Semi-Solid 6061 Aluminum Alloy Slurry Prepared by Serpentine Channel Pouring Process and its Rheo-Diecasting

Semi-solid 6061 aluminum alloy slurry was prepared by a graphite serpentine channel and its rheo-diecasting experiment was carried out on the slurry. The influence of pouring temperature on the microstructure evolution and mechanical properties of the rheo-diecasting were investigated. The microstructure and fracture mechanism of traditional die cast tensile specimens and rheo-diecast tensile specimens were compared and investigated. The results indicate that the microstructure of rheo-diecast tensile specimens is composed of spherical primary α-Al grains and fine secondary solidified α2-Al grains. When the pouring temperature increased from 660 °C to 720 °C, the average equivalent grain diameter of primary α-Al grains increased from 42 μm to 58 μm, and the shape factor decreased from 0.82 to 0.73. As the pouring temperature increases, the as-cast tensile strength and elongation of tensile specimens both increase first and then decrease. When the pouring temperature was 690 °C, the best mechanical properties were obtained, with as-cast tensile strength of 142.93 MPa and as-cast elongation of 4.86%. The fracture mechanism of traditional die casting is mainly ductile fracture, and the fracture mechanism of rheo-diecasting is a mixed fracture of intergranular fracture and ductile fracture.

FEM Simulation of the Riveting Process and Structural Analysis of Low-Carbon Steel Tubular Rivets Fracture

Riveted joints are a common way to connect elements and subassemblies in the automotive industry. In the assembly process, tubular rivets are loaded axially with ca. 3 kN forces, and these loads can cause cracks and delamination in the rivet material. Such effects at the quality control stage disqualify the product in further assembly process. The article presents an analysis of the fracture mechanism of E215 low-carbon steel tubular rivets used to join modules of driver and passenger safety systems (airbags) in vehicles. Finite element method (FEM) simulation and material testing were used to verify the stresses and analysis of the rivet fracture. Numerical tests determined the state of stress during rivet forming using the FEM-EA method based on the explicit integration of central differences. Light microscopy (LM), scanning electron microscopy (SEM) and chemical composition analysis (SEM-EDS) were performed to investigate the microstructure of the rivet material and to analyze the cracks. Results showed that the cause of rivet cracking is the accumulation and exceeding of critical tensile stresses in the rivet flange during the tube processing and the final riveting (forming) process. Moreover, it was discovered that rivet fracture is largely caused by structural defects (tertiary cementite Fe,Mn3CIII along the boundaries of prior austenite grains) in the material resulting from the incorrectly selected parameters of the final heat treatment of the prefabricate (tube) from which the rivet was produced. The FEM simulation of the riveting and structural characterization results correlated well, so the rivet forming process and fracture mechanism could be fully investigated.