Microstructure history effect during sequential thermomechanical processing

Abstract

The key to modeling the material processing behavior is the linking of the microstructure evolution to its processing history. This paper quantifies various microstructural features of an aluminum automotive alloy that undergoes sequential thermomechanical processing which is comprised hot rolling of a 150-mm billet to a 75-mm billet, rolling to 3 mm, annealing, and then cold rolling to a 0.8-mm thickness sheet. The microstructural content was characterized by means of electron backscatter diffraction, scanning electron microscopy, and transmission electron microscopy. The results clearly demonstrate the evolution of precipitate morphologies, dislocation structures, and grain orientation distributions. These data can be used to improve material models that claim to capture the history effects of the processing materials.

Introduction

Due to the increase in fuel costs in recent years, aluminum usage in automotive applications has increased in an attempt to produce more fuel-efficient lightweight vehicles. Therefore, more aluminum alloys are being developed and commercialized to meet the rising demand from automakers. The aluminum alloy 6022 was developed by Alcoa in the late 1980s primarily for automotive closure panel applications. The automotive body panels are fabricated using a stamping process of aluminum sheets. The aluminum sheets are manufactured through a rolling process which reduces the aluminum cast slab from thick billets to thin gauge sheets. The aluminum alloy 6022 displayed enhancements in formability and hemming performance, superior surface quality, and excellent paint-bake response which are vital for performance in the stamping process [1]. This material also displayed improved corrosion resistance over other commercially available alloys. Previous studies have been conducted on thin gauge aluminum alloy 6022 sheets investigating various mechanical behaviors of the material. Tensile and fatigue properties of aluminum alloy 6022 were studied and compared with other commercial aluminum alloys showing a high fatigue strength, good ductility, and moderate tensile strength [2]. Tensile ductility at elevated temperatures tests were conducted on several commercial aluminum alloys which showed lower ductilities in the 6000 series alloys than the 5000 series alloys due to constant-structure dislocation creep caused by undissolved Si [3]. A study on the effects of artificial aging on shear deformation resulted in artificial aging treatments that could be useful industrially for various applications [4]. Hardening characteristics causing springback are of interest during the stamping process of the automotive panels [5]. Studies have also been conducted investigating the effects of Cu content and aging treatments on the precipitation characteristics of aluminum alloy 6022 sheets which shows a higher Cu level always resulting in a higher hardness [6], [7], [8].

The control of the evolution of microstructures during the manufacturing process is critical for determining quality and properties of the finished product. Hot and cold rolling, the critical processes in the manufacturing of commercial aluminum sheets, can significantly impact the final texture of the cold rolled sheet. The dynamic restoration mechanisms involved during hot deformation are directly associated with the evolution of the microstructures and textures. Ren and Morris [9] showed that for higher strain rates, the texture development was governed by dynamic recrystallization (DRX), which tends to have an equiaxed grain structure, randomized texture, and well-recovered subgrain structure. On the other hand, at lower strain rates, dynamic recovery (DRV) was the main restoration mechanism for the material.

During the hot deformation of aluminum alloys, substructures of dislocations and subgrains are formed by a combination of dislocation generation, annihilation and rearrangement. After a critical strain, a dynamic equilibrium is achieved between the rates of dislocation generation and annihilation and a steady state subgrain size r, which is uniquely related to the flow stress σ by a relationship of the form

$$\sigma =\frac{KGb}{r}$$

is maintained [10], [11], [12], [13], where G is the shear modulus, b the magnitude of the dislocation Burgers vector and K a dimensionless constant.

This paper differs from previous reported microstructural analyses that have focused on one or two thermomechanical processing stages. In this study, the microstructure evolution of aluminum 6022 alloy during a sequence of thermomechanical processing has been investigated. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were used to characterize the evolution of dislocation structures, precipitate morphologies, and dominant texture component at each manufacturing stage. Horstemeyer and Wang [14] described a multiscale modeling methodology that is capable of capturing the material processing history effects. They elucidate the point that the microstructural features need to be quantified first, as proposed in the current study, before a microstructure-based internal state variable model can be employed [14], [15], [16], [17].