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The Evolution of Technology for Materials Processing Over the Last 50 Years: The Automotive Example

March 1, 2007

By Taub, Alan I; Krajewski, Paul E; Luo, Alan A; Owens, John N

Author’s Note: The automotive business is an ideal choice to examine the dramatic impact of improved materials and manufacturing processes on an industry. Automakerstodayareabletocombinehigh-tech materials originally applied in aerospace and other industries with the high-volume manufacture of a mass-marketed consumer product. This paper will detail how many of the changes to vehicles that have resulted from these influences over the past 50 years have been enabled By significant advances in materials and processes.

THE FIRST 50 YEARS

The basic elements of the automobile had matured substantially by the middle of the twentieth century-the first motor car was patented by Karl Benz in 1886 and the basic principle of the automobile manufacturing process, the moving assembly line, was first put into practice in 1913.1

By the 1950s, vehicle engineering and manufacturing processes had developed to the point where annual freshening of body styles was possible even with the requirement of efficiently ramping up new models to hundreds of thousands of u n its per year for each body type.2 In that same timeframe, the gasoline internal combustion engine had also advanced significantly. General Motors (GM), in fact, introduced the first overhead valve, high-compression eight- cylinder (V8) engine in the 1953 Buick Roadmaster and the small- block V8 in a 1955 Chevrolet. That first small block displaced just 4.3 liters and produced up to 195 horsepower, or 45 horsepower per liter. Ninety million engines and lour generations later, the small block today displaces up to 6.0 liters and achieves ~400 horsepower, or roughly 65-70 horse power per liter.3 The substantial improvement in power density is even more impressive when one considers that smog-forming emissions (hydrocarbons and nitrogen oxides) have been reduced by more than 99 percent at the same time.4

The large economies of scale available to the industry made vehicles affordable to growing numbers of people. Henry Ford designed the Model T to be “a motor ear for the great multitude . . . so low in price that no man making a good salary will be unable to own one,”5 and A lf red Sloan stated that “the ideal toward which (GM) is striving is to have “a car for every purse and purpose’ and to make every car represent maximum value to the purchaser at its respective price.”6 By 1955, the cost to buy a baseline Chevrolet Bel Air ($1,725) represented just 20 weeks of U.S. wages.7 Affordability promoted personal mobility and by the mid-1950s, the North American auto industry was producing almost 10 million units per year (compared to 14.6 million units today).8 The trend has continued-despite dramatically increased vehicle content, it takes only 24 weeks of a U.S. median family income to purchase a vehicle today.9 Global vehicle ownership has grown even more impressively- the industry produced just over 13.5 million units in 1950 compared to almost 66 million units today.10 In 1950, there were less than 60 million vehicles on the planet and only two percent of the world’s population were vehicle owners. Today, the global vehicles in use stand at 800 million, which translates to approximately 12 percent of the world’s population owning automobiles.11

INDUSTRY CHALLENGES IN PAST 50 YEARS

Beginning in the 1960s, the auto industry has had to deal with a number of significant “externalities” that have driven discontinuous changes in the automobile. These external pressures have included emissions concerns, issues related to energy consumption and availability vehicle safety, and, more recently growing consumer demand for more personalized products. Main of the changes to our vehicles that resulted from these key influences have been enabled by advances in materials and manufacturing processes (Figure 1).

At the start of the 1950s, design and performance were the key differentiators of the product in the marketplace. This worked well for the industry until the mid-1960s, when Ralph Nailer and other consumer advocates began to draw increased attention to automobile safety.12 With the heightened locus on safety, new features like energy-absorbing systems, structures, and materials were introduced into the vehicle to protect occupants in the event of a crash. These changes resulted in more demanding vehicle requirements and increased engineering complexity.

