The aim of this paper is to report the very first in situ observations of the deformation behaviour of an Al-Cu alloy in the semisolid state by using ultrafast, high-resolution X-ray microtomography. It is shown that this deformation is non-homogeneous and involves an accumulation of liquid at an intergranular surface nearly perpendicular to the strain axis. Once the liquid is no longer able to feed such a region, micropores form and grow at this surface, finally leading to a crack.

This paper describes the development of a semi-automated friction stir welding (FSW) technique for joining 38 mm nominal outer diameter (OD) tubes of 6082-T6 aluminium alloy with 3.5 mm nominal wall thickness. Process development for this application of FSW required the integration of a rotational axis with the control system of the FSW machine as well as the development of a secondary human-machine interface (HMI) for this axis and also necessitated the design of a retracting tool pin in order to eliminate the exit hole of the pin. Issues relating to tube support, variation in wall thickness and out-of-roundness during welding are not straightforward for thin tube. The main topics covered in this paper are FSW process design and optimisation together with a discussion of the resulting weld zone microstructure. This welding work underpinned a substantial international network project whose aim was to prove the viability of FSW for making high performance joints in extruded aluminium tubes of diameter suitable for fabricating structural components, e.g. for ground vehicles, and to systematically investigate the fatigue performance, fracture paths, associated microstructural changes and mechanical properties under the type of multiaxial loading (biaxial tension-torsion) that tubes might experience in service.

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Titanium is a versatile biocompatible metal that is desirable in additively manufactured medical implant devices. However, additively manufactured parts have particular microstructures, porosity, residual stress, and surface conditions which can have a strong impact on fatigue performance. Implants have an added complexity from the saline operating environment and the associated impact on the safe design life. Equally, direct energy deposition induces a complex thermal history which, if not carefully controlled, can significantly alter the mechanical/material properties of the component. This study investigates the decrease in fatigue life, in an in vitro body fluid simulation using Ringer's solution, observed in Ti-6Al-4V specimens extracted from coupons manufactured by directed energy deposition. An interrupted deposition strategy was employed to control build regularity, which appeared to influence certain mechanical properties, including corrosion fatigue life. An ≈50% decrease in fatigue life was observed in Ringer's solution at 6 Hz loading frequency, clearly important in designing implants.

In this manuscript the development of the process parameters to friction stir weld 5.15mm thick AA5182-H111 at a feed rate of 1500mm/min is discussed. Compared to a weld made at 200mm/min the weld-pitch had to be reduced from 2.0rev/mm to 0.3rev/mm and down force increased from 27kN to 59kN to create a weld with tensile-and yield strength exceeding that of the parent material, whilst elongation was only marginally reduced. The low weld pitch coupled with the high feed rate resulted in a reduction in the weld temperature and an increase in the process reaction forces. A lower down force was sufficient to consolidate the stir zone and result in ultimate tensile strength yield strength exceeding that of the parent material. However, elongation was reduced since the low weld pitch also reduced the effectiveness of the flutes on the pin to break up and disperse the oxide layer, rendering a pseudo bond along the ‘Lazy S’. This pseudo bond was eliminated through an increase in the down force.

Titanium is a versatile biocompatible metal that is desirable in additively manufactured medical implant devices. However, additively manufactured parts have particular microstructures, porosity, residual stress and surface conditions which can have a strong impact on fatigue performance. Implants have an added complexity from the saline operating environment and the associated impact on the safe design life. Equally, direct energy deposition induces a complex thermal history which, if not carefully controlled, can significantly alter the mechanical/material properties of the component. This study investigates the decrease in fatigue life, in an in-vitro body fluid simulation using Ringer’s solution, observed in Ti-6Al-4V specimens extracted from coupons manufactured by directed energy deposition. An interrupted deposition strategy was employed to control build regularity, which appeared to influence certain mechanical properties, including corrosion fatigue life. An ≈50% decrease in fatigue life was observed in Ringer’s solution at 6 Hz loading frequency, clearly important in designing implants.

In industrial applications tight control during weld set-up equates to increased manufacturing cost. Solid state welding processes, particularly friction sir welding, do not generally make use of filler metals and hence a weld gap will have an influence on joint quality. In the current study the influence of a weld gap of 20% of the plate thickness (i.e. 1mm) in friction stir welded (FSW) joints was compared dynamically to welds made without a gap. These results were benchmarked against samples joined through MIG welding, a widely used method to join aluminium plates in industry. Tests revealed that the introduction of a 1mm gap caused a steeper slope of the stress-life curve of the friction stir welded joints when compared to flaw free welds made without a weld gap. Crack initiation in the 1mm gap FSW samples occurred at the advancing side weld edge where the tool shoulder interacts with the material, while the flaw free ‘zero gap’ welds failed in the parent material from the marks induced during oxide removal. Intermittent root flaws were, however, present in the welds made without a weld gap which decreased the life of the samples as the area of the flaw was increased. MIG welded samples always failed from the weld toe and the fatigue life of these samples were considerably lower than that of the 1mm gap friction stir welded samples. In the friction stir welded samples containing root flaws, a flaw across the entire sample width was required to reduce the life of the friction stir welds to that reached by the MIG welded samples.

This paper reports on the initial process development towards evaluating pin-less friction stir welding as a feasible alternative joining technology for Ti-6Al-4V sheets. Initial research results were derived from “bead on plate” welds with a pin-less tool on 3 mm Ti-6Al-4V sheets to assist with decision making for joining 1 mm Ti-6Al-4V sheets. The study evaluates the shared influence of tool tilt angle and shoulder plunge depth on weld profile and process energy input for a pin-less tool. An experimental approach was followed, performing bead on plate welds with three tool tilt angles (0°, 1° and 2°) respectively at shoulder plunge depths from 0.15 mm to 0.25 mm in 0.05 mm increments. From critically evaluating the hardness profile and heat effected area below the tool shoulder, the shared influence of tool tilt angle and shoulder plunge depth could be analysed to give an indication of the best parameter combination for performing solid state welds on 1 mm Ti-6Al-4V sheet. Additionally, the thinning effect on the weld region was evaluated and compared to welds made with a pin. Future work will include quantifying surface residual stresses on the material and the relationship these stresses exhibits with the varying tool tilt and shoulder plunge depth. This work will assist in selecting weld parameter combination that will minimize tensile residual stresses in critical areas. Knowledge gained from this study forms the basis for the overall process development for joining 1 mm Ti-6Al-4V sheets by pin-less friction stir welding.