Formability of AA7075 Aluminum Sheet at Elevated Temperatures for Automotive Hot Stamping Applications

Abstract

Sheet metal forming is a process in which sheet materials are stretched, drawn and bent into complex shapes by use of a forming press fitted with a die set and punch suitable for forming a part. There are several ways to form sheet including hot and cold stamping. If the material to be formed is ductile, commonly, the cold stamping process is utilized. However, if the material has low formability in its room temperature state, hot stamping can be employed to avoid fracture and significant part spring-back. In this thesis the hot stamping feasibility of aluminum AA7075-F (as-fabricated temper) was investigated. The formability of AA7075-F sheet was assessed for elevated temperature forming by means of the in-plane Marciniak test. Although common at room temperature, development of an elevated temperature Marciniak test was necessary. ABAQUS® FE suite was used to create an accurate finite element model (FEM) to assist in understanding material flow during experimental testing. Upon development of an elevated temperature Marciniak test, strain maps were produced through digital image correlation (DIC) to create forming limit curves (FLC). In addition, the resulting samples were observed for their microstructure, plastic flow, necking and fracture behaviour. Macroscopic strain field inhomogeneity, surface roughening characteristics, and ductile void damage as a function of strain, strain path and temperature were also analyzed in an attempt to correlate formability and fracture behaviour to the material microstructural, damage development and surface characteristics in the deformed state. FLC prediction of AA7075-F sheet were made using the laboratory-based FE simulation models of in-plane Marciniak test. For this purpose, strain-rate dependant data for AA7075-F sheet was obtained through elevated temperature tensile tests for use as material model input data. The predicted FLCs were compared with the experimental FLCs, and good general agreement was observed. Lastly, a die was designed to replicate the hot stamping die quenching process through forming annealed AA7075 sheet. Tensile samples were cut from hot stamped parts to evaluate the process feasibility. Supplementary tests were performed to evaluate the impact of cooling rate on material strength.