Following close on the heels of the calls for enhanced safety, the Organization of Petroleum Exporting Countries oil crises of 1973 and 1979 drove oil and gasoline prices sharply higher and the industry needed to increase vehicle fuel economy. One important avenue for improving vehicle efficiency, and thus fuel economy, is mass reduction. For a mid-sized family car weighing 1,450 kg, it takes a 45-kg reduction in mass to achieve a 0.6 miles-per-gallon improvement in fuel economy.13

To more easily reduce vehicle weight, the industry moved cars from the body-on-frame (BOF) architecture to the more weight- efficient body-frame-integral (BFI) architecture. With the BFI architecture, all body panels contribute to vehicle stiffness and performance. At the same time, lightweight materials began to he substituted for the low-carbon steel that dominated the vehicle structure. With this new architecture and additional engineering safety requirements, it became more difficult to make major styling changes and, therefore, it began to lake longer to refresh vehicle designs.

With today’s more fragmented automotive market, major body architectures are now suited for a wide range of vehicles-from low- volume sports cars to high-volume BFI passenger cars and BOF trucks- requiring many different materials and manufacturing processes (Figure 2). While mans cars have BFI structures today, trucks continue to maintain the BOF architecture because this structure is ideally suited to their load and lowing requirements as well as the proliferation of body styles in this segment of the marketplace. In fact, there are many factors that are used to determine the best body architecture for a vehicle platform. The range of architectures that has been used on aluminum-intensive vehicles, for example, is due to the fact that aluminum is highly amenable to alternate manufacturing processes, as will be discussed in a following section of this paper.

In the 1990s, the era of “global hypercompetition” dawned. The growth of the global industry as well as a greater number of major global manufacturers led to more models being offered in each market and, inevitably, to dramatically fewer units per model compared to the much high numbers produced in earlier decades. In the United States and Europe, for example, the industry is rapidly approaching 400 entries, up from under 300 in the mid- 1990s-a 25 percent increase in ten years. In China, the shift is even more striking, with the number of entries forecast to quadruple to over 250 entries by 2010. With more entries, sales per entry are trending downward in the more mature markets like the United States and Europe, moving from above 50,000 in 1995 to closer to 45,000 units today. And while sales per entry are on the rise in the emerging markets, reflecting rapid growth in these areas, the only developing country where sales are expected to top 40,000 per entry by 2010 is China.14

The combination of more complex engineering requirements and a highly competitive marketplace has driven the need for more efficient and faster vehicle engineering tools. With the exponential increase in computing power over the past 20 years, there has been a virtual explosion in computer-aided design (CAD) and engineering tools within the industry. As a result, today’s product development process depends much more heavily on virtual prototyping using math- based simulation rather than on testing of physical hardware. The industry has been able to significantly reduce the lead time required to develop its new products and powert rains through application of math-based design, engineering, prototyping, and tooling development. Many manufacturers today also employ reusable design templates to automate design and engineering tasks, further speeding development.15

The proliferation of models has also led to dramatic changes to automobile factories-particularly with respect to flexibility and automation. One major enabler has been the introduction of robotics into the plant. Today a typical assembly plant body shop will have 500-000 robots, mainly used for joining (i.e., spot welding), but also for moving material into place. The paint shop will have another 30-40 robots, and the rest of the plant will have some additional robots for material handling or dispensing.16 More recently, we are seeing flexible fixturing, such as GM’s C-Flex programmable body shop tooling, which allows multiple vehicle body styles to be welded with the same robots.17

To summarize the key forces impacting automobiles: The pressure to improve fuel economy is driving weight efficiency and greater use of lightweight materials. The decreasing number of units per model is driving investment efficiency and faster factory product turnover.

This \paper will review how these forces have been met over the past 50 years with new materials and processes. Since the vehicle is a complex integration of a number of subsystems, each of which has been transformed in response to the change drivers noted previously, we will focus on structural materials, concentrating on chassis, interior, and body applications.

AUTOMOTIVE STRUCTURES

Chassis Applications

Lightweight and efficient chassis structures are important to several key performance attributes including ride, handling, and noise and vibration control. Figure 3 shows how, over the decades, new aluminum/magnesium alloys and their manufacturing technologies have enabled reduced mass and improved performance and productivity in the front cradle structure.

