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Public concern for environmental issues, concern for the future availability of petroleum and increasing fuel costs are creating a demand for more fuel efficient vehicles. In response to these pressures, the automobile industry is vigorously pursuing vehicle light weighting. Vehicle mass is the single greatest factor affecting fuel efficiency. Current-generation alternative power trains, such as gasoline-electric hybrids, significantly increase vehicle mass, which makes reduction of mass in the vehicle structure even more critical. There are three methods to reduce vehicle mass. Vehicle size can be decreased, which involves primarily issues of design and marketing. Less material can be used without changing vehicle size. This requires more efficient structures, which typically requires improved metal forming technologies to produce more complex components. Metals, primarily steels, currently used in vehicle structures can be replaced with light-weight metals, such as aluminum and magnesium alloys. Composite systems, such as fiber-reinforced polymers, are far from being cost competitive with metals for most consumer vehicles and are, thus, not a credible alternative to metals for either manufacturing more complex parts or as a replacement for steels.
Steel has traditionally been the most prevalent material used for structural components in consumer automobiles. New generations of advanced steels, such as high strength low alloy (HSLA) steels, continue to provide decreased vehicle structure mass with improved performance . However, even greater gains in vehicle light weighting can be achieved by using light alloys. Moreover, recent increases in fuel costs make light alloys, which generally have a higher cost than steels, quite attractive for use in consumer vehicles. Aluminum currently offers the greatest potential for improved fuel economy through vehicle light weighting at the lowest cost, compared with other current technologies . The use of aluminum alloys in automotive closure applications, which generally require wrought metal alloys, can reduce mass by 30 to 50% compared with similar steel closures . Mass reduction dramatically affects fuel economy. Approximately 0.4 mpg are gained for every 100 lbs. of weight reduction . Another important benefit of using vehicle light weighting is a reduction in CO2emissions; every kilogram of aluminum which replaces two kilograms of steel can provide a net reduction in CO2 emissions of 10 kg over the life of a vehicle . A major barrier to the introduction of aluminum into the vehicle body-in-white is its low formability compared to steels. While steels can offer a maximum formability of up to 50% in cold stamping, aluminums offer a maximum of only 30%. Overcoming this formability issue is a critical enabler for using aluminum to reduce the mass of vehicle structures. Aluminum otherwise offers the very attractive qualities of good specific stiffness, good specific strength, good weldability and excellent corrosion resistance. By using hot forming technologies, the effective formability of some aluminum alloys can be increased to well over 100%, enabling the forming of complex components impossible to manufacture in steel. Thus, hot forming can enable both insertion of light alloy, i.e. aluminum, components into a vehicle structure and the use of less material by producing more efficient, and more complex, components. Two sheet hot-forming technologies are used to form automotive structural and closure components, superplastic forming (SPF) and quick-plastic forming (QPF) [38, 39].
AA5083 alloy sheet is the most commonly used material for hot forming of light panel structures . This alloy is a non-heat-treatable alloy with a nominal composition of Al-4.5Mg-0.7Mn-0.1Cr-0.25Zn-0.5Si-0.4Fe-0.1Cu in wt. percentage . AA5083 offers many desirable properties for use in automobile body structures, such as low density, high strength, good corrosion resistance and good weldability. This alloy can be processed to have a fine grain size and excellent superplastic response [41, 42]. Such fine-grained microstructure not only promotes superplastic performance during deformation at elevated temperatures, but also provides good mechanical strength at room temperature. The high Mg concentration in AA5083 provides solid-solution strengthening at room temperature and can produce solute-drag creep at elevated temperatures.
Fine-grained AA5083 aluminum sheet is used for hot-forming automotive body panels by gas pressure in the SPF and QPF processes. The fine-grained AA5083 alloy for hot forming applications is achieved through thermo-mechanical processing, which includes large cold rolling strain. Thickness reduction during final cold rolling is approximately 75% for production of fine-grained AA5083 sheet. Recrystallization after cold rolling, which typically occurs during heating prior to hot forming, occurs by particle-stimulated nucleation (PSN) of recrystallization . This reaction involves the nucleation of new grains in the highly deformed regions around dispersed particles. The high Mn content of AA5083 is required to produce a significant volume fraction of Al6Mn particles, pro-eutectic products, with sizes of greater than approximately 2 Î¼m to locally concentrate matrix strain during cold rolling and activate the PSN mechanism. A fine-grained (<10 Î¼m) microstructure with random recrystallization texture is achieved with uniformly-distributed constituent particles [43, 24]. In addition to the rather coarse particles, pro-eutectic products, of Al6Mn, Mn additions also create fine precipitates which pin grain boundaries and stabilize grain size at elevated temperatures . These precipitates do not otherwise provide any significant strengthening at room temperature. In addition, superplastic performance can be improved by lowering the amount of Fe and Si impurities  or by promoting grain-boundary-sliding (GBS) creep through Cu addition . The most common particle observed in AA5083 is Al6(Mn,Fe) with sizes of the order of one micrometer . Other particles commonly reported by literatures include Mg2Si  and (Fe, Mn)3SiAl12 (50-100nm) [45, 46].
