Heat treatment and additive manufacturing techniques to improve sheet metal formability

Advanced high strength steel sheets (AHSS) are increasingly used in the automobile industry to increase crash safety and fuel efficiency. However, AHSS typically exhibit lower global and local formability. Global formability is usually represented by forming limit diagrams (FLD). The local formability or stretch flangeability (which represents the resistance to edge cracking under edge stretching conditions in stampings) is determined by hole expansion testing with the results indicated as hole expansion ratios (HER). Some categories of AHSS exhibit good global formability but poor local or stretch formability (poor stretch flangeability leading to edge cracking). In hole expansion testing, a standard specimen with a 10 mm diameter hole is expanded using a conical punch till the onset of cracking and the percentage increase in hole diameter is referred to as the hole expansion ratio. One potential solution is to locally modify the microstructure using localized application of heat sources or by local thickening of the vulnerable areas of the sheet by the deposition of metal powders using additive manufacturing techniques. Previous research at IIT Madras has shown that local heat treatment of hole edges using a micro plasma torch can improve hole expansion ratios in a dual phase (DP 980) steel. The HER can also be improved by local reinforcement of the hole edges by local deposition of reinforcement layers using additive manufacturing (AM) techniques.

In this project, the main objective is to investigate potential improvements in formability of sheet steel using experimental and finite element analysis (FEA). The methodology will involve identification of local heat treatment or AM techniques (AM processes as well as processing parameters) for specific low ductility AHSS steel grades combined with local microstructure analysis to link microstructure modification and/or local strengthening to material forming limits and failure modes in both hole expansion and global formability testing. The FEA will involve development of finite element models that incorporate the influence of microstructure modification/ local strengthening on the prediction of forming and fracture behavior.