Characterisation of quasi‐stationary temperature fields in laser welding by infrared thermography

Abstract

In this work, high-speed thermography is shown to effectively capture quasi-stationary temperature fields during the laser welding of steel plates. This capability is demonstrated for two cases, with one involving the addition of a ferritic-bainitic filler wire, and the other involving the addition of a low-transformation-temperature (LTT) filler wire. The same welding parameters are used in each case, but the temperature fields differ, with the spacing between isotherms being greater in the case where the low-transformation-temperature filler material is added. This observation is consistent with the differences in the extent of the heat-affected zone in each sample, and the shape of the weld pool ripples on the weld bead surfaces. The characterization of temperature fields in this way can greatly assist in the development of novel methods for reducing residual stresses, such as the application of low-transformation-temperature filler materials through partial-metallurgical injection (PMI). This technique reduces or eliminates tensile residual stresses by controlling the temperature fields so that phase transformations take place at the optimum times, and success can only be guaranteed through precise knowledge of the temperature fields in the vicinity of the welding heat source in real time.Keywords: Cooling rate / heat-affected zone (HAZ) / phase transformation / residual stress / temperature measurement / welding thermal cycle In dieser Arbeit wird Hochgeschwindigkeits-Thermographie zur effektiven Dokumentation quasi-stationä rer Temperaturfelder wä hrend des Laserstrahlschweißens von Stahl vorgestellt. Gezeigt wird dies am Beispiel von zwei reprä sentativen Schweißungen, wobei einerseits ein ferritsch-bainitischer Zusatzdraht und andererseits ein Low-Transformation-Temperature-(LTT) Zusatzdraht verwendet wird. In beiden Fä llen werden die gleichen Schweißparameter verwendet. Im Vergleich wird eine merkliche Abweichung der Temperaturfelder deutlich. Bei Low-Transformation-Temperature-Zusatzdraht vergrö ßern sich die Abstä nde zwischen den Iso-Corresponding author: S. Gach, Welding and Joining Institute (

Surface- and volume-based investigation on influences of different Varestraint testing parameters and chemical compositions on solidification cracking in LTT filler metals

“…In particular, the proof that compressive residual stresses are formed [3,4], the investigation of the mechanisms of stress formation [5,6], and the effect on the fatigue strength [7,8] have been the subject of many research projects. Recent publications also deal with extended topics such as microstructure and the associated mechanical properties [9][10][11][12][13], the behaviour during multilayer welding [14][15][16][17][18], and the application of LTT in beam welding [19].…”

Influence of process variables on the characteristics of friction‐stir‐welded polyamide 6,6 joints

“…While during cooling when the temperature reaches below T g , the amorphous chains lose their mobility and make the polymer rigid, hard and stable. T g of polymers can be affected by the following factors: crystallinity, heating and cooling rate and movement of molecular chains [13]. T g is determined by estimating the midpoint in proximity of the change in specific heat from differential scanning calorimetry plot.…”

“…The plate temperature was optimized as 240°C for 10 % talc filled polypropylene and 260°C for 20 %, 30 % and 40 % talc filled polypropylene with the help of AN-OVA [12]. The temperature field during laser welding of steel plates was recorded with an infrared camera which recorded the maximum temperature to be in the range of 400°C [13]. The investigation sought in this present work helps to evaluate the efficiency of the process through analysis of temperature developed during welding; hardness of the welded joint, scanning electron micrograph morphological studies, differential scanning calorimetric analysis (DSC) and Fourier transform infrared spectroscopic (FTIR) analysis so that an understanding on the friction stir welding of thermoplastic polymers might be obtained.…”

Reduction of distortion by using the low transformation temperature effect for high alloy steels in electron beam welding

“…The higher the banitic or martensitic content in the microstructure, the more the residual stress dependents on the transformation temperature [8]. In order to use the LTT effect in beam welding, investigations were carried out by Francis et al [26] and Gach et al [27]. Here, a comparison of conventional filler material and LTT filler material in carbon manganese steel was conducted.…”