The main aim of of this study was to characterize the microstructures and mechanical properties of Cu-AISI4140 steel solid-state joints created via spark plasma welding (SPW), with and without the use of molds, with and without interlayers, under various temperatures, durations, and pressures. To investigate the influence of molds on the Cu-steel joining process, SPW was conducted at 650°C for 30 minutes under a pressure of 20 MPa, both with and without molds. The effects of temperature, duration, and pressure were explored by performing SPW at 650°C, 700°C, and 750°C for durations of 5, 15, 30, and 60 minutes under pressures of 20 and 40 MPa. Additionally, to assess the impact of interlayers, SPW was carried out at 700°C for 15 minutes under a pressure of 20 MPa with a nickel interlayer. Microstructural examinations of the joints were conducted using scanning electron microscopy (SEM) and optical microscopy. Elemental distribution profiles, point chemical analyses, and elemental mapping were generated using energy-dispersive X-ray spectroscopy (EDS). Mechanical properties of the joints were eva‎luated through hardness and tensile tests. Phase identification at the joint interface was performed using grazing incidence X-ray diffraction (GIXRD) on fracture surfaces after tensile testing. Fracture surfaces were examined via field emission scanning electron microscopy (FESEM). The results revealed unidimensional diffusion of Cu atoms into the steel during the SPW process, occurring predominantly within jointed areas. SPW facilitated the forced mixing of Cu into steel and Fe into copper, with no intermetallic compound identified at the joint interface. Employing a mold during SPW reduced unjointed areas and micropores at the joint interface, resulting in increased joining strength from 42 MPa to 90 MPa. Elevating the applied pressure from 20 MPa to 40 MPa at 650°C enhanced joininig strength to 106 MPa, without significantly affecting the diffusion-affected zone (DAZ). Increasing the temperature from 650°C to 700°C significantly increased joining strength to approximately 179 MPa, accompanied by a transition from brittle-ductile to ductile failure. However, further temperature increase to 750°C resulted in a decrease in joining strength to around 157 MPa due to phase transformation in the steel and reduction in the diffusion coefficient of Cu in the austenite phase. Examination of the fracture surfaces indicated brittle failure in the elastic zone for joints created over a 5-minute duration. As the process time increased to 15 and 30 minutes, brittle and ductile ruptures occurred, respectively, before reaching the maximum stress in the engineering stress-strain curve. In the joint created over a 60-minute duration, rupture did not occur at the joint interface before reaching the maximum stress in the engineering stress-strain curve. Instead, with the development of micro-necking at numerous areas of the joint interface, ductile failure occurred after reaching the maximum stress in the engineering stress-strain curve. Additionally, the strength of the joint created over a 60-minute duration approached that of copper. By applying the SPW process with the interlayer of nickel, significant diffusion areas formed on both sides of the interlayer and increased the joining strength from 130 to 166 MPa. However, the presence of the nickel interlayer led to the formation of a brittle intermetallic compound, FeNi3, at the interface of the interlayer-steel and brittle rupture during tensile testing.

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احمد كرمانپور

فتح اله كريم زاده , مسعود عطاپور , قاسم ديني