The first all-wrought aluminum cradle was introduced by GM in the 1999 Chevy Impala. It consisted of 15 extruded sections and two stampings and weighed 18 kg; in comparison, a typical steel sheet construction has about 48 parts weighing 28 kg. Advanced robotic aluminum welding technology (i.e., pulsed-gas metal arc welding) was used to join the complex extrusions and this occurred along a welding line of 40 robotic welders in four weld cells.18

Additional developments include thin-wall and hollow aluminum casting technology, which have made the aluminum cradle design more efficient and greatly reduced welding requirements. A good example is the hollow easting/extrusion welded cradle for the next- generation Cadillac CTS, which will start production in July 2007. The method used to produce the thin-wall (4 mm) hollow casting for this cradle is a modified low-pressure permanent mold process (vacuum riserless casting/pressure riserless casting) in which a steel mold is bottom-fed from a crucible. The vacuum pulls gases from the mold and begins the fill. Pressure is then added to fill the cavity and teed shrinkage, producing extremely sound castings with very low porosity levels (

Another significant process development is high-pressure die casting, which offers attractive flexibility in design and manufacturing. With the excellent die filling characteristics of magnesium and aluminum alloys, this process allows large, thin- wall, and complex light-metal castings to be economically produced, enabling the replacement of steel structures made of numerous stampings and weldments. Despite high productivity (a typical cycle time of about one minute), the biggest drawbacklolheconventional high-pressure die casting process is the porosity that results from entrapped gases as a result of injecting the molten metal at high velocity into the die cavity. The porosity issue is less serious for thin-wall sections (

When thicker walls are needed for stiffness and/or durability in chassis and body applications, the effect of porosity on mechanical properties (especially ductility and fatigue strength) becomes more significant. Vacuum die casting is characterized by the use of a controlled vacuum to extract gases from the die cavities, runner system, and shot sleeve during processing. This technology stretches the capabilities of conventional die casting while preserving its economic benefits. The first-in-industry one-piece magnesium die- cast cradle for the 2006 Chevrolet Corvette Z06 weighs only 10.5 kg, a 35 percent mass savings over the aluminum cradle it replaced. As shown in Figure 4, advanced simulation of die fill und solidification has proven to be a useful tool in guiding design and process control for large and small die castings.

Interior Applications

The instrument panel (IP) beam is the must important interior structural part. As shown in Figure 5, a traditional sheet steel design of ten years ago consisted of about 30 parts.21 In 1996, GM introduced the world’s largest magnesium die casting, a one-piece full IP beam for the GMC Savana and Chevrolet Express vans. In addition to mass reduction (12.3 kg in magnesium versus 18.2 kg in sheet steel), the magnesium IP design provided performance improvements such as enhanced crashworthiness, reduced vibration, and cost savings due to parts consolidation. More efficient magnesium IP designs in recent GM models have achieved even greater mass savings and pan consolidation, such as a 5.8-kg beam casting for a GM mid-size car.

The use of magnesium in IP beams, however, is currently facing strong competition. Instrument panel designs using steel tubes are only slightly heavier than magnesium die castings but can be significantly less expensive. Tubular designs in magnesium are also being explored for future IP development. A hybrid IP design using a magnesium tube over-molded with die castings is 55 percent lighter than a steel tubular design and 30 percent lighter than a one-piece magnesium die-casting design with equivalent stiffness.22

Body Applications

Over the last 50 years, passenger car body structures have moved from all mild-steel sheet construction to mixed materials (Figure 6)- a transition that also has been enabled by advanced manufacturing technologies. In the 1990s, the development of aluminum vacuum die casting, extrusion, and welding/joining technologies-along with the creation of special cast aluminum alloys such as AURAL-2 and Magsimal 59-were key enablers for aluminum-intensive vehicles like the Audi A8/A2, Acura NSX, and Jaguar XJ.

Over the same timeframe, the steel industry has made significant advances in steel alloys. Many advanced high-strength steel grades were developed for automotive applications, including high-strength low-alloy (HSLA) steel, dual-phase steel (a mixture of primarily soft ferrite and hard martensite, possibly with some bainite), and transformation-induced plasticity steel with very high tensile strength (600 MPa) and elongation (36 percent). In many cases, these steels offer only slightly less mass reduction compared to aluminum but at a fraction of the cost. Advanced high-strength steel has made significant inroads, with application in many light vehicles such as the BMW 7-Series and Lexus LS430.

