John E. Carsley

An Experimental Study on Robustness and Process Capability of the Warm Hydroforming Process

The warm sheet hydroforming process was investigated to determine the optimal process conditions of temperature, pressure, and pressurization rate for maximum formability of AA5754-O using an experimental stretch forming die shape. The optimal process conditions were evaluated to determine the robustness and process capability based on physical measurement of formed parts including thickness strain, cavity fill ratio, and radius of curvature. For the simple die shape investigated, a temperature of 268°C, a pressure of 25 MPa, and a pressurization rate of 0.22 MPa/s provided the most balanced combination of uniform thickness strain with the greatest cavity fill ratio and sharpest radius. Temperature had a greater effect on measured properties than either pressure or pressurization rate, although the effect of pressure increased as temperature decreased. The procedures demonstrated in this experimental study could be used to optimize process parameters for robust operation of production applications for more complex automotive body panels fabricated by the warm hydroforming process.

Postanneal Mechanical Properties of Prestrained AA5182-O Sheets

The effects of different prestrain levels, paths, and subsequent annealing on the postannealing mechanical properties of AA5182-O were investigated. Aluminum sheet specimens were prestrained in uniaxial, plane strain, and equibiaxial tension to several equivalent strain levels, annealed at 350 °C for short (10 s) and long (20 min) durations and then tested for postannealing mechanical properties, including tensile properties, anisotropy, and forming limits. The tensile properties, R-values at 0, 45, and 90 deg relative to the sheet rolling direction, and forming limit diagrams (FLDs) exhibited dependencies on prestrain and annealing history. The importance of the process variables and their effects were identified via designed experiments and analysis of variance. Three-dimensional digital image correlation, which captured the onset of local necking, was employed in the FLD development. Texture in the as-received and deformed sheets was investigated with electron backscatter diffraction and provided a means for linking prestrain and static recovery or recrystallization with microstructure. This guided the understanding of the mechanical property changes observed after preforming and annealing. Ultimately, the expanded forming limit curve demonstrated the advantage of annealing in extending the formability of strained.

Comparing Laser Welding Technologies with Friction Stir Welding for Production of Aluminum Tailor-Welded Blanks

A comparison of welding techniques was performed to determine the most effective method for producing aluminum tailor-welded blanks for high volume automotive applications. Aluminum sheet was joined with an emphasis on post weld formability, surface quality and weld speed. Comparative results from several laser based welding techniques along with friction stir welding are presented. The results of this study demonstrate a quantitative comparison of weld methodologies in preparing tailor-welded aluminum stampings for high volume production in the automotive industry. Evaluation of nearly a dozen welding variations ultimately led to down selecting a single process based on post-weld quality and performance.

Recovery quantification and onset of recrystallization in aluminium alloys

Novel forming processes for light metal alloys utilize recovery and recrystallization to extend their total elongation and enhance formability. To attain optimum efficiency in such processes, it is necessary to understand and quantify the kinetics of recovery and recrystallization in work-hardened metal alloys. An electron backscatter diffraction based method, using local average orientation spread, is shown to identify the end of recovery as well as the onset of recrystallization. Local average orientation spread results from dislocation flux and storage during plastic deformation and hence, captures the evolution of static recovery process. The method has been demonstrated using pre-strained Al–Mg alloys. The recovery kinetics is shown to be consistent with results from dislocation density based recovery models. In addition, a direct observation of the coexistence of static recrystallization and recovery illustrates competing processes for energy minimization.

Annealing response of AA5182 deformed in plane strain and equibiaxial strain paths

The annealing response of AA5182 Al–Mg alloy deformed to an effective prestrain of  = 0.15 via plane strain and equibiaxial strain paths is compared. The comparison is done at two temperatures namely, 623 and 673 K. The recovery, recrystallization and grain growth behaviour of this alloy is studied by electron backscatter diffraction and dislocation density estimation using X-ray line broadening analysis. It is found that recrystallization is slower in equibiaxial deformation condition compared to that in plane strain deformation condition during annealing. Significant recrystallization is observed after annealing for 60 s at 673 K and for 480 s at 623 K following plane strain deformation. Furthermore, significant recrystallization is associated with lower grain growth at 673 K (∼55 μm) as opposed to that at 623 K (∼75 μm). The results are explained on the basis of differences in both the strain paths.

