B.L. Xiao

Achieving high property friction stir welded aluminium/copper lap joint at low heat input

Abstract Aluminium and copper plates with 3 mm thickness were successfully friction stir lap welded at a lower rotation rate of 600 rev min−1 using a larger pin 8 mm in diameter. Good metallurgical bonding on the Al/Cuinterface was achieved due to the formation of a thin, continuous and uniform Al–Cu intermetallic compound layer. Furthermore, many Cu particles consisting of pure Cu and intermetallic compound layers were generated at the lower part of the nugget zone, forming a composite structure with increased hardness. A lower rotation rate resulted in a decrease in annealing softening in the heat affected zone (HAZ), and a larger diameter pin increased the Al–Cu bonding area. These factors resulted in that the friction stir welded lap joint exhibited a high failure load of 2680 N with failure in the HAZ on the aluminium side.

Friction Stir Welding of Discontinuously Reinforced Aluminum Matrix Composites: A Review

Friction stir welding (FSW) is considered a promising welding technique for joining the aluminum matrix composites (AMCs) to avoid the drawbacks of the fusion welding. High joint efficiencies of 60%–100% could be obtained in the FSW joints of AMCs. However, due to the existence of hard reinforcing particles in the AMCs, the wearing of welding tool during FSW is an unavoidable problem. Moreover, the low ductility of the AMCs limits the welding process window. As the hard materials such as Ferro-Titanit alloy, cermet, and WC/Co were applied to produce the welding tools, the wearing of the tools was significantly reduced and the sound joints could be achieved at high welding speed for the AMCs with low reinforcement volume fraction. In this article, current state of understanding and development of welding tool wearing and FSW parameters of AMCs are viewed. Furthermore, the factors affecting the microstructure and mechanical properties of the joints are evaluated in detail.

A three-dimensional realistic microstructure model of particle-reinforced metal matrix composites

A new and robust methodology is presented for the complete computer simulation of large three-dimensional (3D) microstructures of particle-reinforced metal matrix composites (PRMMCs), by integrating the boundary representation scheme, the random cutting algorithm and the random sequential adsorption algorithm. The methodology allows large realistic 3D microstructure models to be generated that can be used for multi-scale investigation of PRMMC structure and design. The effect of the simulation parameters on the simulated microstructure is investigated by applying a quantitative metallographic analysis of the distribution functions of aspect ratio, diameter and the area of reinforcements. Simulated large realistic homogenous 3D microstructures of PRMMC are in close agreement with the experimental microstructures.

Realising equal strength welding to parent metal in precipitation-hardened Al–Mg–Si alloy via low heat input friction stir welding

ABSTRACT Two millimetres thick Al–Mg–Si (6061Al-T6) alloy plates were friction stir welded at various welding conditions. Under a low rotation rate of 400 rev min–1 with rapid water cooling, the softening zone in the joint disappeared and a nanostructure with an average grain size of 80 nm was obtained in the stir zone (SZ). Therefore, a weld with equal strength to the parent metal (PM) was successfully achieved with the fracture occurring in the PM. Further, the average microhardness and ultimate tensile strength (UTS) of the SZs increased with the decreasing rotation rate and increasing cooling speed. The average microhardness and UTS of the SZ with nanostructure reached up to 134 HV and 505 MPa, respectively; though the initial strengthened precipitates disappeared. This work provides an effective strategy of achieving high property joints and enhancing the mechanical properties of precipitation-hardened Al alloys.

Material flow and void defect formation in friction stir welding of aluminium alloys

ABSTRACT An ingenious experimental programme by combining artificially thickened oxide layer as marker material and ‘stop-action’ welding were used to study the material flow and defect formation in friction stir welding of aluminium alloys. The results showed that material flow around the pin on the advancing side (AS) was severer than that on the retreating side (RS) and the fastest velocity of material flow in the middle stir zone (SZ) was 43.9 mm s−1. Moreover, the material under the RS shoulder included extruded metal only and the material under the AS shoulder included extruded and rotated metal. Lastly, instantaneous void occurrence and insufficient inflow material were reasons for the preferential formation of void defects in the top SZ on the AS.