GM’s Epsilon body structure, shown in Figure 6, is a high-volume global architecture represented by the Chevrolet Malibu in North America. Introduced in 2003, this was the first GM vehicle that made extensive use of advanced high-strength steels. The body structure used in GM’s current midsize cars (Malibu, Pontiac G6, and the new Saturn Aura) includes five percent HSLA and 12 percent dual-phase steels. It is expected that the body structure for future- generation midsize cars will contain 35 percent dual-phase and 8 percent martensitic steels.

Sheet Aluminum in Closures

Vehicle closure panels such as hoods, doors, decklids, and fenders have typically been made from cold-rolled steel sheet, which has excellent strength and formability. These characteristics, in combination with relatively low cost, make this material very amenable to high-volume stamping.

While steel is pervasive in vehicle closures, the use of lightweight sheet aluminum has been increasing over the past 30 years. Aluminum closures provide 35-50 percent mass reduction com pared to similar steel closures. However, unlike the cast aluminum applications described in the previous sections, the penetration of sheet aluminum has not been as widespread because the raw material is two to three times more costly than steel and it also has significantly lower formability. A complex part like a door or liftgate inner panel has to be redesigned or made from multi-piece assembly because aluminum cannot he conventionally stamped into intricate shapes.

Despite these challenges, sheet aluminum has a long history in the automobile industry. The 1909 Model T Touring Car was made with aluminum body panels, and aluminum hoods were commonly used in many Model Ts (Figure 7).23 Many closure panels were made with aluminum sheet in this time period, using 1xxx or 3xxx series alloys, which have good formability but low strength. The common theme in these early automotive applications was low-volume, niche products that were labor intensive and emphasized craftsmanship.

As automotive volumes increased after World War I, steel became the material of choice in the automobile industry. Steel sheet was used almost exclusively until the oil crises of the mid-1970s drove automakers to explore decreased mass as a method for improving fuel economy. Aluminum sheet began to be reintroduced in the late 1970s, primarily for hoods since their relatively simple design could easily accommodate aluminum stamping. There were many applications for a few years, but the industry switched back to steel for economic reasons by the mid-1980s.26 In the 1990s, the use of aluminum sheet for closures, especially hoods, increased again. This shift was spurred by the number of aluminum-intensive vehicles being developed across the industry.

The growth in aluminum sheet applications over the past 30 years has largely been enabled by manufacturing improvements rather than material improvements. In fact, the common automotive aluminum sheet materials-AA6111, AA5754, AA5182, and AA5030-have existed since 1982, 1970, 1967, and 1954, respectively.27 Most of the manufacturing improvements have been in conventional stamping. Since the characteristics of aluminum are different from those of steel, the stamping industry had a steep learning curve in areas such as finite-element modeling, die design, formability analysis, part handling, lubrication, die coating, die maintenance, and scrap segregation.

Although these improvements have facilitated aluminum usage, the fundamentally lower formability of aluminum compared to steel remains a challenge. To address this issue, alternate forming processes continue to he investigated. These include superplastic \forming,28 quick plastic forming (QPF),29 hydroforming,30 thermohydroforming,31 warm forming,32 roller hemming,33 retrogression heat treatment,34 electromagnetic forming,35 and preform annealing.36

As examples of these technologies, QPF and roller hemming each address different formability problems and provide solutions that are enabled by simulation and robotics technology. In the case of QPF, the technology was developed to take advantage of the enhanced plasticity of certain aluminum alloys at elevated temperatures. The technology for roller hemming was originally developed as a method to reduce tooling investment and provide increased manufacturing flexibility; however, it ended up providing an advantage with aluminum because it modified the strain path and improved formability.

