Development of an aluminum sheet alloy with improved formability
- PDF / 2,457,342 Bytes
- 11 Pages / 594 x 774 pts Page_size
- 26 Downloads / 219 Views
I.
INTRODUCTION
O N E of the limitations for using aluminum alloys in auto-body sheet applications is a lower formability compared to drawing quality steels, especially in plane strain conditions, where most failures occur. Ill This paper describes the development of an aluminum alloy with improved formability by first understanding the microstructural factors which control deformation behavior and optimizing those factors which enhance formability. A fundamental requirement of the microstructure with regard to formability is to provide a sustained amount of strain hardening by dislocation accumulation and dynamic recovery t2'3j in order to delay the onset of the diffuse instability condition of d~r/de = 0". Dynamic recovery should cause rearrangement of dislocations into stable low-energy configurations and should be achieved without significant dislocation annihilation. The difference in deformation behavior between different metals is related to (1) the rate at which dislocations are generated vs the rate at which they are annihilated and (2) the precise way in which they are rearranged by dynamic recovery. These processes are controlled by solutes, second-phase particles, and grain size, which, in addition, also control the extent of strain between localization and fracture. The superior formability of steel is a result of higher overall strain hardening due to enhanced dynamic recovery (mostly by dislocation rearrangement), which provides a more gradual decrease of strain-hardening rate, d~/de, with strain, e. 141 Both materials have similar yield and ultimate tensile strengths.
A. Effect of Solute and Second-Phase Particles 1. Strain hardening and dynamic recovery Solutes (e.g., Mg and Cu) cause very high strainhardening rates by the simple processes of interaction with dislocations. ~2'3j The ability of the deformed substructure to recover, however, is severely restricted if the solutes are mobile, whereby they cause dislocation pin-
ANIL K. SACHDEV, Staff Research Engineer, is with the Metallurgy Department, General Motors Research Laboratories, Warren, MI 48090. Manuscript submitted December 9, 1988. METALLURGICAL TRANSACTIONS A
ning. Although solutes enhance strain hardening by rapid dislocation accumulation, they also increase flow stress, the combination of which does not generally improve formability. The effect of Mg and Cu on decreasing plane strain limit strain was shown in Reference 5. In spite of this decrease, the addition of solute in aluminum is perhaps the most effective method for increasing overall strength while maintaining reasonable levels of formability. Second-phase particles (dispersoids) about 100 to 500 nm in diameter also have an important effect on strain-hardening behavior. These particles cause rapid increases in initial work-hardening rates due to the generation of additional dislocations at particle-matrix interfaces and also due to high elastic back stresses. I6'7'81 If the particles can be distorted or sheared or cause local recovery processes (in their vicinity), a rapi
Data Loading...
