Experimental Investigation on Drilling and Welding of Engineering Materials using Laser

Abstract

Conventional drilling of advanced engineering materials is extremely difficult because of occurrence of rapid tool wear and frequent tool breakage. Likewise, conventional welding processes poses difficulty in fabrication works made of engineering materials such as stainless steel, titanium alloy and shape memory alloy due to excessive heat generation causing high risk of contamination and distortion of weldment. Therefore, Light Amplification by Stimulated Emission of Radiation (laser) is gaining popularity for micro-drilling as well as welding because a high intensity heat source is applied at the precise location to achieve the desired output. The present study focusses on application of laser during micro-drilling and welding of similar/dissimilar materials. In order to gain insight into effect of process parameters on output measures during microdrilling and welding on thin foils of 0.5 mm thickness, extensive experimental investigation is performed using millisecond pulsed Nd:YAG laser. During micro-drilling, it is observed that spatter area and heat affected zone (HAZ) increase with increase in laser current and pulse width because increase in heat input causing more material to melt but sufficient time is not available for completely flushing away the molten material. As a result, heat is not properly dissipated resulting in increase in spatter formation and heat affected zone. It is also observed that increase in pulse frequency and gas pressure leads to decrease in spatter area because of formation of laser supported absorption (LSA) wave which blocks the input energy to penetrate adequately. During laser welding of similar materials, it is found that micro-hardness value in the fusion zone of the weldment is much higher than the base material during welding of stainless steel in comparison to welding of titanium alloy. This phenomenon may be attributed to higher cooling rate observed in case of stainless steel as compared to titanium alloy. Drilling of titanium alloy and stainless steel have potential application in manufacturing of medical implant, compressor blading in gas turbine, turbochargers and steam turbine valve seat. Similarly, welding of thin sheets has potential applications in automobile, razor blades, jet planes, electrical circuits and micro-electromechanical systems.
Artificial intelligence (AI) techniques such as adaptive neuro-fuzzy inference system (ANFIS) and multi-gene genetic programming (MGGP) are used to predict the performance measures such as circularity (at entry and exit), heat affected zone, spatter area and taper for laser drilling process. Similarly, artificial intelligence techniques are used for the prediction of performance measures like bead width, heat affected zone, surface roughness and welding strength during laser welding.
Comparative study of AI models suggests that MGGP predicts the performance measures in an effective manner as root mean square error (RMSE) for testing data is less as compared to ANFIS in both laser drilling and welding operations and can be potentially used for accurate prediction of desired output.
Welding of dissimilar materials such as nitinol, a shape memory alloy, with stainless steel and titanium alloy is one of the challenging tasks because of formation of brittle intermetallic compounds. Therefore, the study is further extended to find the feasibility of welding of nitinol with stainless steel and titanium alloy separately providing copper foil as interlayer during fibre laser welding. Analysis of physical and mechanical properties of the weldments reveals crack-free surface in the weld pool and less percentage of porosity. The study indicates that minimum tensile strength of the welded joint is more than the ultimate tensile strength of the weakest intermediate material i.e. copper. The study examines the possibility of using high melting point intermediate layer material during welding of nitinol with other materials to avoid formation of unwanted phases.
Feasibility of welding of titanium alloy with stainless steel sheets having five millimetre thickness using CO2 laser providing copper as interlayer using electroplating process is explored in this work. Presence of copper provides compatibility between titanium alloy and stainless steel during laser welding process and reduces brittle intermetallic compounds. The study suggests that the presence of interlayer during joining of dissimilar materials provides stable, crack-free and less brittle joints as compared to joining of dissimilar materials without interlayer. The study also examines the possibility of providing interlayer using coating technology during dissimilar metal joining. Welding of dissimilar engineering materials helps to provide hybrid system performance finding potential application in the field of various industrial applications like seismic damping device, petrochemical devices, aerospace equipment and medical equipment