A Novel Joint of 18-8 Stainless Steel and Aluminum by Partial Welding Process to Ni-Plated Carbon Fiber Junction

Abstract

A novel joint of 18-8 stainless steel and aluminum (Al) connected by a 20 × 10 × 0.3 mm nickel (Ni)-coated carbon fiber (CF) cross-weave cloth junction utilizing extremely large friction force by broad interface of 6 µm diameter CF has been successfully developed for potential aerospace application. This was done by a partial welding process of the CF junction to 18-8 stainless steel and Al separately taking advantage of the different welding temperatures of 660°C for Al, and about 1400 to 1450°C for 18-8 stainless steel. First, the 18-8 side is fabricated by inserting half the CF junction length (10 mm) into a cut slit in 18-8 rod half length, followed by the welding and rapid solidification. Next, the remaining exposed CF junction half length is inserted into Al rod, also followed by welding and rapid solidification, resulting in the finished [18-8/CF/Al] joint. The partial welding was performed by novel method of spot electron beam allowing capillary action of molten metals into the CF junction for high CF surface contact area. When the carbon fibers were electroplated with Ni prior to the partial welding, tensile stress, σ_b of Ni-plated carbon fiber [18-8/Ni–CF/Al] joint was 36 MPa, about 2.5 times larger than that without Ni plating [18-8/CF/Al] joint at 14 MPa. Fracture energy estimated by integrated area under the stress-strain curve was substantially improved in [18-8/Ni–CF/Al] over [18-8/CF/Al] joints. This shows the partial welding performed by spot electron beam allows joining metals with different melting temperatures with the carbon fiber junction. XRD (X-ray diffraction) tests revealed improvements in [18-8/Ni–CF/Al] joint are by Ni plating the carbon fibers minimizing typical brittle Al₄C₃ carbide formation by acting as a barrier, and rapid solution hardening by nickel addition. EPMA (Electron probe microanalysis) showed the Ni coating also acts to protect carbon fibers against impingement from the hot molten metals during welding, along with prevention of mutual diffusion between Al or Fe with carbon fibers increasing strength of the [18-8/Ni–CF/Al] joint.

Tensile stress-strain curves of joints of (18-8/Ni–CF/Al) with Ni-coating to carbon fibers, and (18-8/CF/Al) without Ni-coating, respectively, along with SEM image of Ni-coated carbon fibers. Results showed significant increase in tensile properties by Ni-coating.