Origin of Insignificant Strengthening Effect of CNTs in T6-Treated CNT/6061Al Composites

Carbon nanotube (CNT)-reinforced 6061Al (CNT/6061Al) composites were fabricated via powder metallurgy combined with friction stir processing (FSP). CNTs were dispersed after FSP and accelerated the precipitation process of the CNT/6061Al composites. However, the strengthening effect of CNTs on the T6-treated materials was insignificant, while the composites under the FSP and solution treatment conditions exhibited increased strength compared to the matrix. Precipitate-free zones (PFZs) were detected around CNTs in the T6-treated CNT/6061Al composites, and a model was proposed to describe the effect of PFZs on strength. The calculations indicated that the strength of PFZs was similar to that of the T6-treated 6061Al. As a result, the strengthening effect of CNTs on the T6-treated CNT/6061Al composites was insignificant.

Effect of Processing Parameters on Plastic Flow and Defect Formation in Friction-Stir-Welded Aluminum Alloy

The effect of processing parameters on material flow and defect formation during friction stir welding (FSW) was investigated on 6.0-mm-thick 2014Al-T6 rolled plates with an artificially thickened oxide layer on the butt surface as the marker material. It was found that the “S” line in the stir zone (SZ) rotated with the pin and stayed on the retreating side (RS) and advancing side (AS) at low and high heat inputs, respectively. When the tool rotation rate was extremely low, the oxide layer under the pin moved to the RS first and then to the AS perpendicular to the welding direction, rather than rotating with the pin. The material flow was driven by the shear stresses produced by the forces at the pin–workpiece interface. With increases of the rotation rate, the depth of the shoulder-affected zone (SAZ) first decreased and then increased due to the decreasing shoulder friction force and increasing heat input. Insufficient material flow appeared in the whole of the SZ at low rotation rates and in the bottom of the SZ at high rotation rates, resulting in the formation of the “S” line. The extremely inadequate material flow is the reason for the lack of penetration and the kissing bonds in the bottom of the SZ at extremely low and low rotation rates, respectively.

Fabrication of metal matrix composites via friction stir processing

Abstract Friction stir processing (FSP), a development based on friction stir welding, produces severe plastic deformation and material mixing in the produced zone at high temperature and therefore has been developed as a new technique for fabricating metal matrix composites. In this study, three types of composites, carbon nanotubes (CNTs) reinforced 2009Al (CNT/2009Al), shape memory alloy particles (NiTi) reinforced 6061Al (NiTi/6061Al), and in situ Al3Ti particles reinforced pure Al (Al3Ti/Al) composites were fabricated by means of FSP. It was indicated that FSP induced the uniform distribution of CNTs and NiTi particles in the aluminum matrix and promoted the reaction between Ti and Al. The CNT/2009Al and Al3Ti/Al composites exhibited a good combination of strength and ductility. Furthermore, NiTi/6061Al composite had good damping properties and mechanical properties due to the shape memory characteristic of NiTi particles. The mechanisms responsible for the dispersion and damage of CNT and NiTi particles and the accelerated reaction between Ti and Al were analyzed and discussed.

Fabrication of Carbon Nanotube Reinforced Aluminum Matrix Composites via Friction Stir Processing

In this study, CNTs reinforced pure Al, 6061Al and 2009Al composites with uniformly dispersed CNTs were successfully fabricated by a combination of powder metallurgy and subsequent multi-pass friction stir processing (FSP). As the FSP pass increased, the CNT clusters were broken down due to the shear flow of the Al matrix during FSP, however, the CNTs were cut short. After 4-pass FSP, the CNTs were individually dispersed in the aluminum matrix along grain boundaries resulting in a much finer grain size. Although the CNTs were shortened and some Al4C3 formed in the matrix, the layer structures of the CNTs were well retained and the CNT-Al interface was good bonded. Compared to the Al matrix, the strength, especially the yield strength of the composites increased significantly.

The Minimum Representative Volume Element Size for Coefficient of Thermal Expansion of Particle Reinforced Metal Matrix Composites

A 3D realistic microstructure based computational homogenization model is proposed, in order to determine the temperature dependent effective coefficient of thermal expansion of particle reinforced metal matrix composites The model employed three-dimensional realistic microstructures with different domain sizes, where particles had random shape, sharp edges and were randomly distributed. The unit cell microstructure based model and classical analytical models were also presented for comparison. As an illustration of the model, a 17% vol. SiCp reinforced 2124Al composite was investigated. Its minimum RVE size is found to be δ = 15, where δ is called the size ratio and defined by the ratio between the side length of microstructure and the mean particle radius.