In the conventional brazing method, a filler material is required. The filler material melts at the brazing temperature and bonds the members. This study demonstrates a new brazing method where the aluminum members are bonded without a filler material by using the liquid phase provided from the Al–Si alloy single-layer material. In the new method, the Al–Si alloy single layer material partially melts and provides the liquid phase during brazing. Generally, the amount of liquid phase is influenced by the Si content. Therefore, we investigated the influence of Si content on the bonding ability and deformation resistance in a single-layer material. The bonding ability was evaluated by using a test piece made of fin and base materials, and the deformation resistance was evaluated by a sagging test. The bonding ability increased with Si content, but the deformation resistance decreased. We clarified that the trade-off between these properties can be avoided by using Al–2.5 mass%Si alloy at 600°C brazing. We also investigated the influence of Si content on the metal structure and changes in the metal structure during brazing. Accordingly, we clarified that a spherical liquid phase formed from the Si phase and a laminar liquid phase formed through the grain boundaries.

The clustering behavior during natural aging (NA) and subsequent artificial aging (AA) at 170°C in Al–Mg–Si alloys with the different Mg/Si ratio is evaluated using three-dimensional atom probe (3DAP). The internal composition of clusters formed during short-term NA is strongly associated with the alloy composition and prolonged NA leads to the formation of clusters with a Mg : Si ratio close to 1. The number density of clusters decreases with no dependence on the cluster size, and Si-rich clusters with Mg/(Mg+Si) values inside clusters below 0.4 show little variation during AA for 1.2 ks. It is believed that Mg–Si clusters with ranging from 0.4 to 0.6 in the Mg/(Mg+Si) value inside clusters can both dissolve and grow during AA. It is revealed that the delay in age hardening in the early stage of AA is closely related to the number density of the Si-rich and Mg–Si clusters.

Lap joint sheets of 6061-T6/SPCC and 6061-T6/DP590 (Dual phase) steel were fabricated by magnetic pulse welding (MPW). Strong lap joints were achieved at discharge energy W of >2.0 kJ and gap length d of 1.0 mm for the 6061-T6/SPCC, and W of 3.0 kJ and d of 1.4 mm for the 6061-T6/DP590 steel, respectively. This result suggested that the high-collision speed is required for lap joint of the 6061-T6/DP590 steel compared with that of the 6061-T6/SPCC. Weld interface showed wavy joint interface and weld width of the lap joint sheets tend to increase with increasing of discharge energy for MPW. An intermediate layer consisted of FeAl, Fe₂Al₅ and FeAl₃ was recognized at the weld interface discontinuously, due to localized melting and a subsequent high rate cooling of molten Fe and Al confined to the weld interface. Furthermore, work hardening by accumulated plastic strain and grain refinement of Al and Fe at the welded interface were recognized by SEM-EBSD. From microstructure observation, strong lap joint of the 6061-T6/DP590 steel by MPW was thought to be due to an increase in weld width, an anchor effect, and strengthening of the weld interface by work hardening and grain refinement.