Certain magnesium alloy systems can obtain superior mechanical strength through age hardening. However, conventional ageing heat treatments for these alloys are rarely more complex than a single isothermal hold – especially when compared to those of aluminium alloys – so age hardenable magnesium alloys may not be achieving their full strengthening potential. The use of preageing, where a lower temperature heat treatment is utilised before a hotter secondary ageing step, has proven successful in boosting the strength of magnesium alloys previously, but these trials are few in number, and the testing conditions are limited. In this work, a wide range of preageing temperatures and times were trialled on commercial and experimental magnesium alloys to determine the effectiveness of the strengthening technique. The results showed that preageing can produce a significant boost in hardness, can reduce total ageing times, and can provide a degree of mechanical property customisation through control of precipitate habit plane and morphology. However, the effectiveness of the technique is alloy-system dependent, where the fundamental precipitate nucleation and phase evolution is a key contributor to the success of preageing; as is the energy barrier to nucleation in an alloy system, whether that be controlled by alloy chemistry or thermomechanical processing. The results are discussed in the context of alloy design and industrial processing.

High-deposition-rate, directed-energy-deposition additive manufacturing (DED-AM) processes typically produce Ti-6Al-4-V (Ti64) components with coarse columnar β-grain structures that lead to undesirable mechanical anisotropy, as well as a fine heterogeneous lamellar transformation microstructure, which is very different to that seen for standard wrought products. This arises because of the intrinsic lack of constitutional undercooling at the solidification front, and the subsequent high cooling rates and rapid thermal cycling experienced by the deposited material. In this work, the more refined primary β-grain solidification structures and textures seen in components built with the novel coaxial electron beam wire DED AM (CEWAM) process have been characterised in detail, for the first time, with the aim of investigating the potential for this technology to directly replicate the β-annealed damage-tolerant microstructure used in large Ti64 aerospace forgings. Due to its different lower energy density solidification conditions, it has been confirmed, by electron backscatter diffraction (EBSD) analysis and β-grain reconstruction in three orthogonal cross-sections, that the CEWAM process changes the melt conditions to promote β-grain nucleation ahead of the solidification front, which can result in a highly refined, equiaxed, β-grain structure. However, the conditions for refinement were marginal and a mixed grain structure was commonly observed in thicker sections. Additionally, the subsequent grain-growth stability during β-annealing was investigated. It is shown that an equivalent microstructure can be achieved to that seen in a standard β-forged component, by grain structure homogenisation and slow cooling through the β transus, to promote α colony nucleation, allowing direct part substitution. This was made possible by the refined primary β-grain structure achieved during deposition with the CEWAM solidification conditions which, importantly, are also shown to lead to a weaker texture than in a typical forging.

This paper shows the potential of stationary shoulder friction stir welding (SSFSW) for producing higher quality T-section joints relative to a conventional friction stir welding (FSW) approach. The residual stress distributions and their relationship to the weld zone microstructure and hardness distributions in SSFSW T-joints were investigated, as a function of welding travel speed. The final longitudinal residual stress fields were asymmetric, although individual weld zones showed similarities to those for conventional butt SSFSWs. The thermal excursion and plastic strain arising from the second pass lowered the residual stresses seen from the first pass, so that the largest tensile stresses (similar to 160 MPa) were found close to the nugget from the second weld pass. The asymmetry in hardness distribution was caused by the thermal field of the second pass which thermally treated material in the first pass and resulted in areas of age hardening and increased over ageing, depending on the position of overlap of the thermal fields. The effects of the second weld pass on the first pass were more apparent when a lower travel speed was used owing to the increase in heat input and duration of the thermal cycle.

The residual stresses generated in stationary shoulder friction stir welds (SSFSWs) produced in a typical high strength aluminum alloy (AA7010) in 6.3mm thick plate has been mapped over full weld cross sections, using the contour method, and compared to those introduced by conventional friction stir welding (FSW) for welding speeds ranging from 100 to 400mm/min. Compared to in conventional FSW, as a consequence of the material flow being affected by only a rotating probe, the SSFSW process produced a narrower and more uniform weld nugget and heat affected zone profile through the plate thickness. For both processes, ‘M’ shaped residual stress distributions were determined. However, the peak stresses measured in the SSFSWs were slightly lower than those found in the conventional FSWs and the width of the tensile region was appreciably reduced when using a stationary shoulder welding tool. This is shown to be resulting from a more focused temperature distribution obtained from using only a rotating probe to generate heat in the SSFSW process. In both processes, increasing the welding speed led to a narrower residual stress profile, but higher peak tensile residual stresses.

