Magnesium casting alloys have many desirable attributes that make them attractive materials for automotive applications. Some of these attributes include; high specific strength and stiffness for weight reduction, improved sound dampening capability compared to steel and aluminum, and electromagnetic interference shielding ability. Laser welding, which is a highly desirable methods of joining automotive body materials, due to its ability to provide high productivity and structural integrity has been considered for magnesium. However, there is a major challenge for laser welding of Mg alloys: the propensity of porosity formation within the weld. In this paper, the porosity formation mechanism and the factors that influence the porosity formation in a lap joint will be discussed first. Second, a new technology based on dual beam welding will be introduced and elaborated upon, along with recent preliminary results which show that this newly developed technology can significantly reduce porosity and achieve improved quality welds at higher welding speed.

Laser lap welding quality is a nonlinear response based on a host of material and process variables. This is inclusive of both categorical and numeric variables. An adaptive neuro fuzzy inference system in combination with a classification and regression tree is used to characterize this process. This characterization is shown to be more accurate than that achieved by a nonlinear regression model.

A novel concept for dual beam laser lap joining of Zinc coated steel sheets without intended gap at the interface is presented. This concept utilizes two laser beams with an angle between them. This concept is first proved by combining two laser focusing heads (twin-head system) through use of a custom designed fixture. Welding trials were conducted with a robot by mounting the whole beam delivery set-up at the end of the robot arm. The effects of various welding parameters were studied through both experimental trials as well as modelling of the keyhole geometry. Based on successful concept demonstration, a prototype head was designed and built which enabled the realization of this beam configuration through a single laser source. Excellent quality welds were obtained with both the twin-head system and the new dual-beam head on a variety of zinc-coated sheet combinations.

A significant breakthrough was recently achieved at DaimlerChrysler for laser lap joining of zinc coated steel sheets without a gap at the sheet interface [1]. Using this new process, two types of high strength steels and one low strength steel were welded and evaluated. The evaluation showed that all three steels were successfully welded with the new method. With the same thickness stack-up (1.2 mm /1.2 mm), it was found that the hot dipped galvanized (HDGI) dual phase “ DUAL-TEN™ 800” steel exhibited better weldability based on the size of the processing window when compared to hot dipped galvannealed (HDGA) dual phase “ DUAL-TEN™ 600” and HDGA interstitial free (IF) steels. Laser beam to steel interaction was studied for the three steels, with and without coating, by irradiating the laser beam on each sheet surface at different traveling speeds. Laser keyhole profiles were predicted for different welding conditions using an internally developed finite element modeling method. The preliminary results suggest that the major factor contributing to the difference in weldability is the coating type (HDGI vs. HDGA), with the HDGA coating leading to a narrower processing window than the one for the HDGI.