The effectiveness of combining rolling deformation with wire-arc additive manufacture on β-Grain refinement and texture modification in Ti-6Al-4V

In Additive Manufacture (AM), with the widely used titanium alloy Ti–6Al–4V, the solidification conditions typically result in undesirable, coarse-columnar, primary β grain structures. This can result in a strong texture and mechanical anisotropy in AM components. Here, we have investigated the efficacy of a new approach to promote β grain refinement in Wire–Arc Additive Manufacture (WAAM) of large scale parts, which combines a rolling step sequentially with layer deposition. It has been found that when applied in-process, to each added layer, only a surprisingly low level of deformation is required to greatly reduce the β grain size. From EBSD analysis of the rolling strain distribution in each layer and reconstruction of the prior β grain structure, it has been demonstrated that the normally coarse centimetre scale columnar β grain structure could be refined down to < 100 μm. Moreover, in the process both the β and α phase textures were substantially weakened to close to random. It is postulated that the deformation step causes new β orientations to develop, through local heterogeneities in the deformation structure, which act as nuclei during the α → β transformation that occurs as each layer is re-heated by the subsequent deposition pass

Modelling and visualisation of material flow in friction stir spot welding

The material flow in friction stir spot welding of aluminium to both aluminium and steel has been investigated, using pinless tools in a lap joint geometry. The flow behaviour was revealed experimentally using dissimilar Al alloys of similar strength. The effect on the material flow of tool surface features, welding conditions (rotation speed, plunge depth, dwell time), and the surface state of the steel sheet (un-coated or galvanized) have been systematically studied. A novel kinematic flow model is presented, which successfully predicts the observed layering of the dissimilar Al alloys under a range of conditions. The model and the experimental observations provide a consistent interpretation of the stick-slip conditions at the tool-workpiece interface, addressing an elusive and long-standing issue in the modelling of heat generation in friction stir processing.The authors wish to thank the EPSRC for funding this research through the following grants: Friction Joining – Low Energy Manufacturing for Hybrid Structures in Fuel Efficient Transport Applications (EP/G022402/1 and EP/G022674/1); and LATEST2, Light Alloys Towards Environmentally Sustainable Transport (EP/H020047/1).This is the author accepted manuscript. The final version is available from Elsevier at http://dx.doi.org/10.1016/j.jmatprotec.2015.06.02

Thermal Modeling of Al-Al and Al-Steel Friction Stir Spot Welding

This paper presents a finite element thermal model for similar and dissimilar alloy friction stir spot welding (FSSW). The model is calibrated and validated using instrumented lap joints in Al-Al and Al-Fe automotive sheet alloys. The model successfully predicts the thermal histories for a range of process conditions. The resulting temperature histories are used to predict the growth of intermetallic phases at the interface in Al-Fe welds. Temperature predictions were used to study the evolution of hardness of a precipitation-hardened aluminum alloy during post-weld aging after FSSW.The work described herein has been sponsored by the UK Engineering and Physical Sciences Research Council (EPSRC) via the following grants: Friction Joining—Low Energy Manufacturing for Hybrid Structures in Fuel Efficient Transport Applications (EP/G022402/1 and EP/G022674/1), and LATEST 2: Light Alloys Towards Environmentally Sustainable Transport, 2nd Generation Solutions for Advanced Metallic Systems (EP/H020047/1).This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s11665-016-2225-

Mechanical performance and microstructural characterisation of titanium alloy-alloy composites built by wire-arc additive manufacture

A first stage study has been performed to investigate the potential for exploiting high deposition rate WAAM to print dual-alloy microstructures. Samples were built using alternating feed wires of commercially-pure Ti and Ti–6Al–4V. A high level of dilution occurred during deposition accompanied by effective liquid-phase mixing, producing a regular distribution of solidified melt tracks of approximate bimodal composition each less extreme than that of their respective constituent feed wires. The yield strength of the dual alloy composite material was approximately midway between that of the two alloys from which it was produced and exhibited a double inflection yield behaviour. Overall, because of the relatively coarse length scale there was not a significant property advantage in tensile loading above that of a chemically homogenous material, thus the main advantage of printing alternate alloys at this length scale is likely to reside more with increasing crack path tortuosity during fracture or fatigue loading. Importantly, the deposited material was found to have a refined β-grain structure suggesting that the composition gradients introduced by dual-alloy printing can disrupt the epitaxial columnar growth normally seen in WAAM deposits

Quantification of strain fields and grain refinement in Ti-6Al-4V inter-pass rolled wire-arc AM by EBSD misorientation analysis

Inter-pass deformation is an effective method for refining the coarse β-grain structure normally produced in high-deposition-rate additive manufacturing processes, like wire-arc additive manufacturing. The effectiveness of applying contoured surface rolling deformation tracks to each added layer has been studied by developing, and applying, a large-area SEM-based strain mapping technique. This technique is based on calibration of the average point-to-point Local Average Misorientation (LAM) of α-phase lamellar variants in EBSD orientation data to the local effective plastic strain. Although limited in the strain range that can be measured, the technique has proven to be very effective for identifying the size and depth of the plastic zone induced by surface rolling, as well as the local strain distribution, up to a saturation limit of ~12%. The strain fields mapped showed a close correlation to the region and level of recrystallization that occurred in the deformation zones during rapid re-heating through the β transus. The β recrystallization identified was consistent with the local strain distribution within the plastic zones measured by the LAM method and previous work on the recrystallization mechanisms operating in WAAM inter-pass deformation processes

A Lang Park mystery: Analysis of remains from a 19th century burial in Brisbane, Queensland

Salvage excavation of the Suncorp Stadium (Lang Park) redevelopment site in Brisbane revealed almost 400 graves. Originally known as the North Brisbane Burial Grounds, it was the site of Brisbane's principal cemetery between 1843 and 1875. A grave in the Anglican section of the cemetery yielded several teeth and associated non-dental bone fragments, and stature data derived from the coffin indicate a child burial. Observation of the stages of tooth eruption, resorption, and formation revealed evidence for two children, one aged approximately three years old and the other aged 12. An examination of the coffin furniture showed that the coffin was bought by a wealthy Anglican family, and DNA analyses suggest that the older individual was of Eastern European descent. These results suggest the burial of the older child in the same grave as the younger was most likely clandestine, and highlight the importance of post-excavation analyses to the interpretation of Australian cemeteries