By means of ultrasonic soldering with a low temperature solder, 6061 aluminium alloy was joined. The following results were obtained. The effect of ultrasound on soldering greatly promotes processes of chemical interactions between solder components and the 6061 aluminium alloy due to thermal effects. The crystallisation behaviour of the fusion zone is divided into four steps. The speed of growth for serration shaped reaction layer is different. The shear strength of the joint is improved significantly, with the maximum shear strength approaching 54·36 MPa.