Metallic multi-material structures are heterogeneous structures characterized by changing composition, microstructures, and site-specific characteristics, advantageous for numerous applications where multifunctionality is desired. Metallic multi-material structures are known as bimetallic structures (BSs), which are functionally graded materials (FGMs). In recent years, wire-arc additive manufacturing (WAAM) advanced as a promising additive manufacturing process to realize the fabrication of these structures due to its high deposition rate, cost-effectiveness, and material utilization efficiency. This review presents a comprehensive overview of the recent progress, processing strategies, and scientific challenges in WAAM of multi-material structures. The paper begins with an introduction to multi-material structures, followed by a bibliometric analysis of the current research landscape. Conventional and additive manufacturing fabrication approaches are presented. The review highlights key developments in processing strategies and critically evaluates research studies on WAAM of BS and FGMs. Major scientific challenges, including porosity, lack of fusion, residual stresses, cracking, material compatibility, and brittle intermetallic phase formation, are critically analyzed. Additionally, modeling, simulation, and process automation issues are discussed as barriers to industrial-scale implementation. The paper concludes with an outlook on future research directions to address existing challenges and accelerate the adoption of WAAM for complex multi-material components.

While binder jetting (BJ) additive manufacturing (AM) holds considerable promise for industrial applications, defects often compromise part quality. This study addresses these challenges by investigating binding mechanisms and analyzing common defects, proposing tailored solutions to mitigate them. Emphasizing defect identification for effective quality control in BJ-AM, this research offers strategies for in-process rectification and post-process evaluation to elevate part quality. It shows how to successfully process metallic parts with complex geometries while maintaining consistent material properties. Furthermore, the paper explores the microstructure of AISI M2 tool steel, utilizing advanced image processing techniques like digital image analysis and SEM images to evaluate carbide distribution. The results show that M2 tool steel has a high proportion of M₆C carbides, with furnace-cooled samples ranging from ~2.4% to 7.1% and MC carbides from ~0.4% to 9.4%. M₆C carbides ranged from ~2.6% to 3.8% in air-cooled samples, while water-cooled samples peaked at ~8.52%. Sintering conditions also affected shrinkage, with furnace-cooled samples showing the lowest rates (1.7 ± 0.4% to 5 ± 0.4%) and water-cooled samples showing the highest (2 ± 0.4% to 14.1 ± 0.4%). The study recommends real-time defect detection systems with autonomous corrective capabilities to improve the quality and performance of BJ-AM components.

The versatility of titanium (Ti) allows it to be employed in various industries, from aerospace engineering to medical technology, highlighting its significance in modern manufacturing and engineering processes. Spark plasma sintering (SPS) is currently being explored to enhance its properties further and broaden its application range. The current study focuses on exploring and optimizing the effect of SPS temperature (800, 900, 1000, 1100, 1200, and 1400 °C) on pure Ti sintered at 60 MPa in a controlled argon environment with a dwell time of 5 min. All the prepared samples were highly dense with a relative density above 99%, but exhibited significant variations in grain size (10 to 57 µm), tensile yield strength (488 to 700 MPa), ultimate tensile strength (597 to 792 MPa), and ductility (4 to 7%). A microstructural investigation was performed using XRD, SEM, and EDS to predict the influence of sintering temperature on the formation of different phases. The XRD patterns of all sintered samples showed the presence of single-phase α-Ti with hexagonally close-packed Ti. This work is a step forward in optimizing SPS-processed Ti’s physical and mechanical properties for enhanced structural and biomedical applications.

Titanium (Ti) is widely used in structural, maritime, aerospace, and biomedical applications because of its outstanding strength-to-weight ratio, superior corrosion resistance, and excellent biocompatibility. However, the lower surface hardness and inferior wear resistance of the Ti and Ti alloys limit their industrial applications. Coating Ti surfaces can initiate new possibilities to give unique characteristics with significant improvement in the Ti component’s functionality. The current research designed and synthesized titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN), tantalum nitride (TaN), and niobium nitride (NbN) ceramic coating layers (400 µm) over a Ti substrate using a spark plasma sintering process (SPS). The coatings on the Ti substrate were compact and consolidated at an SPS temperature of 1500 °C, pressure of 50 MPa, and 5 min of holding time in a controlled argon atmosphere. Microstructure investigation revealed a defect-less coating-substrate interface formation with a transition/diffusion zone ranging from 10 µm to 20 µm. Among all of the ceramic coatings, titanium carbide showed the highest improvement in surface hardness, equal to 1817 ± 25 HV, and the lowest coefficient of friction, equal to 0.28 for NbN.

β-titanium (β-Ti) alloys are used in various biomedical applications, especially for orthopedic implants, due to their superior biocompatibility, excellent corrosion resistance, and enhanced mechanical properties. However, the inferior tribological properties of β-Ti alloys lead to fretting wear and a strong tendency to seize, which is a major concern in orthopedic applications involving continuous friction. This work aims to address this issue by incorporating biocompatible nitrides in Ti-Nb-Zr-Ta (TNZT) β-Ti alloys. TNZT composites comprising 2 wt.% of biocompatible nitrides (TiN, NbN, ZrN, and TaN) were prepared using high-energy ball milling followed by spark plasma sintering. All the nitrides improved the hardness and wear resistance of TNZT alloys and showed excellent biocompatibility. TNZT-2 wt.% TiN showed the average highest hardness of 311.8 HV and the lowest coefficient of friction of 0.659, suggesting the highest efficiency of TiN in improving the tribological performance of TNZT alloys. The underlying mechanisms behind the superior performance of nitride-reinforced TNZT composites are discussed in detail. The effect of TiN concentration was also studied by preparing TNZT composites with 5 and 10 wt.% TiN, which showcased a higher hardness of 388.5 HV and 444.3 HV, respectively. This work will aid in producing superior β-Ti alloys for advanced orthopedic applications.

A novel approach is introduced, utilizing low concentrations of hexagonal boron nitride (hBN) and fabricating titanium matrix composites (TMCs) through ball milling and spark plasma sintering (SPS). The ball-milled Ti-x wt.% hBN (x: 0.1, 0.25, and 0.5) powders sintered at 60 MPa pressure and a 5-min holding time for temperatures ranging from 900 to 1200 °C. The in-situ formation of titanium boride whiskers (TiBw) and Ti(N) solid solution occurred from the Ti and hBN particle reaction during sintering. The XRD pattern of the hBN-added sample shows α-Ti similar to pure Ti without the reaction phase due to a lower hBN fraction in Ti. However, the XRD peak shift toward a lower diffraction angle for the Ti-hBN sample confirms the formation of Ti(N) within the Ti matrix. Microstructure analysis reveals significant grain refinement with increasing hBN fraction; the grain sizes for Ti, Ti-0.1hBN, Ti-0.25hBN, and Ti-0.5hBN are 36, 22, 20, and 18 μm, respectively. The presence of TiBw and Ti(N) leads to a grain refinement effect in higher hardness and enhanced strength in composite samples. The Ti-0.25 wt% hBN sample sintered at 1200 °C exhibited an optimal combination of relative density (99.73 %), hardness (341.8 ± 6 HV), yield strength (1042 ± 21 MPa), compressive strength (1840 ± 23 MPa), and elongation (34.4 ± 1.5 %). The biocompatibility is confirmed through cell adhesion, viability, and cytotoxicity studies, highlighting these composite's excellent biocompatibility and potential for orthopedic implant application.