Citation: Boruah, D.; McNutt, P.; Sharma, D.; Begg, H.; Zhang, X. Understanding the Effect of Substrate Preheating Temperature and Track Spacing on Laser Assisted Cold Spraying of Ti6Al4V. Metals 2023, 13, 1640. https://doi.org/10.3390/ met13101640 Academic Editor: Alessio Silvello Received: 8 August 2023 Revised: 11 September 2023 Accepted: 20 September 2023 Published: 25 September 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). metals Article Understanding the Effect of Substrate Preheating Temperature and Track Spacing on Laser Assisted Cold Spraying of Ti6Al4V Dibakor Boruah 1,2, * , Philip McNutt 1 , Deepak Sharma 3 , Henry Begg 1 and Xiang Zhang 2 1 Surface, Corrosion and Interface Engineering, TWI Ltd., Cambridge CB21 6AL, UK; henry.begg@twi.co.uk (H.B.) 2 Faculty of Engineering, Environment and Computing, Coventry University, Coventry CV1 5FB, UK 3 Materials Innovation Centre, School of Engineering, University of Leicester, Leicester LE1 7RH, UK * Correspondence: d.boruah@twi.co.uk Abstract: In this study, laser-assisted cold spray (LACS) of titanium alloy Ti6Al4V onto Ti6Al4V substrates has been investigated in two phases: (i) single-track deposits on substrates preheated to 400 ◦ C, 600 ◦ C, and 800 ◦ C, respectively, and (ii) single-layer (multi-track) deposits on substrates preheated to 600 ◦ C with three different track spacings (1 mm, 2 mm, and 3 mm). Cross-sectional microstructures of the single-track deposits showed intimate contact at the interfaces, especially extensive interfacial mixing for specimens with substrate preheating at 600 ◦ C and 800 ◦ C. Cross- sectional area porosity content in single layer LACS coatings was found to be around 0.4%, which is significantly lower than the standard or conventional cold spray (CS) process having ~2.3% porosity. The microstructure reveals that the LACS process has improved the adhesion and cohesion of the deposits, in addition to the other advantages of the CS process. The average microhardness values of LACS deposits were found to be in the range of 388–403 HV (the highest hardness with the lowest track spacing), which is approximately 6–10% lower than that of the CS deposits without laser substrate preheating. Tensile residual stresses were found in all three LACS coatings, which was due to elevated process gas temperature along with high heat input during laser preheating of the substrate. It was observed that the higher the track spacing, the higher the stress magnitude, i.e., 31 MPa, 135 MPa, and 191 MPa in the longitudinal direction when deposited with 1 mm, 2 mm, and 3 mm track spacings, respectively. Heat treatments induced varied microstructures in LACS coatings, encompassing fully equiaxed or lamellar α-phase within the β-phase, or a bimodal microstructure, with characteristics linked to track spacing variations. Key contributions of this study include enhanced coating-substrate adhesion through extensive interfacial mixing, a substantial reduction in cross-sectional area porosity compared to CS, insights into the effects of residual stresses, and, ultimately, advancing the comprehension of LACS and its potential advantages over conventional CS process. Keywords: coatings; laser-assisted cold spray; microstructure; residual stress; repairs; titanium 1. Introduction Cold spray (CS) technology is a solid-state material deposition technique wherein powder particles are accelerated to reach a critical velocity by a supersonic jet of preheated compressed gas (usually N 2 and/or He). The high-velocity impact of the sprayed particles on a substrate and associated severe plastic deformation disrupt thin oxide films promoting intimate metallic contact of particles and substrate by creating bonding similar to explosive welding (or explosive bonding), resulting in solid-state deposition of material layers. The ability of CS technology to use lower deposition temperature (always below the melting point of the deposited material) makes it suitable for depositing temperature-sensitive materials, such as nanocrystalline and amorphous materials, as well as oxygen-sensitive materials such as titanium, aluminium, copper, etc. [1–3]. Metals 2023, 13, 1640. https://doi.org/10.3390/met13101640 https://www.mdpi.com/journal/metals