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High entropy alloys (HEAs) have superior mechanical properties that have enabled them to be used as structural materials in nuclear and aerospace applications. As a dissimilar joint design is required for these applications, we created a dissimilar joint between CoCrFeMnNi-HEA and duplex stainless steel (DSS) through laser beam welding; a technique capable of producing a sound joint between the two materials. Microstructure examination using SEM/EBSD/XRD analysis revealed that the weld metal (WM) exhibits an FCC phase regardless of the postweld heat treatment (PWHT) temperature (800 and 1000 °C) without forming detrimental intermetallic compounds or microsegregation. The heat-affected zone of the CoCrFeMnNi-HEA showed CrMn oxide inclusions while that of the DSS showed no inclusions. Moreover, a lower hardness was recorded by the WM compared to the base metal after welding. After PWHT, the hardness of the WM, CoCrFeMnNi-HEA, and DSS decreased with an increase in the PWHT temperature. However, the decrease in the hardness of the HEA was more significant than in the WM and DSS. The cause for this reduction in hardness was attributed to recrystallization and grain growth. In addition, a strength of 584 MPa with low ductility was recorded after welding. The obtained strength was lower than that of the BMs, but comparable to that of the welded CoCrFeMnNi-HEA. The application of PWHT resulted in over a 20% increment in ductility, with only a marginal reduction in strength. The deformation mechanism in the as-weld joint was mainly dominated by dislocation while that for the PWHT joint was twinning. We propose laser beam offset welding as a technique to improve the mechanical properties of the dissimilar joint, which will be the subject of future studies.

The microstructures and mechanical properties of joints between 17-4PH stainless steel and pure V were investigated. During laser welding of these joints, the laser beam was first placed at the middle of the joint interface (LW-0) and then shifted by 0.1 mm (LW-0.1) or 0.2 mm (LW-0.2) towards the 17-4PH side. The effects of post-weld heat treatment on the mechanical properties of the welded joints were examined. The microstructure of the LW-0 joint, which was mainly composed of (FeV)ss due to uniform mixing of Fe and V, changed to an (Fe)ss-rich phase at the 17-4PH//FZ side and (FeV)ss at the FZ//V side upon shifting the laser beam towards the 17-4PH side. A small fraction of σ-phase was formed, which induced crack initiation and propagation, when the laser beam was not shifted. The grain morphologies in the fusion zones were highly dependent on the Fe–V composition. At a V content above 28 at.%, (FeV)ss readily forms and a coarse grain structure was observed; below this concentration Fe-rich phases formed with a columnar grain structure upon solidification. The hardness and tensile properties of the joints were dependent on the V concentration in the melt zone. The hardness increased in the order of LW-0.2 LW-0.1 > LW-0. The hardness significantly increased after annealing due to the precipitation of the σ-phase, which was dispersed throughout the fusion zones of LW-0 and LW-0.1, or at the FZ//V interface for LW-0.2.

The feasibility of using a high-entropy rare-earth oxide (REO) as a top coating material for thermal barrier coatings was explored using the atmospheric plasma spray technique. The microstructure and Vickers hardness of the coating layer were compared to those of an 8 mol % yttria-stabilized zirconia (8YSZ) top coating material. Macroscopic observations revealed the formation of a well-coated surface with no surface defects or delamination. Scanning electron microscopy images showed the presence of several parallel and vertical microcracks in the REO and 8YSZ coating layers. The origin of these cracks is attributed to differences in the coefficient of thermal expansion, very fast cooling, and process parameters. X-ray diffraction demonstrated the high phase stability and excellent thermal properties of REO due to the absence of phase transformation after plasma spray processing. The measured Vickers hardness of REO was 425 HV, which is lower than that of sintered REO powder and the 8YSZ coating.

In this study, electron beam welding, continuous laser welding, and pulsed laser welding were used to join Ti–6Al–4V alloy and 17-4PH stainless steel with a 1 mm pure V sheet interlayer. In addition, the effect of V composition on the mechanical properties of the welded joint by shifting the laser towards the 17-4PH was examined. Vanadium bonded well with Ti, and the weld metal was characterized by a (βTi,V) solid solution. In the V–steel welded joint, similar phases ((Fe,V)ss) but with different compositions were found in the MZs, which affected the mechanical properties. The cracked region was visible at the V–steel interface of the pulsed-laser-welded joint and contributed to its low tensile strength. Electron beam welding showed a good intact joint with no defects, and failure occurred in the vanadium interlayer. A common MZ was created for the three metals in EB1, which formed Fe2Ti and FeTi intermetallic compounds that acted as crack initiation sites and led to the failure of the welded joint during machine processing. Variation in V composition had a significant impact on the mechanical properties of the welded joints. Hardness increased incrementally with the V concentration, and joints with a composition of less than 35 at% V, which corresponded to α-Fe + (Fe,V)σ and α-Fe phase formation ranges, had better tensile properties than those with more than 35 at% V, which corresponded to the single (Fe,V)σ phase. However, care should be taken in regulating the laser beam in order to avoid a very low V content, since this could lead to the formation of unfused regions.

