DEVELOPMENT OF THE TECHNOLOGY AND EQUIPMENT FOR LASER AND LASER-ARC WELDING OF ALUMINIUM ALLOYS*

High energy concentration of laser radiation predetermines considerable intensification of the processes of treatment of materials. The concentrated energy input allows materials to be treated at higher speeds and lower residual thermal distortions. Recent achievements in the field of physics and engineering have made it possible to develop new laser radiation sources with much higher energy efficiency. In welding of alloys, especially light ones based on aluminium, computation of parameters of a welded joint and prediction of its chemical composition and mechanical properties present a difficult problem because of the necessity to allow for removal of volatile additions, such as magnesium, lithium or zinc, which are crucial for the entire set of mechanical properties. Part of these additions is evaporated during welding. As a result, chemical composition and mechanical properties of the weld metal may differ from those of the base metal. The Al—Mg system is one of the most promising in terms of development of alloys to be welded. Mechanical properties of the welded joints, as well as properties of the base metal, depend mostly on the magnesium content of an alloy, as strength of the alloy grows with increase of the magnesium content. Lithium also exerts a significant effect on properties of aluminium alloys, as variations in the lithium concentration influence both mechanical properties of the alloys and their sensitivity to cracking. Computer model of laser-arc welding (LAW) based on technologically applicable descriptions of the processes taking place during LAW was used to increase efficiency of development of the LAW technology and equipment [1]. One of the results of many years’ efforts is software LaserCad developed by the Institute of Laser and Welding Technologies of the St.-Petersburg State Polytechnic University for modelling of laser, arc and hybrid welding. The software allows computation of geometric characteristics of the weld, thermal cycles in the weld metal and HAZ, and content of alloying elements in the weld metal. Mathematical modelling of hybrid welding based on the physically adequate model, allowing investigation of relationship between various physical phenomena responsible for development of instabilities in the weld pool, is the most expedient method to analyse causes of the humping effect occurring in the welding process and choose the ways of its elimination. The processes of hybrid LAW with deep penetration, as well as the related processes of laser welding, are often accompanied by formation of porosity and root peaks in the welds [2]. According to current notions about a physical nature of the laser welding processes, this is caused by development of self-oscillations of the keyhole and weld pool in welding with deep penetration [3]. Numerous experimental results confirm that the welding process with deep penetration is not stationary even in the case of stabilisation of all external factors affecting the weld pool [4]. In particular, highspeed filming of laser welding of composite specimens made from metals and optically transparent materials [5] showed a continual variation in shape of the keyhole, quasiperiodic motion of the zone with a maximal intensity in depth of the keyhole, as well as presence of such zones on the rear wall of the keyhole. Filming of the plasma plume also showed the presence of its periodic fluctuations [6]. Comparative investigations of motion of molten metal on the weld pool surface and process of formation of the root peaks confirm correlation between