Two sheet hot-forming technologies superplastic forming (SPF) and quick-plastic forming (QPF) are used to form automotive structural and closure components;
2.1.1 Superplastic Forming
After 40 years of development, SPF is now considered a standard process in several industries, including aerospace and transportation . SPF depends on the phenomenon of superplasticity, the capability of certain alloys to undergo extensive, neck-free, tensile deformation at elevated temperature (typically T > 0.5 Tm, Tm is the melting temperature of the material) prior to fracture . Superplastic behavior is commonly associated with a high strain-rate sensitivity, m, as in equation ÃÆ’=Ã°¾Ã°'Å¡ where ÃÆ’ is the flow stress, is true strain rate and K is a constant which depends on test temperature, material and microstructure. The m values for most metals are less than 0.3, whereas superplastic materials achieve m values of 0.5 or greater. Superplastic deformation is associated with GBS creep, which requires a fine grain size, typically less than 10 Î¼m. A typical SPF process uses hot gas pressure to form a sheet into a single-sided die of desired shape, as shown in Fig. 2.
A schematic of the SPF process
The single-sided tool is far less expensive than matched tooling for low volume production, for which capital investment must be minimized. SPF has been extensively used in the aerospace industry because small production quantities are required and complex parts must be produced in materials with relatively low room-temperature formability . SPF has the ability to achieve large strains to failure in aluminum sheet alloys for the manufacture of complex panels. In addition to the low-capital-investment requirements of SPF, the high forming temperature associated with this process results in stress relieving, which eliminates spring back, a common and costly problem encountered with conventional cold-stamping process. However, within the automotive industry, the primary problems with SPF technology are manufacturing cost and productivity which greatly depend on forming cycle time. Beside the requirement of costly SPF-grade fine-grained aluminum alloys, SPF involves operating at an elevated temperature of 500Â°C or higher and a relatively slow strain-rate of <10-3S-1. These requirements make SPF an expensive process with long forming time and, thus, only suitable for low-production.
2.1.2 Quick Plastic Forming
QPF was recently developed by General Motors as a hot blow-forming technology that adapted the SPF process to produce aluminum closure panels at high volumes . The QPF process uses commercial-grade AA5083 sheets and operates at lower temperatures (~450Â°C) and faster strain rates (10-1~10-3S-1) than traditional SPF process. Under these conditions, dislocation creep phenomena, specifically solute-drag (SD) creep, become more important and contribute significantly to deformation . It has been shown that the conditions for QPF operation produce material deformation which is controlled by both GBS and SD creep [41, 49]. Although the formability of AA5083 in QPF is less than in SPF, the QPF process is capable of producing complex automobile parts at remarkable rates. Several production closures have been successfully produced using this technology, including the Oldsmobile Aurora deck lid, Chevrolet Malibu Maxx lift gate, and Cadillac STS deck lid, with over 300,000 panels produced through the end of 2005 . Comparing with SPF technology, the required cycle time for QPF has been reduced from ~30 minutes per part to 2ââ‚¬"3 minutes per part , which make the QPF technology viable for commercial forming of complex parts in large quantities at fast production rates.
2.2 Deformation Mechanisms
There exist a variety of creep deformation mechanisms, described in great detail by Nieh et al , such as diffusional creep, GBS creep, dislocation creep, and etc. More than one creep mechanism can be active at the same time. If several mechanisms are independent and operating in parallel, then the steady-state creep rate is given by
Where ÃµÃ°'- is the creep rate for the ith mechanism. The fastest mechanism will dominate the creep behavior. For mechanisms operating in sequence, then the steady-state creep rate is given by and the slowest mechanism will control the rate of creep deformation.
Two deformation mechanisms, GBS and SD creep, are known to govern the plastic deformation of AA5083 in hot forming processes over the strain rates and temperatures of interest for QPF . In conventional superplastic materials GBS creep dominates deformation at slow strain rates and high temperatures, and SD creep dominates deformation at fast strain rates and low temperatures. The creep behaviors of AA5083 materials are summarized in Fig. 3 , for which the logarithm of the Zener-Hollomon parameter, Z, is plotted against the logarithm of Youngââ‚¬â„¢s modulus compensated stress.