Paul E. Krajewski

Overview of Quick Plastic Forming Technology

General Motors has developed Quick Plastic Forming (QPF) as a hot blow forming process capable of producing aluminum closure panels at high volumes. This technology has been successfully implemented for automotive liftgates and decklids with complex shapes. This talk will review key elements of the QPF process, describe some of the technical achievements realized in this process, and identify areas for future research in process, material, and lubricant development.

Mass Decompounding and Vehicle Lightweighting

Although mass reduction can be associated with additional costs, a decision to lightweight a structural subsystem may, depending on when in the vehicle development process the decision is taken, result in secondary (additional) mass savings such that the value of lightweighting is substantially increased. This paper overviews a method to estimate the potential for secondary mass savings in different vehicle subsystems. We close by describing current research efforts aimed at developing new lightweight product solutions for both body and powertrain applications along with commensurate manufacturing processes.

Tribological issues during quick plastic forming

Quick plastic forming (QPF) was developed as a high-volume, hot blow forming process for automobile components, enabling larger volume applications than traditional superplastic forming (SPF). One critical aspect of the process is the tribological interaction between the forming tool and the aluminum blank, as this impacts formability, surface quality, and tool durability. While QPF has been successfully implemented for automobile components, many opportunities exist for improving the tribological condition during the process, including the die coating or treatment, the lubricant, and the fundamental understanding of aluminum/iron adhesion under QPF conditions (450 °C). This work reviews key tribological issues affecting QPF and identifies areas where additional research is required.

Deformation and Failure of a Superplastic AA5083 Aluminum Material with a Cu Addition

A modified AA5083 aluminum sheet material containing a Cu addition of 0.61 wt pct has been investigated under conditions relevant to commercial hot-forming technologies. This material was produced by continuous casting followed by industrial hot and cold rolling into sheet. Deformation and failure mechanisms at elevated temperatures were investigated through mechanical testing, thermal analysis, and microscopy. The effects of Cu addition are evaluated by comparisons with data from AA5083 sheet materials without Cu addition, produced both by continuous and direct-chill (DC) casting techniques. At low temperatures and fast strain rates, for which solute-drag (SD) creep governs deformation, the Cu addition slightly increases tensile ductility at 450 °C but does not otherwise alter deformation behaviors. At high temperatures and slow strain rates, for which grainboundary-sliding (GBS) creep governs deformation, the Cu addition decreases flow stress and, at 450 °C, improves tensile ductility. A strong temperature dependence for tensile ductility results from the Cu addition; tensile ductility at 500 °C is notably reduced from that at 450 °C. The Cu addition creates platelike particles at grain boundaries, which produce incipient melting and the observed mechanical behavior.

Forming limit diagrams for AA5083 under SPF and QPF conditions

Forming Limit Diagrams (FLD’s) for AA5083 aluminum sheet were established under both Superplastic Forming (SPF) and Quick Plastic Forming (QPF) conditions. SPF conditions consisted of a strain rate of 0.0001/s at 500°C, while QPF conditions consisted of a strain rate of 0.01/s at 450°C. The forming limit diagrams were generated using uniaxial tension, biaxial bulge, and plane strain bulge testing. Forming limits were defined using two criteria: (1) macroscopic fracture and (2) greater than 2% cavitation. Very little difference was observed between the plane strain limits in the SPF and QPF conditions indicating comparable formability between the two processes with a commercial grade AA5083 material.

The effect of particle reinforcement on the creep behavior of single-phase aluminum

