Superplastic-like forming of lightweight alloys

Abstract

Conventional superplastic forming (SPF) is attractive for the current industrial fabrication of precision, large and complex-shaped sheet-metal components. This process requires a fine-grained material and is carried out at high temperatures (typically above 500°C for aluminum alloys and 900°C for titanium alloys) and slow forming rates (mostly strain rate slower than 10-3 s-1). To address these limitations, an advanced sheet-forming process has been designed by combining hot drawing and superplastic forming (namely gas forming) in a one-step operation to establish a fast forming technology. The process together with a non-isothermal heating system is referred to as superplastic-like forming. The aim of this work is to investigate (i) the relationship between process parameters, material draw-in and thickness distribution and (ii) the microstructural evolution, deformation mechanism and post-forming properties during forming. Target materials are commercial grade AA5083 and Ti-6Al-4V alloys. Tensile tests were performed for both materials to determine the optimal deformation conditions, i.e. temperature and strain rate. The flow stress data of AA5083 were used in the calibration of a material model for finite element modeling (FEM). The results showed that AA5083 can deform with reasonable flow stress and tensile elongation at 400°C and a strain rate of 2×10-3 s-1 and that Ti-6Al-4V alloy possesses good forming capability at 800°C and a strain rate of 10-3 s-1. A rectangular die cavity with multiple steps was used to demonstrate different amounts of material draw-in and surface expansion of the formed part in superplastic-like forming. The punch geometry was designed and validated through a parametric study during hot drawing. The measured drawing limits were used to determine the sheet size for hot drawing. The optimal dimensions of AA5083 and Ti-6Al-4V sheets are 200×200 and 210×210 mm2, respectively. The non-isothermal heating system is expected to maintain the sheet at a lower global (furnace) temperature. Meanwhile, a higher local temperature in specific zones, i.e. die radii, ensures that the material there has more ductility to flow. For AA5083, the entire forming was conducted at a global temperature of 400°C, but the material close to the die radii was selectively heated to 420°C. During superplastic-like forming of Ti-6Al-4V alloy, the global and local temperatures were 800 and 820°C, respectively. After completion of forming AA5083 in 8 min, a final part with maximum percentage thinning of 40% at the outward corners and 137% surface expansion was achieved. The forming of Ti-6Al-4V alloy was completed in 16 min, exhibiting a maximum percentage thinning of 54% at the outward corners and the surface expansion of 130%. Stress gradients and the corresponding strain rate differences at the outward corners in the forming sheet led to thinning gradient and plastic straining as a result of geometric inhomogeneity as demonstrated through finite element simulations of hot drawing and gas forming. Cracks and oxidation on the Ti-6Al-4V sheet surface are the other reasons accounting for the thickness reduction at the outward corners. Fairly uniform microstructure was achieved with the process. The main deformation mechanisms for AA5083 during hot drawing are subgrain boundary migration and subgrain rotation, and they change into subgrain boundary migration and grain growth during gas forming. The hot-drawing phase involves purely dislocation-based deformation, while the gas-forming phase involves dislocation creep with hardening and recovery contributions. A physical model based on the concept of dislocation density is constructed to describe the deformation behavior of AA5083. Simulations of the superplastic-like forming are promising and match the experimental measurements with reasonable agreement, illustrating that critical attention should be paid to the die radii during the process and die design.