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Crack-free dissimilar TNM-Ti6242 joints were successfully obtained by laser welding with preheating temperatures of 400 °C, 600 °C and 800 °C, in spite of the differing thermophysical properties of the used alloys. The microstructure of the joints was investigated by optical microscopy, SEM, EDX, EBSD and the local mechanical properties were determined by microhardness measurements. The microstructure coarsens with increasing preheating temperature. Homogenous mixture of both alloys in the fusion zone was achieved, even so local microsegregations of Ti, Al and Nb occurred. The fusion zone mainly consists of α2 (Ti3Al). A transition zone, present as a β/βO seam, was revealed for preheating temperatures of 400 °C and 600 °C. For preheating to 800 °C the β/βO seam was penetrated with α2 lamellas. Since the alloying elements of the other alloy were detected in the heat-affected zone, diffusion processes occurred. The average microhardness in the fusion zone of the dissimilar TNM-Ti6242 joints is independent of the preheating temperature and is higher than the base material microhardness. Comparing the microhardness in the fusion zone of similar Ti6242 and TNM joints with dissimilar TNM-Ti6242 joints, the microhardness values of the dissimilar TNM-Ti6242 joints range between the values of the similar Ti6242 and TNM joints. However, the values in the fusion zone are closer to the microhardness values of the similar TNM joint, which is caused by the mainly α2 containing fusion zone of the dissimilar TNM-Ti6242 joints.

Two approaches were used for laser beam welding of similar and dissimilar joints of AA7075 and AA5182 that aim to overcome the weldability problems of high-strength Al-Zn-Mg-Cu alloys. The first approach implies the use of vanadium foil as additional filler material, while the second implies the use of a fiber laser with a large beam diameter and a top-hat beam profile. Although both approaches result in an improved weld quality, in terms of weld appearance, porosity and cracking, the resulting mechanical properties differ considerably. The addition of vanadium leads to a local increase of microhardness in the fusion zone. However, the tensile strength of these joints is lower as for fiber laser welded joints. In direct comparison fiber laser welded joints exhibit also higher formability as the joints welded with vanadium foil. The highest formability is obtained for dissimilar joints with the medium-strength Al-Mg alloy. Due to the unavoidable softening in the weld zone of heat treatable aluminum alloys, the formability of the joints is inferior in comparison to the base materials. In addition, the positive effect of post-weld heat treatment, surface milling and warm forming on the resulting mechanical properties of similar and dissimilar joints is discussed.

This work deals with the effects of laser beam power, focus position and advance speed on the geometry, microstructure and mechanical properties such as the tensile strength and microhardness of autogenously fibre laser beam welded Ti-6Al-2Sn-4Zr-2Mo (denoted as Ti-6242) butt joints used for high temperature applications. The Ti-6242 sheet employed here is characterized by a globular (α+β) microstructure. Laser beam welded butt joints consisted of a martensitic fusion zone, inhomogeneous heat affected zones and equiaxed base materials. The microhardness increased from 330 HV 0.3 in base material to 430 HV 0.3 in fusion zone due to the martensitic transformation. Butt joints showed the base material level of strength in tensile test. The local increase in microhardness provided a shielding effect that protected the Ti-6242 butt joint against mechanical damage during the static tensile load test. The predicted critical total underfill depth that does not reduce the tensile strength of the weld was determined to be 25% of the specimen thickness.

Laser beam welding of butt joints made of the newly developed high-strength Al-Zn alloy PA734 is conducted. A new approach of the Helmholtz-Zentrum Geesthacht is used to solve the problems of weldability and softening. The results of the fatigue, fatigue crack propagation and fracture toughness tests are discussed relating to the microstructural characteristics and the mechanical properties of the laser welded joints and compared to the base material. The obtained data can be used for the assessment of the damage tolerance behaviour of the laser welded integral aircraft structures made of Al-Zn alloys.