Bilal Ahmad

Cold spray (CS) is a solid-state additive deposition technique of growing interest for repairing and protecting aerospace structural components. This study investigates how variations in CS repair thickness influence the fatigue behaviour of AA7075 coatings on AA7075-T6 substrates, simulating real-world post-processing through controlled material removal. Results show that CS coatings reduced overall fatigue life; however, thicker coatings (> 0.5 mm) showed only a moderate (~30%) reduction compared to uncoated material, while thinner coatings (≤ 0.5 mm) exhibited a greater (~70–85%) reduction when considering a redefined stress state. Failure mechanisms are thickness-dependent: thin coatings fail predominantly through CS cracking, which induces sharp, geometric stress concentrations at crack tips and accelerates substrate failure, while thicker coatings fail via interfacial delamination. Mirror and camera-assisted in-situ crack observation, supported by fractography, confirmed that in thick coatings cracks initiated at spray-induced substrate craters, whereas in thin coatings initiation occurred within the CS during the first loading cycle at surface or near-surface pores. These findings highlight the importance of optimising CS repair thickness, and interfacial morphology, as well as considering in-service bending and shear stress distributions, to maximise fatigue durability in aluminium repairs.

This paper presents a study on residual stress measurement in wire-arc additively manufactured (WAAM) titanium samples using the non-destructive method of phased array ultrasonics. The contour method (CM) was used for the verification of the phased array ultrasonic results. This allowed for a comparison of measurement methods to understand the effects on the distribution of residual stress (RS) within Ti-6Al-4V samples and the effectiveness of measurement of residual stress using phased array ultrasonics. From the results of the experiments, the phased array ultrasonic data were found to be in good agreement with the CM results and displayed similar residual stress distributions in the samples. The results of the individual elements of the phased array were also compared and an improvement in accuracy was found. From per-element results, anomalies were found and could be mitigated with the ability to average the results by using phased array ultrasonics. Therefore, based on these results, there is a strong case for the benefits of using phased array ultrasonics as a method of residual stress measurement for WAAM Ti-6Al-4V components over other existing residual stress measurement techniques.

This study aims to demonstrate the capability of the digital image correlation (DIC) technique for evaluating full-field residual stresses in wire and arc additive manufactured (WAAM) components. Investigations were carried out on WAAM steel parts (wall deposited on a substrate) with two different wall heights: 24 mm and 48 mm. Mild steel solid wire AWS ER70S-6 was used to print WAAM walls on substrates that were rigidly clamped to H-profiles. DIC was used to monitor the bending deformation of WAAM parts during unclamping from the H-profiles, and residual stresses were calculated from the strain field captured during unclamping. Residual stresses determined from the proposed DIC-based method were verified with an analytical model and validated by the results from established residual stress measurement techniques, i.e., the contour method and X-ray diffraction.

Wire + arc additive manufacturing (WAAM) is a modern manufacturing process that has opened new possibilities for rapid builds and reductions in material wastage. This paper explores residual stress in WAAM Ti-6Al-4V walls built using three different deposition strategies: single bead, parallel path, and oscillation path. The effect of interlayer hammer peening and interlayer temperature was investigated for the single bead walls. We also examined the residual stress in compact-tension (C(T)) coupons extracted from large builds (walls) with crack orientation either parallel with or perpendicular to the build direction. This type of sample is often used for the measurement of the fatigue crack growth rate. The contour method was used for experimental determinations of residual stress. In addtion, residual stress in the C(T) coupons was estimated by finite element (FE) analysis. A good agreement was achieved between the contour method and FE analysis. The oscillation-path wall had the lowest residual stress values. For the single bead walls built with various process conditions, residual stress was significantly reduced after removing the substrate. A interlayer temperature of 110 °C resulted in much higher residual stress values in the wall (both tensile and compressive) compared to the continuous build, with much higher interlayer temperature.

STEM and TEM analysis of cross-sections of the metal/oxide interface have been used to examine the transient scales (<200nm) formed on an iron-aluminium-silicon alloy. This alloy behaved differently to most alloys of its type, forming a flat adherent scale rather than a convoluted poorly adhered scale. Analysis showed that the unusual oxidation behaviour was due to the segregation of an impurity element (uranium) to the metal/oxide interface. The analysis also showed how the scale developed during the early stages of oxidation.

Development of residual stress of high magnitude, to the extent of material yield strength and in some cases higher than yield strength, is one of the major challenges faced by components produced using the wire arc additive manufacturing (WAAM). This study focuses on aluminum alloy 5183 with respect to the residual stress formation and distribution in WAAM builds. Residual stresses were determined using the contour method. The effects of processing conditions, such as substrate thickness, interlayer temperature, and deposit height on the magnitude and distribution of residual stresses were investigated. Substrate thickness was found to have a major influence on the residual stress distribution along deposit height. Tensile residual stress up to the value of the material yield strength was present. The majority part of the deposit showed tensile stress while substrate showed compensating compressive residual stress. Lower interlayer temperature samples exhibited residual stresses of higher degree of magnitude compared with sample produced using higher interlayer temperature. Deposit height, i.e., total number of layers affected stress distribution pattern similar to substrate thickness.

Residual stresses were characterised in a wire-feed additively manufactured titanium alloy component. A numerical simulation based on the inherent strain method was used to model residual stresses arising from the manufacturing process. The contour method was used to experimentally determine the residual stress field. High tensile residual stresses were seen at and around the interface of the substrate and the deposited metal. Compressive residual stresses were present in the substrate and at the top of the deposit. The satisfactory correlation was achieved between the results from the numerical simulation and the contour method, except for the location of the root of the deposit. The effect of pre-heating the sample substrate on the residual stress distribution is also discussed.