Aluminum and magnesium were joined through diffusion bonding using Ni interlayer. The microstructure and mechanical performance of the Al/Ni/Mg joints at different temperatures was investigated by means of scanning electron microscope (SEM), electro-probe microanalyzer (EPMA), X-ray diffraction (XRD), Vickers hardness testing, and shear testing. The results show that the addition of Ni interlayer eliminates the formation of Mg–Al intermetallic compounds and improves the bonding strength of the Al/Mg joints. The Al/Ni/Mg joints are formed by the diffusion of Al, Ni and Mg, Ni. The microstructure at the joint interface from Al side to Mg side is Al substrate/Al–Ni reaction layer/Ni interlayer/Mg–Ni reaction layer/Mg substrate multilayer structure. The microhardness of the Mg–Ni reaction layer has the largest value of HV 255.0 owing to the existence of Mg2Ni phase. With the increase of bonding temperature, the shear strength of the joints increases firstly and then decreases. The Al/Ni/Mg joint bonds at 713 K for 90 min, exhibiting the maximum shear strength of 20.5 MPa, which is greater than that of bonding joint bonded directly or with Ag interlayer. The fracture of the joints takes place at the Mg–Ni interface rather than the Al–Ni interface, and the fracture way of the joints is brittle fracture.

We report on an investigation of the influence of reinforcement particle size on the microstructure and mechanical behavior of Al metal matrix composites. In our work, Al 7075/B4C composites containing three types of B4C particle sizes (56.9 µm, 4.2 µm and 2.0 µm) were synthesized and studied. For a constant value of volume fraction of B4C, the composite with coarse reinforcement particles exhibited a relatively homogeneous and discrete distribution of the B4C particles while the composites with fine reinforcement exhibited agglomeration of the B4C particles. The composite with the smallest B4C particles possessed the highest yield strength and fracture strength. Quantitative analysis of the strengthening mechanisms revealed that smaller B4C particles lead to larger values in strain gradient strengthening as well as CTE mismatch strengthening, which are significantly correlated to the geometrically necessary dislocations caused by the presence of B4C. In addition, the different spatial distributions of the B4C particles contributed to different fracture mechanisms in the composites.

•W powders were uniformly coated with highly purified Cu by electroless plating method.•The physical properties improve greatly with increasing Cu content in the coating.•Ideal Cu network structure provides a perfect path for electron transportation.•Strong interfacial bond between Cu and W improves the stress transfer.The effects of interfacial bond and homogeneous microstructure on the physical properties of Cu–W composites have been investigated. To acquire strong interfacial bond and homogeneous microstructure, different modified W powders have been designed, which W powders were coated with different Cu contents using electroless plating method. The results showed that by increasing the Cu content of the coating, the microstructure of Cu–W composites becomes homogeneous, and the physical properties, including thermal, electrical and mechanical properties, improved greatly. When 20Cu@W composite powders were used to fabricate Cu–W composites, the physical properties reached the optimal values: The thermal conductivity was 239 W/(m·K) which was close to the theoretical vaule of 240 W/(m·K), the electrical conductivity was 50.6%IACS, coefficient of thermal expansion was the minimum value of 7.3 × 10− 6/K, the bending strength and Vickers hardness were 976.7 MPa and 224.8 HV, respectively. These optimal values were much higher than those of mixed Cu–W composites. The properties enhancement of Cu–W composites is attributed to the strong interfacial bond between Cu and W and homogeneous microstructure. This enhancement effect was strengthened by increasing the coating's Cu content, resulting in the continuous improvement of the physical properties.

Plasma-activated sintering (PAS) has been applied, for the first time, to join magnesium and aluminum using a CuNi/Ag/CuNi sandwich structural interlayer. A cleaning effect and high efficient plasma heating mode in PAS have contributed to forming a strong interfacial diffusion bond under low temperature 673 K (400 °C) and short dwell time (0.6 ks). The designed interlayer provides a diffusion barrier effect and an enhanced physical contact between the interfaces. Strong bonding has been achieved without forming the brittle Mg-Al intermetallics.

Diffusion bonded Mg–Al joints with and without an interlayer were produced. An Ni foil interlayer eliminated the formation of Mg–Al intermetallic compounds, while addition of an Al thin film to the interlayer improved the properties of the Mg–Ni intermetallic compounds. The shear strength of the joints was improved by the addition of the Ni foil and Al thin film interlayer.

Diffusion bonding aluminum and magnesium using a Ni interlayer was investigated for the first time. The bonding trials were carried out at 440 °C for different time under 1 MPa in a vacuum of 6 × 10− 3 Pa. The interfaces were investigated by means of SEM, EPMA, XRD and Vickers hardness test. The results showed that dissimilar metals of Mg/Al could be successfully joined by diffusion bonding with a Ni interlayer and the Mg–Al intermetallic compounds were impeded. From Al side to Mg side, there was Al substrate, Al–Ni reaction layer, Ni interlayer, Mg–Ni reaction layer, Mg substrate. The microhardness in the Al–Ni reaction layer and Mg–Ni reaction layer increased sharply, which were 201.5 Hv and 244.1 Hv. The fracture occurred in the intermetallic compound layer of the Mg–Ni reaction layer, where the Mg2Ni phase appeared on both Al and Mg surfaces.Highlights► Using a higher melting-point interlayer to diffusion bonding Al–Mg for the first time. ► Al and Mg were bonded successfully by diffusion bonding using Ni interlayer. ► The brittleness Mg–Al intermetallic compounds were impeded. ► The fracture occurred in the Mg–Ni interface rather than in the Al–Ni interface.

•A bilayer composite was synthesized using plasma activated sintering.•Precipitation behavior of the bilayer composites was discussed.•Densification mechanisms of the bilayer composites were discussed.•Fracture mechanisms of the bilayer composites were discussed.In the present study a novel bilayer composite consisting of a tough layer (AA7075) and hard layer (AA7075/B4C composite), was successfully synthesized using plasma activated sintering. No obvious defects such as porosity, cracks, or delamination were observed in the as-processed bilayer composite. Transmission electron microscopy results suggest that dislocation density in the vicinity of B4C particles is higher relative to that of the unreinforced layer, as imaged with the [011] zone axis. The precipitation behavior is significantly different in the two layers, despite identical process conditions. Moreover, no obvious precipitates free zones were observed in the interfacial region between the two layers. To provide insight into the mechanical behavior of the bilayer composites, we measured the bending strength on two different load-bearing surfaces. Our results indicate that when the AA7075/B4C layer was loaded under tensile conditions, the AA7075/B4C composite was prone to crack formation and catastrophic failure. In contrast, when the AA7075 layer was loaded in tensile, the bilayer composite achieved a notable bending strength value of 1234 ± 72 MPa, which is higher relative to that of aforementioned case (938 ± 85 MPa). Interestingly, no interfacial cracks or delamination were observed under these testing conditions. In addition, the fracture mechanisms that are active in the bilayer composites for the load bearing conditions studied were discussed in detail.