The effect of TiC particle reinforcement on the creep behavior of Al (99.8) and Al-1.5Mg is investigated in the temperature range of 150 °C to 250 °C. The dislocation structure developed during creep is characterized in these materials. The addition of TiC increases creep resistance in both alloys. In pure aluminum, the presence of 15 vol pct TiC leads to a factor of 400 to 40,000 increase in creep resistance. The creep strengthening observed in Al/TiC/15p is substantially greater than the direct strengthening predicted by continuum models. Traditional methods for explaining creep strengthening in particle-reinforced materials(e.g., threshold stress, constant structure, and dislocation density) are unable to account for the increase in creep resistance. The creep hardening rate(h) is found to be 100 times higher in Al/TiC/15p, than in unreinforced Al. When incorporated into a recovery creep model, this increase inh can explain the reduction in creep rate in Al/TiC/15p. Particle reinforcement affects creep hardening, and thus creep rate, by altering the equilibrium dislocation substructure that forms during steady-state creep. The nonequilibrium structure generates internal stresses which lower the rate of dislocation glide. The strengthening observed by adding TiC to Al-1.5Mg is much smaller than that found in the pure aluminum materials and is consistent with the amount of strengthening predicted by continuum models. These results show that while both direct (continuum) and indirect strengthening occur in particle-reinforced aluminum alloys, the ratio of indirect to direct strengthening is strongly influenced by the operative matrix strengthening mechanisms.

High-Temperature Forming of a Vehicle Closure Component in Fine-Grained Aluminum Alloy AA5083: Finite Element Simulations and Experiments

Fine-grained AA5083 aluminum-magnesium alloy sheet can be formed into complex closure components with the Quick Plastic Forming process at high temperature (450oC). Material models that account for both the deformation mechanisms active during forming and the effect of stress state on material response are required to accurately predict final sheet thickness profiles, the locations of potential forming defects and forming cycle time. This study compares Finite Element (FE) predictions for forming of an automobile decklid inner panel in fine-grained AA5083 using two different material models. These are: the no-threshold, two-mechanism (NTTM) model and the Zhao. The effect of sheet/die friction is evaluated with five different sheet/die friction coefficients. Comparisons of predicted sheet thickness profiles with those obtained from a formed AA5083 panel shows that the NTTM model provides the most accurate predictions.

Recrystallization and grain growth phenomena in a particle-reinforced aluminum composite

Recrystallization and grain growth in a 2219/TiC/15p composite were investigated as functions of the amount of deformation and deformation temperature. Both cold and hot deformed samples were annealed at the normal solution treatment temperature of 535 °C. It was shown that large recrystallized grain diameters, relative to the interparticle spacing, could be produced in a narrow range of deformation for samples cold-worked and those hot-worked below 450 °C. For cold-worked samples, between 4 to 6 pct deformation, the recrystallized grain diameters varied from 530 to 66 μm as the amount of deformation increased. Subsequent grain growth was not observed in these recrystallized materials and noncompact grain shapes were observed. For deformations greater than 15 pct, recrystallized grain diameters less than the interparticle spacing were observed and subsequent grain growth produced a pinned grain diameter of 27 μm. The pinned grain diameter agreed well with an empirical model based on three dimensional (3-D) Monte Carlo simulations of grain growth and particle pinning in a two-phase material. Tensile properties were determined as a function of grain size, and it was shown that grain size had a weak influence on yield strength. A maximum in the yield strength was observed at a grain size larger than the normal grain growth and particle-pinned diameter.

Magnesium alloys for lightweight powertrains and automotive structures

Abstract: This chapter introduces magnesium, the lightest of the structural automotive metals. it provides an overview of alloy nomenclature and properties, and the major casting, sheet forming, and extrusion processes. Descriptions of automotive magnesium applications produced by each process are provided and there is a summary that describes the challenges in alloy and process development that need to be overcome if the magnesium content in automotive sub-system applications is to be increased.

Local Thinning at a Die Entry Radius During Hot Gas-Pressure Forming of an AA5083 Sheet

Splitting at regions of local thinning below die entry radii is a critically important mechanism of failure in hot gas-pressure forming of sheet materials. Local thinning is controlled by sheet-die friction and die geometry, as well as sheet material properties. In this study, local thinning is investigated at a particularly severe die entry radius during hot forming of a fine-grained AA5083 sheet at 450°C. Particular emphasis is placed on the relationship between local thinning and sheet-die friction conditions. A simple analysis of the mechanics of this thinning phenomenon is presented. Finite element simulation results are presented for different sheet-die friction conditions. Sheet thickness profiles measured from parts produced in forming experiments using three different lubrication conditions are compared with predictions from simulations. Simulation predictions agree well with experimental data for the occurrence and location of thinning below a die entry radius. Additional insights into sheet-die friction for controlling local thinning and preventing premature necking failure are detailed.