The new process of ‘stationary shoulder’ friction stir welding (SSFSW) has been directly compared to conventional (friction stir welding) using welds produced in a high strength AA7050-T7651 aluminium aerospace alloy. The process window for each approach was first compared using torque–rotation rate decay curves. Under optimum process conditions, SSFSW had a ∼30% lower heat input than FSW and the stationary shoulder resulted in narrower welds with a reduced heat affected zone (HAZ) width. The SSFSW welds also had more uniform through thickness properties and performed better than conventional FSWs in cross-weld tensile tests. In addition it is demonstrated that the SSFSW process resulted in a far superior surface finish, although the stationary shoulder led to surface ‘speed cracking’ under certain welding conditions. The reasons for these benefits are discussed aided by thermal and hardness modelling.

Static Shoulder Friction Stir Welding (SS-FSW) is a modification to conventional FSW that was originally developed to improve the weldability of titanium alloys by reducing through thickness temperature gradients. Surprisingly, to date, there have been no published systematic studies comparing SS-FSW to FSW for aluminium welding. This may be because the high conductivity of aluminium means the heat input produced by the shoulder is thought to be beneficial. In the work presented when welding a high strength 7050 aluminium alloy, even in a relatively thin 6 mm plate, it is shown that SS-FSW has several advantages; including a reduction in the heat input, a massive improvement in surface quality, and a more uniform through thickness temperature distribution, which leads to narrower welds with a reduced heat affected zone width and more homogeneous through thickness properties. The reasons for these benefits are discussed.

Stationary (or Static) Shoulder Friction Stir Welding (SS-FSW) is a variant of FSW that was developed primarily to improve the weldability of titanium alloys by reducing the through thickness temperature gradient. Surprisingly, SS-FSW has been largely ignored by the Al welding community because it is widely supposed a rotating shoulder is an essential aspect of the process and that the higher conductivity means the surface heating effect of the shoulder is generally beneficial. In the work presented it is shown that SS-FSW has major advantages when welding high strength aluminium alloys; including a reduction in the heat input, a massive improvement in surface quality, and a narrower and more symmetric temperature distribution, which leads to narrower welds with a reduced heat affected zone width and lower distortion. The reasons for these benefits are discussed based on a systematic study aimed at directly comparing both processes.

The manufacture of Ti- composite hyper-joints for aerospace applications requires arrays of pins to be attached to a metal component’s surface. Here, the feasibility of using percussive arc micro-welding for this purpose has been explored, applied to dissimilar Ti alloys, to allow advantage to be taken of tailored pin properties. To simulate the process, Timetal21s wire pins were welded to a Ti-6Al-4V baseplate under optimized parameters. Analysis of the welds indicated the presence of martensitic microstructures and chemical inhomogeneity in the melt pool, as a result of the rapid weld cycle (5 ms). The performance of the welded pins was assessed by a micro-tensile and a pin shear/bend test, assisted by FE modeling.

Dissimilar joining of aluminum to steel sheet in multimaterial automotive structures is an important potential application of ultrasonic spot welding (USW). Here, the weldability of different zinc-coated steels with aluminum is discussed, using a 2.5-kW USW welder. Results show that soft hot-dipped zinc (DX56-Z)-coated steel results in better weld performance than hard (galv-annealed) zinc coatings (DX53-ZF). For Al to hard galv-annealed-coated steel welds, lap shear strengths reached a maximum of ~80% of the strength of an Al-Al joint after a 1.0 s welding time. In comparison, welds between Al6111-T4 and hot dipped soft zinc-coated steel took longer to achieve the same maximum strength, but nearly matched the Al-Al joint properties. The reasons for these different behaviors are discussed in terms of the interfacial reactions between the weld members.

The ability to join dissimilar materials in the automotive industry will result in more efficient multimaterial structures. However, welding of aluminium (Al) to magnesium (Mg) alloys is problematic because of the rapid formation of brittle intermetallic phases at the weld interface. Ultrasonic welding (USW) is a solid-state joining technology that may offer a potential solution, but USW of Al to Mg is currently not well understood. Here, we have investigated the effect of process variables and energy input on joint formation between Al-6111 and Mg-AZ31 alloys, and we report on the optimum welding conditions, heat generation, and the formation of a significant intermetallic reaction layer. Furthermore, the factors influencing the interface reaction rate and the advantages of precoating the Mg with Al are discussed.