Journal of Welding Science

Surface roughness in the welding processes is one of the important parameters in the laser welded metal connections which affects laser beam absorption directly. When the laser beam is irradiated to the surface of the base metal, the surface roughness plays an important role in the amount of beam absorption and the amount of melting achieved and directly affects the penetration depth. The main purpose of this study is to investigate the effect of roughness mentioned above in the equal parameter for this widely used aluminum alloy. Microstructural Surveys were performed on three different roughness levels of the sample and the results obtained from the analysis of samples by optical microscope (OM), atomic force microscope (AFM) and Scanning electron microscopy (SEM) analysis showed that, increasing the surface roughness up to Ra = 0.16 micrometer, caused the greater degree of beam engagement by the surface grooves, hence more concentration of the beam photons and more melting obtained, so the depth of penetration increases by consuming a lower amount of energy.
 

In this study, non-homogenous welding of nimonic 75 superalloy to Monel 400 with 1 mm thickness was investigated with pulsed Nd:YAG laser welding. The mechanical properties of the joint were analyzed with optical and scanning electron microscope, X-ray diffraction, micro-hardness test and tensile test. In the case of non-homogeneous welding of Nimoinc 75 superalloy to Monel 400, defects such as liquation cracks and porosity in the welded samples were observed. these defects were removed with increasing the preheating temperature and decreasing the heat input. The results showed the voltage, pulse width, pulse frequency and welding speed should be selected as 500 volts, 9 milliseconds, 3 Hz and 0.9 mm/s respectively to reach the proper penetration depth. Also, the investigations show that the welding structure is composed of austenitic matrix containing columnar dendrites and some cellular areas. The mechanical properties of the weld metal were reduced after joining and segregation causes a change in the amount of elements and the appearance of intermetallic compounds in the spaces between dendrites and cells. All non-homogeneous samples broke during the tensile test from the weld metal area.

In this study, 1 mm thick austenitic stainless steel 316L sheets were used for experimental testing. The experimental welding process was carried out using a Nd:YAG pulsed laser welding machine, and the welding simulation was performed using the SYSWELD software with a three-dimensional model for thermodynamic and mechanical analysis. The simulation results showed over 90% correlation with the experimental results. Analysis of experimental and numerical data revealed that at a constant voltage of 440 volts, decreasing the welding speed from 2 to 0.5 mm/s increased the overlap rate of pulses from 67% to 93% and the maximum average power density (EPPD) from 5963 to 21831 W/mm². Additionally, increasing the voltage from 440 to 480 volts at a constant speed of 1 mm/s raised the heat input from 114 to 138 J/mm and the weld depth from 0.56 to 0.66 mm. Due to the high cooling rate, the grain size of the weld metal became finer than the base metal (63% reduction in grain size). Two phases, austenite and ferrite, were observed in the weld metal, and the solidification mode was predicted to be FA.With an increase in welding speed from 0.5 mm/s to 2 mm/s at a constant voltage of 440 volts, the maximum tensile residual stress increased from 96 to 260 MPa due to reduced pulse overlap (from 93% to 67%), uneven heat distribution in the part, and the generation of thermal stresses. Furthermore, increasing the welding voltage from 440 to 480 volts at a constant speed of 1 mm/s caused the maximum tensile residual stress to rise from 124 to 152 MPa. The maximum hardness of the weld metal increased from 180 to 215 Vickers as the welding speed rose due to the prevention of carbon diffusion and an increased growth rate. However, with an increase in welding voltage and heat input (from 57 to 69 J/mm), the hardness decreased from 225 to 215 Vickers due to a reduction in thermal gradients and grain growth.