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The resistance spot welding with intermediate frequency and direct current (RSWIFDC) was conducted to solve the weldability of the Al alloy TL091, The high-quality joint bearing dynamic load is realized for the first time by using the technique with the power of 2 kHz and the double-transformer based on the rapid response speed and stable welding current. By optimizing the welding process parameters, the strength, hardness distribution, microstructures and fracture morphologies of the joints were analyzed in details. The experimental results showed that the joints welded by the technique had better comprehensive mechanical properties, higher tensile strength and plasticity than that of the joints obtained by other spot welding. Shear fracture of the joints was seen at the nugget instead of the HAZ, thus indicating that the strength at the HAZ is higher than that at the nugget. The plastic ring of the joint was narrower as we desired. It was concluded that, by developing this advanced welding method and optimized welding parameters, high-quality joints were acquired to meet with higher design requirements of body-in-white of Al alloy under dynamic load. This is of great significance to the wide application of Al alloy body.

The effects of annealing conditions on the microstructure and deformation mechanism of 0.17C-9Mn-3.5Al steel have been investigated. Through austenite reverted transformation (ART) annealing, a multiphase microstructure composed of d-ferrite, a-intercritical ferrite, g-austenite, and a'-martensite is obtained in as-forged specimens. For the specimen annealed at 750 8C for 60 min, a high volume fraction (42.19%) of retained austenite is attained, yielding a large total elongation (TE) of %38% and tensile strength (Rm) of %810 MPa and a two-stages work hardening behavior. The work hardening behavior analysis using the differential C-J model clarifies that the multistage can be mainly attributed to the discontinuous transformation induced plasticity (TRIP) effect, which is related to the volume fraction of retained austenite and its different stability levels as well as cooperative deformation of the multiphase microstructure. Thus, the active TRIP effect predominately affects the high strain stage, resulting in a higher work hardening ability and a increased plasticity.[ Ã ] X.