•Elimination of distortion through the application of rolling after deposition.•Reduced residual stresses – particularly adjacent to the baseplate.•Grain refinement when the rolled material was ...reheated by the subsequent deposition pass.•A simple analytical model for predicting the residual stress and distortion in AM parts.
Parts manufactured by Wire and Arc Additive Manufacture (WAAM) can have significant residual stress and distortion, as well as large grain sizes. To overcome these problems, each layer on a linearly deposited steel WAAM part was rolled with either a ‘profiled’ roller, which had a similar shape to the deposited layer, or a ‘slotted’ roller, in which a groove prevented lateral deformation. Both rollers reduced the distortion and surface roughness, but the slotted roller proved more effective – eliminating the distortion. The residual stresses in the rolled WAAM parts were measured and were lower than those in the unrolled control specimen – particularly adjacent to the baseplate. Rolling also induced additional grain refinement when the rolled material was reheated during the subsequent deposition pass. The application of rolling may be a key technology for enabling implementation of WAAM on large-scale structures.
Many additively manufactured (AM) materials have properties that are inferior to their wrought counterparts, which impedes industrial implementation of the technology. Bulk deformation methods, such ...as rolling, applied in-process during AM can provide significant benefits including reducing residual stresses and distortion, and grain refinement. The latter is particularly beneficial for titanium alloys where the normally seen large prior β grains are converted to a fine equiaxed structure – giving isotropic mechanical properties that can be better than the wrought material. The technique is also beneficial for aluminium alloys where it enables a dramatic reduction in porosity and improved ductility.
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The stability of Plasma transferred arc Wire + Arc Additive Manufacture is critically dependent on the transfer mode of the wire into the melt pool which can either be in permanent ...contact (TM1), intermittent (TM2) or fleeting (TM3) and it’s governed by the position of the wire. Analysis of the voltage waveforms in conjunction with video imaging was used to understand the preferred mode for process control and how these transfer modes are affected by convex and concave melt pools shapes. The study indicated that stable transfer can be more easily achieved with the convex weld pool, because it minimises the extent of the undesirable TM3 where droplet transfer occurs in an erratic fashion. Furthermore, even though the operational range of TM2 is limited, a control algorithm could be more easily developed for this transfer mode. Finally, video imaging showed how the melt pool oscillation interacts with droplet detachment under some operational conditions.
The mechanisms of post-weld rolling and how it reduces and eliminates residual stress and distortion are poorly understood. Finite element analysis was applied to two different methods of rolling: ...rolling the weld bead directly with a single roller and rolling beside the weld bead with a dual flat roller. The models showed that both rolling techniques were able to induce compressive stress into the weld region, which increased with rolling load. The distribution of stress was sensitive to the coefficients of friction between the workpiece and the roller and the backing bar. High friction coefficients concentrated the plastic deformation and compressive stress within the centre of the weld bead. Distortion can be eliminated by rolling; however, the experiments indicated that this was only achieved when applied to the weld bead directly.
In-process deformation methods such as rolling can be used to refine the large columnar grains that form when wire + arc additively manufacturing (WAAM) titanium alloys. Due to the laterally ...restrained geometry, application to thick walls and intersecting features required the development of a new ‘inverted profile’ roller. A larger radii roller increased the extent of the recrystallised area, providing a more uniform grain size, and higher loads increased the amount of refinement. Electron backscatter diffraction showed that the majority of the strain is generated toward the edges of the rolled groove, up to 3 mm below the rolled surface. These results will help facilitate future optimisation of the rolling process and industrialisation of WAAM for large-scale titanium components.
This paper describes the application of the computational fluid dynamics (CFD) code, FLUENT, to modelling the 3-dimensional metal flow in friction stir welding (FSW). A standard threaded tool profile ...is used for the analysis and features such as the tool rake angle, heat generation and heat flow are included. The primary goal is to gain a better understanding of the material flow around a complex FSW tool and to demonstrate the effect of the tool rake angle, and weld and rotation speed. The model captured many of the real process characteristics, but gave poor predictions of the welding forces. The model also generated an excessive amount of heat, which led to a large over-prediction in the weld temperature. These shortcomings can be overcome by using a viscosity relationship that includes material softening near the solidus or material slip at the tool interface.
Numerical models used to simulate LFW rely on the modelling of the oscillations to generate heat. As a consequence, simulations are time consuming, making analysis of 3D geometries difficult. To ...address this, a model was developed of a Ti–6Al–4V LFW that applied the weld heat at the interface and ignored the material deformation and expulsion which was captured by sequentially removing row of elements. The model captured the experimental trends and showed that the maximum interface temperature was achieved when a burn-off rate of between 2 and 3mm/s occurred. Moreover, the models showed that the interface temperature is reduced when a weld is produced with a higher pressure and when the workpieces are oscillated along the shorter of the two interface dimensions. This modelling approach provides a computationally efficient foundation for subsequent residual stress modelling, which is of interest to end users of the process.
Linear friction welding (LFW) is a solid-state joining process that is finding increasing interest from industry for the fabrication of titanium alloy (Ti–6Al–4V) preforms. Currently, the effects of ...the workpiece geometry on the thermal fields, material flow and interface contaminant removal during processing are not fully understood. To address this problem, two-dimensional (2D) computational models were developed using the finite element analysis (FEA) software DEFORM and validated with experiments. A key finding was that the width of the workpieces in the direction of oscillation (in-plane width) had a much greater effect on the experimental weld outputs than the cross-sectional area. According to the validated models, a decrease of the in-plane width increased the burn-off rate whilst decreasing the interface temperature, TMAZ thickness and the burn-off required to remove the interface contaminants from the weld into the flash. Furthermore, the experimental weld interface consisted of a Widmanstätten microstructure, which became finer as the in-plane width was reduced. These findings have significant, practical benefits and may aid industrialisation of the LFW process.
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•The workpiece geometry has a significant effect on Ti–6Al–4V linear friction welds.•A decrease of the in-plane width decreases the interface temperature.•A decrease of the in-plane width decreases the burn-off to remove the contaminants.•A decrease of the in-plane width can refine the interface microstructure.
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This paper investigates a resistance spot welded reinforced adhesive (weld-bonded) joint between 304 stainless steel to carbon fibre reinforced plastic (CFRP), where welds are made ...both with and without the reinforcing carbon fibres present. Successful welds with the fibres present could only be produced with high electrode pinch forces, which helped reduce contamination of the weld nugget. Similar joint strengths were achieved in both cases, however the joints without fibres exhibited an increased strain to failure. Both joints were significantly stronger than either an adhesive joint or a comparable bolt reinforced adhesive joint. These techniques provide an alternative for joining thin metallic components to CFRP structures where increased strength and integrity is required.
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