Microstructure of interpass rolled wire + arc additive manufacturing Ti-6Al-4V components

Mechanical property anisotropy is one of the issues that are limiting the industrial adoption of additive manufacturing (AM) Ti-6Al-4V components. To improve the deposits’ microstructure, the effect of high-pressure interpass rolling was evaluated, and a flat and a profiled roller were compared. The microstructure was changed from large columnar prior beta grains that traversed the component to equiaxed grains that were between 56 and 139 μm in size. The repetitive variation in Widmanstätten alpha lamellae size was retained; however, with rolling, the overall size was reduced. A “fundamental study” was used to gain insight into the microstructural changes that occurred due to the combination of deformation and deposition. High-pressure interpass rolling can overcome many of the shortcomings of AM, potentially aiding industrial implementation of the process.EPSRC, AirBu

Investigation of low current gas tungsten arc welding using split anode calorimetry

Most previous split anode calorimetry research has applied high weld currents which exhibit pseudo Gaussian distributions of arc current and power density. In this paper we investigate low current arcs and show that both the current and power distributions have minima in the centre – varying significantly from the expected Gaussian profile. This was postulated due to the formation of the arc with the copper anode and the tungsten cathode. Furthermore, a number of parameters were varied including the step size between measurements, anode thickness and anode surface condition as well as cathode type and tip geometry. The step size between measurements significantly influenced the distribution profile and the anode thickness needed to be above 7 mm to obtain consistent results

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

Resistance estimation for temperature determination in PMSMs through signal injection

Real-time thermal management of electrical machines relies on sufficiently accurate indicators of temperature within a machine. One indicator of temperature in a permanent-magnet synchronous motor (PMSM) is the stator winding resistance. Detection of PMSM winding resistance in the literature has been made on machines with relatively high resistances, where the resistive voltage vector is significant under load. This paper describes a technique applied to sense the winding resistance where the resistance is low and hence the resistive is voltage difficult to detect. A current injection method is applied which enables the resistance to be determined, and hence the winding temperature in non-salient machines. This method can be applied under load, and in a manner that does not disturb shaft torque, or speed. The method is able to distinguish between changes in the electro-motive force (EMF) constant and the resistive voltage. Simulated results on an experimentally verified model illustrate the effectiveness of this approac

The tyrosine kinase inhibitors methyl 2,5-dihydroxycinnamate and genistein reduce thrombin-evoked tyrosine phosphorylation and Ca2+ entry in human platelets

AbstractPlatelet activation is associated with the phosphorylation of a number of platelet proteins at tyrosine residues. The significance of this is unknown. Here we have investigated the effects of two tyrosine kinase inhibitors, methyl 2,5-dihydroxycinnamate and genistein, on thrombin-evoked protein tyrosine phosphorylation and Ca2+ signal generation in fura-2-loaded human platelets. Both compounds inhibited thrombin-evoked tyrosine phosphorylation and reduced the elevation of [Ca2+], in the presence, but not the absence, of external Ca2+. This suggested a selective inhibition of thrombin-evoked Ca2+ entry but not release from internal stores. Both compounds also reduced thrombin-evoked Mn2+ entry. In contrast, selective blockade of protein kinase C with Ro 31/8220-002 potentiated the thrombin-evoked Ca2+ signal. These data are compatible with a role for protein tyrosine phosphorylation contributing to thrombin-evoked Ca2+ entry in human platelets

Improved microstructure and increased mechanical properties of additive manufacture produced TI-6AL-4V by interpass cold rolling

Distortion, residual stress and mechanical property anisotropy are current challenges in additive manufacturing (AM) of Ti-6Al-4V. High-pressure, interpass rolling was applied to linear AM parts and resulted in a change from large columnar prior β grains to a completely equiaxed microstructure with grains as small as 89 μm. Moreover, α laths thickness was also reduced to 0:62 μm. The change in material microstructure resulted in a substantial improvement of all mechanical properties tested, which were also totally isotropic. In rolled specimens, maximum measured strength and elongation were 1078MPa and 14% respectively, both superior to the wrought material. Distortion was reduced to less than half. Rolling proved to be a relatively easy method to overcome some of the critical issues which keep AM from full industrial implementation

Arc instabilities during split anode calorimetry with the TIG welding process

Split anode calorimetry investigations of tungsten inert gas (TIG) arcs were conducted using a large-diameter shielding gas nozzle. Some conditions displayed arc symmetry whilst others proved distinctly asymmetric. The variation of welding current and electrode tip to workpiece distance (ETWD) was studied. Decreasing the ETWD was found to increase the current density towards the arc axis but similar to a previous study of the authors, Gaussian distributions were not observed. The gas nozzle was designed to produce laminar gas flow and sound shielding behaviour; however, anode surface oxidation was found after welding, presumably caused by shielding gas contaminated with oxygen through the welding sequence. Therefore, axial arc symmetry was influenced by random effects. The conditions and reasons for the observed phenomena are explored within this paper

Residual Stress Characterization and Control in the Additive Manufacture of Large Scale Metal Structures

Additive Manufacture of metals is an area of great interest to many industrial sectors. All metal additive manufacturing processes suffer with problems of residual stresses and subsequent distortion or performance issues. Wire + Arc Additive Manufacture (WAAM) is a metal additive manufacture process that is suitable for the production of large scale engineering structures. Paramount to the successful industrial application of WAAM is the understanding and control of residual stress development and their subsequent effects. Vertical inter-pass rolling can be used to reduce these residual stresses, but its potential is limited due to the absence of lateral restraint of the wall. So it deforms the wall in its transverse direction rather than reducing longitudinal tensile residual stresses, which is the main source of the distortion. The potential of a new pinch-roller concept is currently being investigated at Cranfield University with very promising preliminary results: It was possible to entirely eliminate the distortion of a Ti 6Al 4V WAAM wall

Design of an empirical process model and algorithm for the Tungsten Inert Gas wire+arc additive manufacture of TI-6AL-4V components

In the wire+arc additive manufacture process parameters can be varied to achieve a wide range of deposit widths, as well as layer heights. Pulsed Tungsten Inert Gas was chosen as the deposition process. A working envelope was developed, which ensures unfeasible parameters combinations are excluded from the algorithm. Thanks to an extensive use of a statistically designed experiment, it was possible to produce process equations through linear regression, for both wall width and layer height. These equations are extremely useful for automating the process and reducing the buy-to-fly ratio. For a given layer height process parameters can be selected to achieve the required layer width while maximising productivity