A comparative study of additive manufacturing techniques: Residual stress and microstructural analysis of CLAD and WAAM printed Ti–6Al–4V components

Abstract

Nowadays, there is a great manufacturing trend in producing higher quality net-shape components of challenging geometries. One of the major challenges faced by additive manufacturing (AM) is the residual stresses generated during AM part fabrication often leading to unacceptable distortions and degradation of mechanical properties. Therefore, gaining insight into residual strain/stress distribution is essential for ensuring acceptable quality and performance of high-tech AM parts. This research is aimed at comparing microstructure and residual stress built-up in Ti-6Al-4V AM components produced by Wire + Arc Additive Manufacturing (WAAM) and by laser cladding process (CLAD). 2 Introduction Additive manufacturing, often called 3D printing is nowadays among the most studied processes. AM is a key technique of a great potential in reducing high cost of producing conventional components made from relatively expensive materials such as titanium alloys. The worldwide Ti component production is constrained due to the high cost of Ti in comparison to other materials. Therefore, AM techniques aiming towards zero waste manufacturing are identified as potential prosperous routes in broadening Ti parts fabrication that are usually affected by often difficult and extensive machining. Ti is very broadly used in space, aerospace, nuclear, marine and chemical industries by virtue of its desirable properties such as high specific strength combined with excellent corrosion and oxidation resistance [1]. Although Ti is a very cherished material, its use in AM processes is also relatively challenging because of its low thermal conductivity which results in drawbacks such as uneven temperature field and poor interlamination integration [2]. Avoiding extensive machining by a near netshape successive layers fabrication can reduce the Ti parts production cost significantly. The buy-tofly ratio for a part machined from forged billet is typically 10-20 [3] and can potentially drop to nearly 1 when produced by AM techniques. There are numerous AM techniques that are capable of producing complex geometries close to their net-shape. Simply, AM techniques can be classified according to feeding technique, heat source or feedstock material. Powder bed, blown power and wire feed are the main AM techniques using heat sources such as electron beam, laser or electric arc, while the most common feedstock materials are powder or wire. Despite the similarity of AM

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

“…After unclamping, the profile shows a constant drop of residual stress with increasing distance to the substrate, with being compressive in the top layers. This trend is also supported by this model and also reported elsewhere [3], [4], [8]. Side Rolling took place from 3 mm above the substrate to the top of the wall.…”

Stress in Thin Wall Structures Made by Layer Additive Manufacturing

“…The model assumes very simple stress state with only the longitudinal component being nonzero and it is fully characterised by a single parameter, the deposition stress σ d or, equivalently, the curvature κ. The fact that the normal and transverse stresses can be accepted as zero for all practical purposes has a solid experimental corroboration in this study as well as in other studies [4,6]. While not every detail can be derived with the simple analytical model, it is clear that the main trend and magnitude of stress distribution in the wall can be reproduced.…”

“…5. Experimental residual stress distributions in walls two samples, a Ti-6Al-4V thin wall (left, adopted from [6]) and a steel thin wall (right, adopted from [5]). Experimental points are overlapped with simulated stress profiles (blue lines) calculated using "full constrain" modelling approach is most likely unable to accurately characterise the stress field at the joint point with the base plate, as well as at the very top layer of deposited material, which would have a different history to all other deposited layers.…”

“…For experimental studies, a single wall structure (T-shape sample) of a constant thickness and height, built on a rectangular base plate appears to be a standard choice [3][4][5][6]. Depending on the material, exact dimensions of the build-up and the process, the deflection of the back side of the base plate can vary greatly, but a typical reported deflection of the base plate in case of Ti-6Al-4V is 14 mm per 1000 mm longitudinal base length [6]. In addition to causing distortion, residual stresses may have detrimental effects on mechanical properties, especially fatigue behaviour, thereby degrading the performance of the component in service.…”

“…In one case the Ti-6Al-4V alloy 6 mm thin wall was additively manufactured by a similar technique and measured using neutron diffraction [6]. In the other case, a stress analysis of a steel 5 mm thin wall on the steel substrate was carried out also with neutron diffraction [5].…”