Quick Plastic Forming

The phenomenon of superplasticity, in which fine-grained or dual- phase materials exhibit extremely high ductility due to grain boundary sliding, has been known since the famous work of Backofen over 40 years ago.37 Superplastic forming (SPF), which uses gas pressure to form superplastic alloys into single-sided dies, has been used extensively in the aerospace market to make a variety of components for spacecraft and commercial airplanes.38 Since the 1970s, it has also been used by the automotive industry to produce complex, lightweight panels for niche products with annual volumes of less than 1,000 units. An example of the SPF process, where panels are manually extracted from large heated presses, is shown in Figure 8. The technology has been optimized around niche vehicles where tooling costs are extremely low, cycle times are long, labor content and material price are high, and parts can be reworked after production because each vehicle is handcrafted.

Quick plastic funning was developed by GM as a hot blow-forming technology that adapted the SPF process to the mainstream automotive industry. It has been implemented to produce liftgates for the Chevrolet Malibu Maxx and decklids for the Cadillac STS. The goal of QPF technology development was to create a process that would enable dimensionally accurate automotive parts, manufactured at automotive volumes for a competitive price. The requirement for dimensionally accurate parts necessitated the development of improved part release mechanisms, a new seal-head geometry, and, most importantly, a fully automated process.39 A photograph showing the automated part handling system is shown in Figure 9.

A key enabler for automating the QPF process was the development of the integrally heated tool, which eliminated the need for a special heated press. The heated tool allowed the use of conventional material handling equipment and also prevented heat loss during multiple press cycles. The requirement for dimensionally accurate parts also resulted in a lower forming temperature (450C compared to >500C). Decreasing the temperature gave me panel more strength and made it less susceptible to damage or distortion during handling.

Automotive parts are generally not as complex as aerospace parts. Thus, while improved formability was required compared to conventional stamping, the formability did not have to meet traditional SPF levels of up to 1,000 percent. This led to the development of math-based tools such as PAMQPF, a finite-element modeling code that could predict the formability required to make the desired components.40

To manufacture parts at automotive volumes, the process also had to have the ability to produce up to 100,000 panels per year. This required that cycle times be reduced from ~30 minutes per part to 2- 3 minutes per part, which meant that the material needed to be deformed at strain rates of 0.001-0.01 per second. This is at least ten times faster than SPF materials were typically deformed, meaning forming limits and deformation behavior needed to be re- evaluated.41

Finally, the requirement for lower cost drove the development of an improved material for the QPF process. This involved formulating a lower-cost version of AA5083 that was able to tolerate a higher impurity level (e.g., iron). The commercial-purity AA5083 materials developed tor QPF provided almost all of the formability of me high- purity materials but were cost-competitive with commercial aluminum sheet materials.

Roller Hemming

A significant formability challenge for aluminum in vehicle closure applications is the ability to hem the outer panel around the inner panel (requiring a 180-degree bend) to produce a tight, crack-free assembly. The quality of the hem on a closure panel is very important to the appearance of the vehicle since it defines the fit between body panels. In the automotive industry, this has been termed the “jewel effect.” Because aluminum is typically up-gaged compared to steel and has lower formability, the radius at the edge of the hem is larger than is typical for steel, thus giving a less desirable appearance. Improving the hem situation in aluminum has been addressed by a variety of methods, including increasing the hem radius (rope hem), using a down-standing flange instead of a hem, using lower-strength 6xxx series alloys, and applying localized retrogression heat treatment.42 While each of these techniques has demonstrated improvement in hemming, they all have drawbacks that have prevented their widespread use.

Roller hemming uses a rotating tool on the end of a robot arm to provide the mechanical deformation associated with hemming (Figure 10). This represents a significant change from the traditional methods. Hemming was originally a manual operation where the inner and outer panels were located in a fixture and the hem was formed by hammering the flange from the outer panel over. As volumes increased, mechanical hemming was performed first in a press, then with smaller “table-top” units with complex cam systems to bend and flatten the flange into a hem. These processes were expensive, requiring dedicated tooling and presses for each application. With roller hemming, only a simple clamping fixture is needed tor each assembly enabling decreased investment and increased flexibility. The robot can he programmed to follow any desired path and thus the roller hemming cell can be used for a variety of closure panels. This becomes more advantageous as the number of model variants for a particular vehicle increases, effectively reducing the volume requirements for specific closures. The evolution to roller hemming over the past 20 years has very positively impacted hemming force, investment, and plant floor space.

