Aspects of Heat Flow in Laser Processing of Metals

Since the laser was invented in 1960, its use in manufacturing industry has been growing rapidly. Laser processing of metals is based on the flow of heat that is generated by the absorbed laser beam. One outstanding aspect of laser beams is high precision along with high controllability of energy transfer, which includes creative techniques of shaping the beam and in turn the process. The thesis presents six Papers A-F on different metal processing techniques, namely welding, hardening and cutting, the latter combined with additive manufacturing. For each respective technique it was studied how desired properties can be optimized by controlled use of the laser beam and in turn of the temperature field. Addressing their different complexity of the heat transfer, various theoretical and experimental analysis methods were applied. Laser beam welding is usually conducted with standard beam shapes, i.e. Gaussian or top-hat like, which is not always optimal for the process. Identification of an optimised weld pool shape or temperature cycle could increase the quality of welded products or even enable new applications. Papers A and B aim to increase the knowledge for non-standard beam shapes, particularly for single-pulse conduction mode welding. Paper A presents an investigation on an industrial application where a C-shaped weld joint is desired. The sensitivity to and optimization of different C-shaped beam irradiation profiles is discussed. The analysis is mainly carried out by applying Finite Element Analysis, FEA, to calculate the heat conduction contributions, showing unexpected sensitivity in certain regimes. Paper B presents a semi-analytical model for fast calculation of the temperature field from different beam profiles. Examples include multi-spots or the misalignment sensitivity of Diffractive Optical Elements. In Paper C, for laser hardening of 11% Cr ferritic stainless steel the temperature field was studied to enable hardening. It was shown that single-track hardening without sensitisation could be achieved but overlapping tracks had a continuous network of ditched grain boundaries and is thereby at risk for sensitisation. The sensitised area is caused by a reheating cycle. The same mechanism for the same material was studied in Paper D when applying a recently developed drop deposition technique, where additive manufacturing is fed by laser cutting. The same reheating isotherm becomes critical, but here sensitisation tests show a discontinuous network of ditched grain boundaries in the added material. The solid heat-affected zone on the other hand has a continuous network of ditched grain boundaries, which implies a sensitisation risk. The continuous network is however not in contact with the surface. The tested parameters is thus not at risk for intergranular corrosion through sensitisation. For friction stir welding of dissimilar metals, Ti-6Al-4V with AISI 304L stainless steel, Paper E, the influence of a laser-induced preheating temperature field on the tool forces was investigated through numerical simulation. By suitable application of laser preheating, the forces acting on the tool can be substantially lowered, in a robust manner. The temperature field from seam welding induces a residual stress field. In Paper F, for continuous wave laser keyhole welding of high strength steel butt joints, a method is presented to identify the residual stress behaviour of laser welded sheets by measurement of the fatigue crack growth rate during testing, by deriving the crack acceleration. The analysis was confirmed by hole drilling tests and by FEA. The knowledge and methods of the above different experimental and theoretical studies complement each other. They contribute to further optimize certain aspects through laser-induced temperature fields, for different manufacturing techniques