Peter Nerman, Johan Andersson

Abstract

Laser weldability has been investigated for ferritic carbon steel, austenitic stainless steel and aluminium. The ferritic carbon steels were uncoated Docol 600, t=1.25-1.75mm, 10µm Zinc coated Dogal 600, t=1.25mm and uncoated Docol 800, t=1.0mm. The austenitic stainless steels were medium rolled 304, t=1.5mm and soft annealed 304, t=1.0mm and hard rolled 301, t=1.0mm. The aluminium material consisted of a standard profile (width x height) 100x40 mm with 2.5 mm wall thickness in grade AA 6063-T6.

The laser type and power used for aluminium and stainless steels was a 3 kW Nd:YAG with a maximum power of 4 kW while a 6 kW CO2 with a maximum power of 8 kW was used for ferritic carbon steel. The laser welding was performed at weld speeds in the range of 2.5-12.5 m/min and focal lengths of 200 and 270mm for the ferritic carbon steels. The austenitic stainless steel welding was performed at weld speeds of 3.4-9 m/min and focal lengths of 100mm and 200mm. The aluminium welding was performed at weld speeds of 3.5-6 m/min and focal lengths of 100mm and 150 mm.

X-ray analysis, macro- and microstructure investigation, Erichsen testing (formability) or tensile testing and micro-hardness investigation was used to investigate the laser weldability of the materials mentioned above. It was found that the ferritic carbon steels are readily laser welded to Erichsen numbers of 70-80% of that in the parent metal and for medium rolled and soft annealed 304, Erichsen numbers of as high as 100% of that in the parent metal was achieved. The 301 hard rolled material, however, displayed moderate Erichsen numbers of only ~55%.

The results also indicate measures that could be taken to ensure high weld qualities, good formability, i.e. Erichsen numbers. Also, defined intervals of weld speed and focal lengths are outlined that ensures Erichsen numbers of 75-85%. A theoretical approach that possibly could predict what values of weld speed and focal length to choose for reaching Erichsen numbers of as high as 90% are presented. This approach is based on the introduced parameter “combined differential weld hardness x joint width", which is an approximation of the area enclosed by the hardness profile over the weld.

In was also found that the welded joints in aluminium showed low mechanical strength transverse the weld direction (Rm?51% of the base material). This is however normal for a welded joint in this material without a post welds heat treatment.