While not originally developed for aluminum, the strain path used in roller hemming actually is more favorable for aluminum, creating a less severe condition and allowing a flat hem to be produced.39 The rolling motion of the process moves the strain path away from pure plane strain, which is the worst condition tor formability. This effectively decreases the amount of strain at the surface of the bend, thereby allowing more forming before fracture occurs. Recent work has shown that aluminum alloys that exhibit cracking during conventional hemming operations do not exhibit cracking during roller hemming. As a result, this technology has recently seen increased usage in aluminum applications, such as the Audi A8.41 It likely will be a critical enabler for the use of aluminum in future vehicle closures.

HYDROFORMED STRUCTURES

Tube hydroforming is a metal-forming process that uses pressurized fluids such as waler to make various perimeter shapes from tubes. Compared to stampings, hydro formed tubes provide further mass savings for structural components. They are used today for many structural applications, including frame rails, engine cradles, radiator supports, and IP beams.43,44

As shown in Figure 11a, a tube hydroforming process was a key enabling manufacturing technology for the Corvette Z06) aluminum frame. The Corvette’s 4.8 m frame rail is the largest hydroformed aluminum part in the world. The hydroformed roof bow and IP beam also are very complex tubular structures that consolidated many stamped pans for significant mass savings and stiffness improvements.

The Chevrolet SSR, a low-volume sport truck, features full- length hydroformed rails similar to the Corvette frame, but this time made from steel (Figure 11b). A traditional stamped frame with equivalent strength and rigidity would weigh roughly 20 percent more than the hydroformed structure. This improved strength allows the frame to take on the road inputs and allows the suspension to do its job more precisely, resulting in better ride quality.

GM’s truck programs also make extensive use of tube hydroforming to produce steel truck frames. The technology, which is highly automated, has improved quality and manufacturing efficiency. The GMT800 truck frame, introduced in 1999, was the first high-volume application of hydroformed front rails and cross-members. The hydroformed frame doubled the torsional stiffness compared to a conventional stamped truck frame while achieving a 15 percent weight savings and better ride quality. The hydroformed frames are further improved in GM’s newly introduced GMT900 trucks, which have an annual production run of 1.3 million units.

LIGHTWEIGHT POLYMER COMPOSITE MATERIALS

Although automakers have been making composite-intensive prototype and low-volume niche vehicles for decades, broader application of polymer composites has been thought to be too expensive or not suitable for high-volume manufacturing. Of the myriad combinations of polymer type (thermoset or thermoplastic) and forming process (injection molding, compression molding, liquid molding, thermoforming, etc.), the greatest inroads have been made in compression-molded thermosets.

Compression-molded thermosets or fiber-reinforced plastics (FRP) applications have more than a 50-year history in the automotive industry. Most automotive historians credit the 1953 Corvett\e as a landmark example because it was the first widespread application of FRP in a production automobile.45 When introduced to the public at the Motorama Show at the New York Waldorf Hotel, the Corvette show car sported a body almost entirely made of composites (Figure 12). The choice of FRP for the show model was based on expediency; the original production plans called for steel panels formed from kirksite tooling.46

However, the concept of an FRP sports car captured the imagination of the American public and demand for the car caused Chevrolet to scour the FRP industry for capable molding processes. Although the open mold processes developed in the boat-building industry were evaluated, they were found to be too slow. Nevertheless, beginning in June 1953, 300 Corvettes were produced with FRP body panels made in open molds by hand rolling polyester resin into glass-fiber mats. In the meantime, a faster, more productive process was under development to meet the expected demand.

The chosen process was Molded Fiber Glass’s (MFG) matched metal die molding, which was capable of making up to 100 parts per day in a single tool. For simple shaped pans, preforms u ere made from fiberglass mais: tor more complicated or deep-draw parts, preforms were prepared by hand-spraying chopped fiberglass strands onto a shaped screen (Figure 31. The molding was accomplished by placing the fiberglass preform into the tool, hand spreading a measured quantity of polyester resin onto the perform, then closing the mold. After a three-minute cure, the molded part could he pulled from the tool and dellashed with a hand-held electric sunder.