Advances in characterization of sheet metal forming limits

This paper accounts for nonlinear strain path, sheet curvature, and sheet-tool contact pressure to explain the differences in measured forming limit curves (FLCs) obtained by Marciniak and Nakajima Tests. While many engineers working in the sheet metal forming industry use the raw data from one or the other of these tests without consideration that they reflect the convolution of material properties with the complex processing conditions involved in these two tests, the method described in this paper has the objective to obtain a single FLC for onset of necking for perfectly linear strain paths in the absence of through-thickness pressure and restricted to purely in-plane stretching conditions, which is proposed to reflect a true material property. The validity of the result is checked using a more severe test in which the magnitude of the nonlinearity, curvature, and pressure are doubled those involved in the Nakajima Test.

Cold and Warm Hydroforming of AA754‐O Sheet: FE Simulations and Experiments

The sheet hydroforming with punch (SHF‐P) process offers great potential for low and medium volume production, especially for forming: (1) lightweight sheet materials such as aluminum (Al) and magnesium (Mg) alloys and (2) thin gage high strength steels (HSS). Mg and Al alloys are being increasingly considered for automotive applications, primarily due to their lightweight and high strength‐to‐weight ratios. However, there is limited experience‐based knowledge of process parameter selection and tool design for SHF‐P of these materials. Thus, there is a need for a fundamental understanding of the influence of process parameters on part quality. This paper summarizes analyses of the SHF‐P process of AA5754‐O sheet using finite element (FE) simulations. FE simulations and preliminary experiments of SHF‐P were conducted to determine the process parameters (blank holder force versus punch stroke and pot pressure versus stroke) to form a challenging shape (a cylindrical cup with a reverse bulge) successfully at...

Estimation of Metal Hardening Models at Large Strains

Simulation accuracy of large strain deformation of sheet metals, such as that which occurs during hemming and vehicle crash situations, is limited because existing hardening laws (true stress vs. true strain relationships) are extrapolated from uniform elongation data and applied for post-uniform deformation. In this paper, a reverse-engineering method was developed to predict metal hardening laws at large strains beyond uniform elongation for sheet metals. The method required a standard uniaxial tensile test and finite element analyses (FEA), and was implemented as a custom computer code called GMSS (General Motors Stress-Strain). The true stress vs. true strain data pairs are determined when the load and displacement history of a tensile test specimen matches the FEA results using GMSS. Test cases showed that the true stress vs. true strain relationships at very large strains (75% for AA6111 aluminum, and 85% for DP600 steel) could be automatically generated using GMSS. This reverse-engineering method will provide General Motors with an easy-to-use tool for generating very accurate metal hardening laws for post-uniform deformation that can greatly improve the accuracy of FEA for formability (including hemming), and crashworthiness simulations.Copyright

Aluminum Tailor-Welded Blanks for High Volume Automotive Applications

Design of Experiment based approach is used to systematically investigate relationships between 8 different welding factors and resulting weld properties including strength, elongation and formability in 1.2mm-2mm thick friction stir welding of AA5182-O for TWB application. The factors that result in most significant effects are elucidated. The interactions between several key factors like plunge depth, tool tilt, pin feature and pin length on the overall weld quality is discussed. Appropriate levels of factors that lead to excellent weld properties are also identified.

Characterizing the Hemming Performance of Automotive Aluminum Alloys With High-Resolution Topographic Imaging

We used high-resolution quantitative surface analysis to evaluate the surfaces of two aluminum automotive closure panel alloys, which were bent to a 180 deg angle in a simulated hemming test. Maps of the displacements normal to the sheet were superimposed on the topographies to correlate the location of the maximum displacements and the surface morphology. While the alloys had similar mechanical properties, quantitative analyses yielded considerable differences in the deformed surface morphologies. One alloy had a greater density and broader size distribution of constituent particles, which increased the likelihood for particle decohesion. This resulted in large surface displacements that were uncorrelated with the underlying microstructure. While no splitting was observed in either alloy, large uncorrelated surface displacements could indicate the presence of short surface cracks.