The high-temperature oxidation behaviour of CoCrNi, CoCrNiMn, and CoCrNiMnFe equimolar alloys was investigated. All three alloys have a single-phase face-centred cubic structure. Thermogravimetric analyses (TGA) were conducted at temperatures ranging from 800 to 1000 °C for 24 h in dry air. The kinetic curves of the oxidation were measured by TGA, and the microstructure and chemical element distribution in different regions of the specimens were analysed. The oxidation kinetics of the three alloys followed the two-stage parabolic rate law, with rate constants generally increasing with increasing temperature. CoCrNi displayed the highest resistance to oxidation, followed by CoCrNiMnFe and CoCrNiMn exhibiting the least resistance to oxidation. The addition of Mn to CoCrNi increased the oxidation rate. The oxidation resistance of CoCrNiMn was enhanced by the addition of Fe. Less Mn Content and the formation of more Cr2O3 were responsible for the reduction in the oxidation rates of CoCrNiMnFe. The calculated activation energies of CoCrNiMn and CoCrNiMnFe at 800, 850 and 900 °C were 108 and 137 kJ mol−1, respectively, and are comparable to that of Mn diffusion in Mn oxides. The diffusion of Mn through the oxides at 800–900 °C is considered to be the rate-limiting process. The intense diffusion of Cr at 1000 °C contributed to the formation of CrMn1.5O4 spinel with Mn in the outer layer of CoCrNiMn and Cr2O3 in the outer layer of CoCrNiMn.

The interfacial structure and pore formation mechanism of V, which was laser cladded under various conditions with 17-4PH alloys on a Ti-6Al-4V plate, were studied using optical and scanning electron microscopy and an electron probe microanalyzer. Pores were formed in the interfacial regions of the cladded V. Moreover, upon high energy input, more prominent pores were observed, which was confirmed by the effect of V oxide generated during laser cladding. Cracks and delamination occurred when the 17-4PH alloy was additionally cladded due to pore generation owing to the high energy input used for V cladding.

Residual stress is a crucial element in determining the integrity of parts and lifetime of additively manufactured structures. In stainless steel and Ti-6Al-4V fabricated joints, residual stress causes cracking and delamination of the brittle intermetallic joint interface. Knowledge of the degree of residual stress at the joint interface is, therefore, important, however, the available information is limited owing to the joint’s brittle nature and its high failure susceptibility. In this study, the residual stress distribution during the deposition of 17-4PH stainless steel on Ti-6Al-4V alloy was predicted using Simufact additive software based on the finite element modeling technique. A sharp stress gradient was revealed at the joint interface, with compressive stress on the Ti-6Al-4V side and tensile stress on the 17-4PH side. This distribution is attributed to the large difference in the coefficients of thermal expansion of the two metals. The 17-4PH side exhibited maximum equivalent stress of 500 MPa, which was twice that of the Ti-6Al-4V side (240 MPa). This showed good correlation with the thermal residual stress calculations of the alloys. The thermal history predicted via simulation at the joint interface was within the temperature range of 368–477 °C and was highly congruent with that obtained in the actual experiment, approximately 300–450 °C. In the actual experiment, joint delamination occurred, ascribable to the residual stress accumulation and multiple additive manufacturing (AM) thermal cycles on the brittle FeTi and Fe2Ti intermetallic joint interface. The build deflected to the side at an angle of 0.708° after the simulation. This study could serve as a valid reference for engineers to understand the residual stress development in 17-4PH and Ti-6Al-4V joints fabricated with AM.

A lap joint was formed between a CoCrFeMnNi-high entropy alloy (HEA) and duplex stainless steel (DSS) using laser beam welding. The microstructure morphology of the weld varied, with the upper section of the DSS side comprising a primary austenite phase and a secondary delta ferrite phase. An unmixed region that flowed from the DSS and characterized as ferrite was observed as the main metallurgical feature of the weld. The lower section was characterized by single austenite phase with large grains. Hardness fluctuations occurred within the weld, where the unmixed zone exhibited the highest hardness. The tensile shear strength of the joint was 470 MPa.

Modern fuel injectors for gasoline, diesel, and gaseous fuel use dissimilar joints of Inconel 718 and STS 304. To better understand the microstructural evolution and mechanical properties of these types of dissimilar joints, the joints of these alloys were fabricated by employing laser beam welding process. The joint was characterized by electron microscopy and hardness measurement techniques. By employing the laser beam welding, a crack-free deep penetrating joint was obtained. The weld metal was primarily composed of an austenite phase with a distribution of Laves and Ti/Nb carbides in the interdendritic regions. The low heat inputs and fast cooling rates suppressed the Laves precipitation, as its volume fraction was less than that of most IN718 joints fabricated by traditional welding techniques. Partially melted zones (PMZ) were formed on both sides of the joints. The PMZ of IN718 base metal was complex with Nb-rich precipitates detected as δ-Ni3Nb, while that of STS 304 was free of precipitates. No significant grain coarsening or negative metallurgical effects were observed at the heat-affected zones. The fusion zone (FZ) recorded a lower hardness, compared to the parent metal, which is mainly due to the depletion of strengthening elements (Nb, Mo, and Ti) and lack of hardening γ′′ particles. The joint showed a low strength with failure occurring at the FZ. Post-weld solution treatment at 980 °C resulted in the partial dissolution of the Laves phase and complete disappearance of the dendrites, followed by the precipitation of plate- and needle-like δ phase. Post weld heat treatment increased the hardness of the joint. The solution treated + double aged joint showed much higher strength than the directly aged joint. The increased hardness reflects the precipitation of γ′′ in the weld.

Stress – strain values obtained from tensile tests of aluminium and steel is used to evaluate the true stress – true strain values. The Hollomon’s model is then used to predict the strain-hardening behavior in the two specimens. It is clearly seen that the strain-hardening behavior in metals can be described using the Hollomon’s model. However, we have assumed that the onset of strain-hardening is at the yield point up until the ultimate tensile strength. The correlation between the experimental true stress – true strain values of aluminium and the calculated values using the Hollomon equation is much better than that of steel.