Z. Y. Ma

Friction stir welding and processing

Abstract Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships.

Microstructural modification of as-cast Al-Si-Mg alloy by friction stir processing

Friction stir processing (FSP) has been applied to cast aluminum alloy A356 plates to enhance the mechanical properties through microstructural refinement and homogenization. The effect of tool geometry and FSP parameters on resultant microstructure and mechanical properties was investigated. The FSP broke up and dispersed the coarse acicular Si particles creating a uniform distribution of Si particles in the aluminum matrix with significant microstructural refinement. Further, FSP healed the casting porosity. These microstructural changes led to a significant improvement in both strength and ductility. Higher tool rotation rate was the most effective parameter to refine coarse Si particles, heal the casting porosity, and consequently increase strength. The effect of tool geometry was complicated and no systematic trend was observed. For a standard pin design, maximum strength was achieved at a tool rotation rate of 900 rpm and traverse speed of 203 mm/min. Post-FSP aging increased strength for materials processed at higher tool rotation rates of 700 to 1100 rpm, but exerted only a marginal effect on samples prepared at the lower rotation rate of 300 rpm. Two-pass FSP with 100 pct overlapping passes resulted in higher strength for both as-FSP and post-FSP aged conditions.

Creep deformation characteristics of discontinuously reinforced aluminium-matrix composites

Recent developments in the study of creep behaviour of discontinuously reinforced aluminium-matrix composites (DRAMCs) at elevated temperatures are reviewed in this paper. These include the shapes of the creep curves, the origin and characteristics of the threshold stress, the creep strengthening of the DRAMCs, the nature of the rate-controlling processes, the effect of cyclic stress, and creep rupture. The DRAMCs exhibit high values of apparent stress exponent and apparent activation energy for creep. Incorporation of the threshold stress into analyses reduces the high and variable values of apparent stress exponent and activation energy to those anticipated from the creep of pure metals and solid-solution alloys. This indicates that the creep in the DRAMCs is controlled by the plastic flow in the matrix materials

Creep behavior of in situ dual-scale particles-TiB whisker and TiC particulate-reinforced titanium composites

A titanium composite reinforced by in situ dual-scale particle, high-aspect-ratio TiB whiskers and fine TiC particulates was fabricated by a reactive hot pressing technique from a B 4 C–Ti system. The composite was subjected to creep investigations in compression at 873–923 K. This composite exhibited a stress exponent of 4.5–4.6 and a creep activation energy of 298 kJ/mol. By comparison, unreinforced Ti exhibited a stress exponent of 5.2–5.3 and a creep activation energy of 259 kJ/mol. No change in the stress exponent with varying creep rates was observed in both composite and unreinforced Ti under the investigated creep rates. The creep resistance of the composite was more than one order of magnitude higher than that of the unreinforced Ti. The load transfer mechanism accounted for this result. The creep of both composite and unreinforced Ti was controlled by lattice diffusion in the titanium matrix.

Static and cyclic creep behaviour of SiC whisker reinforced aluminium composite

Static and cyclic creep tests were carried out in tension at 573-673 K on a 20 vol.-%SiC whisker reinforced aluminium (Al/SiCw) composite. The Al/SiCw composite exhibited an appaient str ess exponent of 18.1-19.0 at 573-673 K and an apparent activation energy of 325 kJ mol(-1) for static creep, whereas an apparent stress exponent of 19.6 at 623 K and an apparent activation energy of 376 kJ mol(-1) were observed for cyclic creep. A cyclic creep retardation (CCR) behaviour was observed for the Al/SiCw composite. The steady state creep rate for cyclic creep was three orders of magnitude lower than that for. static creep. Furthermore, the steady state creep I rates of the composite tended to clear ease continuously with increasing percentage unloading amount. The static creep data of the Al/SiCw coinposite were rationalised by the substructure invariant model with a true stress exponent of 8 together with a threshold stress. The CCR behaviour can be explained by the storage of anelastic strain delaying non-recoverable creep during the onload cycles.

Effects of macroscopic bulk defects on the damping behaviors of materials

A large number of macroscopic pores or graphite participates were introduced into commercially pure Al and ZA27 alloy by infiltration process to comparatively study the influence of macroscopic defects on the damping behaviors of the materials. The mean diameter of the bulk defects is ( 1.0 ± 0.5) mm, and the volume fractions of pores and graphite particulates are in the range of 50%– 75% and 19%–94%, separately. It is shown that addition of a number of pores or graphite particulates can significantly improve the damping of commercially pure AI, due to the comprehensive effects of the macroscopic and microscopic defects. However, the pores have little effect on the damping capacity of high damping ZA27 alloy, and graphite particulates make the high temperature internal friction peak decrease. It is considered that graphite particulates may repress the intrinsic damping mechanism of ZA27 alloy.