques like the entropy method, the analytical hierarchy method or the digital logic method. To rank the alternatives there are also some different techniques: ELECTRE methods, TOPSIS, GRA and others. Table 1 provides some examples of use of these methods in the literature.

Table 1 – Decision making methods used for materials selection.

Ranking	Weights	Authors
ELECTRE	Entropy	[9]
TOPSIS	Entropy	[10]
TOPSIS	AHP	[11]
WPM	Digital logic	[12]
GRA	AHP	[13]
GRA	NSFDSS
GFDA	AHP	[14]
GFDA	FLA

Another technique that can be used to raise the weights of the attributes and to perform trade-off analysis for materials selection is the Multi Attribute Utility Analysis (MAUA), [5]. Utility analysis affords a rational method of materials selection which avoids many of the fundamental logical difficulties of many widely used alternative approaches. However this method is valid when there is certainty regarding the attribute levels of the alternatives. Whenever that is not the case, another operations research technique, subjective probability assessment (SPA), can be used to address this issue. SPA makes it possible to measure a probabilistic distribution describing the confidence of the decision maker in the levels of attributes for which there is a high degree of uncertainty. These probability distributions can be used in conjunction with MAUA to provide a consistent framework for making materials selection decisions, [15]. In order to obtain the SPA for new materials that the designers have uncertainty regarding attributes, a questionnaire must be used,[16].

All these methods with some advantages and disadvantage in accordance to the specific problem can be applied to the materials ranking concerning technical performance.

As regards economical performance it can be assessed based on cost metric computation. One of the attractions of cost as a basis for decision making is the apparent simplicity of the metric: an economic measure of the resources employed to undertake a set of actions, typically to yield a good or service. However, engineers are usually far less comfortable with cost when tasked with relating it to a set of specific technical or design changes. The difficulty arises from the fact that cost has traditionally been associated with accounting rather than with the engineering field, [17]

The economic performance component can be assessed with the use of predictive process based cost modeling techniques (PBCM), [17]. The use of this type of tools at this stage is of great importance because the PBCM is composed of three interrelated and interdependent models: a process model, an operations model and a financial accounting model. The PBCM explicitly offers the modeler the opportunity to incorporate engineering and operational functionality into estimates of resource requirements, whose technical interactions are largely ignored or oversimplified in other modeling approaches. This cost analysis can be extended to consider not only cost of production (the one directly supported by the producer), but also the life-cycle cost (LCC) of the product, [18]. More and more companies are trying to develop ways to lower the cost of use and disposal of products (e.a. aircraft or automotive industry) as a tradeoff for higher manufacturing costs.

The environmental performance component can be assessed using life cycle assessment tool. The eco-indicator 99 is one of the tools that can be used to rank the alternatives. The eco-indicator 99 uses a cradle to grave approach and consider 11 environmental impact categories in the following three areas: Human health, ecosystem quality and resources. [18],[19], [20] and [21] use the LCA technique for different applications to understand the environmental impact of the materials used to produce a product.

These estimations must be done as soon as possible in the early phases of product development where major improvements can be done to the design.

The major issues faced over the last decade concerning motor vehicles are: environmental constrains, economic demands and performance enhancements, [22]. The huge effort taken by the automobile companies to reduce exhaust emissions obliges to better motors and for higher performance materials that enable to lose weight and aerodynamic drag of the vehicle.

When the 3 components are calculated, the decision can be done using a ternary graphic, where each axis is associated to the importance of each performance component. At this point, the problem is also to determine the weights of each of the components. This decision must be made by each company according to their decision values of the long term revenue.

Detail design phase

When the product reaches the detail design (Fig. 2), a “Materials Selection Engine 2nd loop” is needed. The objective of this phase is essentially to refine and validate the “best material”. The natural evolution of the product development in the phase of detail design can change the ranking obtained in the materials selection 1^st loop. In this stage, for questions of time expenditure, a smaller set of materials can run the materials selection 2^nd loop.

Conclusions

Materials and process selection is a very complex decision making process by itself. There are technological, economic and environmental issues that must be taken in account in the early stages of product design.

The use of material set-based product development strategy increases the number of operations necessary to deliver a final product in the early stages of product design. However, this effort is of paramount importance because it is at these stages that fundamental decisions are made. If the selected material is frozen to early the result can be a poor solution too far from the “best one”.

The proposed methodology is very simple, but hard work is hidden inside, mainly when product complexity increases. When a new product contains different parts made with different materials the work load increases exponentially. The outcome of the proposed methodology is a strategic tool, or a designer thinking guide, for informed decision making regarding materials selection at the different stages of product development.

Acknowledgments

The authors would like to acknowledge the MIT-Portugal Program.

Keywords

Materials selection; Product development, Decision making tools

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Illustrations

Fig. 1. The driving forces that influence the product design [5]

Fig. 2. The design flow chart and the relation with the materials selection engine. Materials and process selection information is required in every step – breath at the upstream, detail at the downstream.