D.R. Ni

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.

Characterization of the Corrosion Product Films Formed on the As-Cast and Friction-Stir Processed Ni-Al Bronze in a 3.5 wt% NaCl Solution

An as-cast Ni-Al bronze (NAB) was subjected to friction-stir processing (FSP). The friction-stir processed NAB (FSPed NAB) exhibited better corrosion resistance in a 3.5 wt% sodium chloride solution. The corrosion product films formed on the as-cast and FSPed NAB were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and electron probe micro-analysis (EPMA). Uncorroded phases were retained in the films. The interfaces between these phases and their surrounding corrosion products allowed the chloride ions in and reduced the film protectiveness. For the as-cast NAB, the un-corroded phases were large in size or continuous in structure, resulting in continuous interfaces. Therefore, the film on the as-cast NAB exhibited relatively inferior protectiveness. The films on the as-cast and FSPed NAB were similar in composition with mainly copper chloride hydroxide Cu2(OH)3Cl in the outer layer, and Al2O3 and Cu2O with the incorporation of Fe and Ni in the inner layer. Stability of the fi...

Comparison of Corrosion and Cavitation Erosion Behaviors Between the As-Cast and Friction-Stir- Processed Nickel Aluminum Bronze

An as-cast nickel aluminum bronze (NAB) was treated by friction-stir processing (FSP). Immersion test and electrochemical measurements under cavitation erosion condition were carried out to investigate the long-term and short-term corrosion behaviors of the as-cast and as-FSP NAB. Cavitation erosion tests were conducted in both distilled water and 3.5 wt% sodium chloride (NaCl) solution. The immersion test indicated that the electrochemical impedance of the as-FSP NAB was much higher, while there was little difference between them in the short-term test. The cumulative mass loss of the as-cast NAB was about 1.5 and 2 times as large as that of the as-FSP in distilled water and 3.5 wt% NaCl, respectively. The higher corrosion resistance of the as-FSP was a result of the refined and homogenized microstructure. Improved mechanical properties and fewer galvanic corrosion sites are attributed to the higher cavitation erosion resistance of the as-FSP NAB.

Corrosion and Cavitation Erosion Behaviors of Friction Stir Processed Ni-Al Bronze: Effect of Processing Parameters and Position in the Stirred Zone

As-cast Ni-Al bronze (NAB) was subjected to friction stir processing (FSP). Different processing parameters including rotating rate, traverse speed, and processing passes were selected to investiga...

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.

Influence of Zn coating on friction stir spot welded magnesium-aluminium joint

ABSTRACT For friction stir spot welded (FSSW) magnesium–aluminium joints, the formation of Mg–Al intermetallics in the hook region and cracking at the interface damaged seriously the strength of Mg–Al joints, resulting in lower joint load of only 0.8 ± 0.2 kN. When adding the hot-dipped Zn coating on the Al substrate surface prior to FSSW, a brazed layer, composed of Mg–Zn and Al–Zn diffusion zones at the edge of the shoulder, and a transition layer, composed of MgZn2, Zn-rich zone and residual Zn in the hook region, were formed in the FSSW Mg–Al joint, eliminating the cracking and Mg–Al intermetallics in the FSSW joint without the Zn coating. The load of the joint with the Zn coating increased to 3.7 ± 0.3 kN.

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.

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.

Bulk NiTip/Al Composites Prepared by Friction Stir Processing

Friction stir processing (FSP) was used to prepare shape memory alloy particles (NiTip) reinforced 6061 Al bulk composites with the aim to avoid deleterious. Al-NiTi interface reaction that occurred in cast and powder metallurgy processes. NiTip were homogeneously distributed in the Al matrix without interfacial reaction. Conventional T6-treatment (solution + aging) showed little effect on the interfacial microstructure. The NiTip in both the as-FSP and T6-treated composites still maintain the intrinsic characteristic of a reversible thermoelastic phase transformation. The shape memory characteristic of the NiTip decreases the CTE of the Al matrix, and an apparent two-way shape memory effect is observed in the cold-rolled composite. The composite owns a good combination of damping and mechanical properties.