Over the next decade, the fundamental process remained unchanged, although impressive productivity and quality improvements were made. Robert Morrison, MFC’s chief executive officer, estimated that between 1953 and 1963 processing costs for the Corvette body panels were reduced by 50 percent.47 A large fraction of these productivity gains came as a result of improved efficiencies in the preform fabrication, which allowed the process to become less labor- intensive. Stronger hydraulic presses with improved controls were also cited as contributing factors.

Despite these productivity gains, the surface quality of the preform-based molded pans was inconsistent and often required significant hand finishing prior to painting. Inconsistencies in the manually sprayed preforms were blamed for main of the surface flaws. The composites industry took two different approaches to these surface quality problems. One approach was to develop a low-shrink resin system that would be less prone to surface waviness and cracking. The second approach was to develop a pre-impregnated Mowable mat system to replace the preform process.48,49 This pre- impregnaled mat material eventually evolved into what is now called sheet molding compound (SMC). As early as 1968, these two approaches coalesced into a low-shrink SMC that was introduced into production in truck cabs by International Harvester.50

Both approaches were used by the Corvette program in the early 1970s.51 In 1971, a low-profile additive for reduced shrinkage was included in the polyester resin used in the preform process for all of the outer surface panels while SMC was implemented in a few parts with shapes diflicult to preform. Over the next few years. Corvette panels were increasingly switched from the preform process to SMC. This changeover was due to continuous improvements to the material and process. With the improxed consistency of SMC, the surface quality improved and the labor required for hand finishing decreased.

In the 1970s, when externalities such as the oil crises and increasingly stringent safety and fuel economy standards required automakers to significantly decrease vehicle mass, the productivity gains achieved with SMC led many manufacturers to turn to composites. Automotive usage of SMC more than doubled, from 20 million kg per year in 1970to nearly 45 million kg per year by the end of the decade (Figure 14).

This dramatic increase in usage would have been even larger had SMCs not been plagued xvith surface Maws such as pits and porosity, which required hand finishing prior to top-coating. In response to this issue, in-mold coating technology was developed in the late 1970s.52 This technology involved injecting a coating over the surface of the SMC while still in the mold. The resulting improvement in surface quality was dramatic, but a high price was paid in the form of extended cycle time.

The late 1970s also saw innovations in hydraulic presses when secondary hydraulic systems were developed to improve platen parallelism (Figure 15). Other press improvements included innovative short-stroke designs that allowed a secondary hydraulic system to drop and lock the upper platen into position just above the tool and a much shorter, high-pressure stroke to be initiated from the lower platen.” In addition to faster closing limes, the lower posit ion of the locked upper platen increased the effective rigidity of the press, enabling thinner parts to he molded with improved tolerances.54

Despite all of the improvements made in composites processing since the first Corvette, comparativeyl long cycle times still limited SMC’ applications to lower-volume vehicles heading into the 1980s. The five-minute cycle time required in MFG’s preform molding in 1953 had only been reduced to three minutes for a typical SMr molding by 1983. Between 1983 and 1988, however, a series of process improvements were developed to reduce cycle time (Figure 14).55

Vacuum-assisted molding was a key technology developed for shorter cycles. Removal of the air ahead of closing the press allowed use of thinner, higher diecoverage charge patterns without fear of air entrapment and blister formation. Thinner charges allow etl for faster mold closing times, which in turn enabled faster chemistries that would otherwise have led to pre-gel. Perhaps even more significantly, vacuum-assisted molding led to improved surface quality and allowed for the elimination of in-mold coating (Figure 16). This alone accounted for a 30 percent cycle time improvement.56

In the mid-1980s, improved microprocessor controls gave SMC presses unprecedented control of platen parallelism and closing speed. These presses thickness and therefore enabled thinner wall sections to be molded.57 In addition to cost and mass reduction, thinner parts also enabled shorter cycle times. By 1988, press improvements in combination with vacuum assisted molding enabled SMC productivity to finally meet the elusive 60-sccond cycle time that translates to better than 250,000 parts per year from a single tool.

