Search

High strength steels are a vital material for aerospace applications but are also prone to damage from fatigue, corrosion, and wear. Additive manufacturing (AM) processes such as laser directed energy deposition (L-DED) offer a means for repairing both the geometry and structure of damaged steels; however, significant variation in tensile properties have been reported following repair. While previous studies have tried to improve performance through postdeposition heat treatment, such practices may not be possible for commercial parts due to risks of distortion and thermal damage to the substrate. Instead, this investigation analyses the role of the intrinsic heat treatment effect on as-deposited tensile properties through a detailed review of both AM and AM repair literature. By assessing a wide variety of high strength steels, the links between conventional heat treatment parameters and steel performance in AM are established, and the role of steel composition understood. This review is supported by additional AM and L-DED repaired samples, with consistent parameters used between steels to ensure similar thermal histories, and eliminate potential discrepancies seen between AM machines. The results demonstrate the effect of intrinsic heat treatment on martensitic and precipitation hardening steels, the role of residual heat and heat extraction through the substrate, and flag potential issues faced by steels at risk of temper embrittlement. Taken together, these findings provide a clear vision for the advancement of AM repair and the optimization of mechanical performance. [ABSTRACT FROM AUTHOR]

Titanium and its alloys have been used in a broad range of products such as biomedical implants due to their high specific strength, corrosion resistance, and biocompatibility. Improvement in microstructure and mechanical properties of commercially pure (CP) titanium is usually performed by cold rolling and controlled atmosphere heat treatment. However, this is an energy-intensive, expensive, and a multi-stage process to achieve the desired properties. Hence, to address these challenges, in this study, the effects of an in situ electro-plastic treatment (ISEPT) on the microstructure evolution of CP titanium have been studied. The deformation load and electric current in this treatment were applied in the same direction (in situ) to maximize the electro-plastic effect. Simultaneous electric current and strain application created a condition for dynamic recrystallization to occur at low temperature and under atmospheric conditions, thus reducing the cost and energy for manufacturing in a single process. The rapid heating and cooling prevented the oxidation of titanium to a large degree, eliminating the requirements for costly inert gas or vacuum. The results showed rapid recrystallization of CP titanium at 3.2 mm/s roller surface speed. Recrystallization led to a reduction in average grain size to 7 μm with 18% increase in microhardness. Pre-cold rolling of the CP-Ti structure enhanced grain refinement due to the introduction of dislocations and the applied electric current interactions. The SEM and XRD investigations revealed the relationship between the ISEPT treatment and the evolution of microstructure in CP-Ti. The effect of specimen geometry on the ISEPT was discussed. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) is a powder bed fusion type metal additive manufacturing process which is being applied to manufacture highly customised and value-added parts in biomedical, defence, aerospace, and automotive industries. Aluminium alloy is one of the widely used metals in manufacturing parts in SLM in these sectors due to its light weight, high strength, and corrosion resistance properties. Parts used in such applications can be subjected to severe dynamic loadings and high temperature conditions in service. It is important to understand the mechanical response of such products produced by SLM under different loading and operating conditions. This paper presents a comprehensive review of the latest research carried out in understanding the mechanical properties of aluminium alloys processed by SLM under static, dynamic, different build orientations, and heat treatment conditions with the aim of identifying research gaps and future research directions. [ABSTRACT FROM AUTHOR]

• Bulk samples in the as-deposited state showed very poor mechanical properties due to low inter-particle bond strength. • Horizontally-built samples showed lowest pitting corrosion resistance compared to other samples. • Considerable anisotropy in both mechanical as well as corrosion properties were observed for samples printed in two different orientations. • Particle shape generated by cold spray process exhibits an influence on the strength of horizontally and vertically build deposits. • Post-build heat treatment significantly improved the strength, ductility, and corrosion resistance of the CSAM parts. Aluminium 6061 alloy (Al6061) is one of the widely used materials in the aluminium alloy family for structural applications owing to its high strength-to-weight ratio and high corrosion resistance. Al6061 components are traditionally manufactured using conventional processes such as casting, forging, rolling, and machining, which typically involved high scrap/waste generation. However, with the advent of advanced additive manufacturing (AM) processes, there is an opportunity to reconsider component design to near-net shapes with reduced material wastage. Although mechanical and corrosion properties of Al6061 fabricated with conventional processes is well understood, very little information is known about AMed Al6061. This study investigates the effect of build orientation and heat treatment on the mechanical and corrosion properties of Al6061 samples manufactured by cold spray additive manufacturing (CSAM) process. To the best of the authors' knowledge, this work is the first to explore both the mechanical and corrosion properties of Al6061 alloy print-parts fabricated using this unique CSAM system. Due to the lack of metallurgical bonding between powder particles, as-deposited samples - regardless of their build orientation, exhibited lower strength and ductility compared to the wrought counterparts. Particle shape generated by cold spray process exhibits an influence on the strength of horizontally and vertically build deposits. Expectedly, heat treatment significantly improved the mechanical properties by inducing metallurgical bonding at prior particle boundaries. As for corrosion resistance, horizontally-built samples exhibited the highest pit depth and pit penetration rate compared to the other conditions, which suggested a higher susceptibility to pitting corrosion owing to more strain-hardened particle sites available for corrosive attack. Heat treatment on as-deposited samples produced strain-free grains; thus, eliminating areas prone to corrosive environmental attack, leading to improvement in pitting corrosion resistance. Investigating post-processing heat treatment is crucial to improve the mechanical properties of as-deposited samples to a level similar to the ones of wrought condition. This research work is essential for selecting the correct processing routes and their parameters to manufacture components exposed to corrosive environments. [Display omitted] [ABSTRACT FROM AUTHOR]

• Tensile strength decreases with repair depth, but fatigue life remains similar. • Additive technique equivalent to current grind-out method for low cycle fatigue. • High cycle fatigue lower due to defects and local softening via in-situ tempering. • Layer delay strategy for in-situ tempering out performs track delay method. • Track delay leads to severe localised cracking at soft bands between tracks. Laser directed energy deposition repair provides significant advantage over current grind-out repair methods for 300 M steel, with samples containing 40% deposit producing equivalent low cycle fatigue behaviour to unfilled 10% grind-outs, though with reduced tensile strength. High cycle fatigue is associated with crack initiation at defects or locally softened regions due to in-situ tempering. While deposition strategy influences the degree of tempering, both layer and track delay strategies produce equally soft regions leading to similar fatigue limits regardless of fill depth. Layer delay specimens perform better overall due to more uniform tempering, which prevents concentrated crack growth and premature failure. [ABSTRACT FROM AUTHOR]