Automation and robotics entered into the SMC molding process in the mid-1980s. Automated charge cutling, robotic charge placement, robotic demolding, and automated routing/ deflashing stations were gradually introduced into SMC plants.58 In terms of annual SMCproductionperworker. in 1985 the average was 12.5 tons; by 1990, this increased to 18 tons per worker.

Automotive applications of SMC continued to increase through the early 1990s. A major driving force was the lower inevstment levels needed for tooling in SMC vs. sheet steel. While the mass savings afforded by SMCs were welcome, this a as not the deciding factor. In the mid-1990s, low-density formulalions of SMC (1.3 g/cc vs. 1.9 g/ cc) were developed by replacing mineral tillers with glass microspheres. These materials replaced conventional SMCs in many structural parts but were not successfully applied to cosmetic surfaces due to the surface defects caused by microspheres at the surface.

By the late 1990s, environmental concerns drove a transition from solvent-applied primers to powder-applied primers in many automobile paint shops. The higher paint hake temperatures that were required with the new primers caused an unacceptable level of surface defects in the available cosmetic grades of SMC. As a result, cosmetic applications of SMCs were eliminated from vehicles destined for powder prime-based paint shops. Efforts are continuing to develop a new SMC formulation capable of withstanding the higher-temperature bakes.59,60

THE FUTURE

While the changes in materials and processes over the past 50 years have been dramatic, the next 50 years promise to be even more exciting as rapid advancements in materials science and engineering enable new automotive materials, novel vehicle designs, and improved fabrication technologies.

The 2000 Chevrolet Corvette Z06 already contains some of these nextgeneration materials. In addition to a lightweight aluminum frame (at 129 kg. 30 percent lighter than the steel frame it replaced ). the Curvette Z06 also uses many advanced materials such as a first-in-industry magnesium engine cradle, magnesium steering column, steering wheel, and brake module support bracket, carbon fiber floor pan (with a “unique in the industry” sandwich structure), front fenders and wheelhouses, and titanium valves and connecting rods (Figure 17). The employment of these advanced materials, in combination with the use of efficient hydroforming and die-casting processes, made this vehicle the most affordable exotic performance car in the world (e.g., 60-mph acceleration in 3.7 seconds) and the first 500-horsepower non-gas-guzzler in the United States.

As we look farther into the future, we also will see expanded use of nano-engineered and smart materials. Some of these materials are making their way in to vehicles today. For example. Figure 18 shows a nano-reinforced TPO composite that GM is using for a number of different part applications on several vehicles.61,62

One trend is clear-vehicles will consist of a more balanced use of many materials in the future, incorporating more lightweight materials such as nanocomposites and aluminum and magnesium sheet (Figure 19).63 In addition, the types of materials employed in automobil\e manufacture may depend more on materials innovations rather than on manufacturing innovations as a result of the dramatic advances being made in materials science today.

References

1. Austin Weber, Assembly, 46 (2) (2003), pp. 62-64, 66-67.

2. J Heasley, The Production Figure Book for U.S. Cars (Osceola, WI: Motorbooks International Publishers, 1977), p. 122.

3. GM News Releases, “Gen-IV V8 Marks 50 Years of Small-block Performance” (28 October 2003); “GM Introduces Gen-IV Small-block V8 for Trucks” (28 October 2003); “New Products Strengthen GM Brands for 2005″ (27 July 2004).

4. GM Public Policy Center, www.gm.com/company/ gmability/ sustainability/reports/05/400_products/8_eighty/482.html.

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Alan I. Taub, Paul E, Krajewski, Alan A. Luo, and John N. Owens are with General Motors Corporation Research & Development, 30500 Mound Road, Warren, Mi 48090. A. Taub can be reached at (586) 986- 4817; fax (586) 986-6347; e-mail alan.taub@gm.com.

Copyright Minerals, Metals & Materials Society Feb 2007

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