AN EXPERIMENTAL AND NUMERICAL INVESTIGATION ON THE JOINING OF ALUMINIUM ALLOY AND STEEL

Abstract

In recent years, the demand for lightweight vehicles has increased in the automobile sector to enhance fuel efficiency and reduce carbon emissions without compromising passengers' safety. Implementing the multi-material design in vehicles using high strength-to-weight ratio materials can solve the problem. The dissimilar metal joining owns the benefits at economic and environmental levels. The lighter vehicles offer better fuel efficiency, ultimately leading to a reduction in CO2 emissions. The aluminium alloys can be used on the body in white (sheets at hood, fenders, doors, boot, and roof) of the vehicle. The significant difference in thermo-mechanical properties, negligible iron solubility in aluminium, severe distortions, porosities, and development of brittle intermetallic compound (IMC) at the interface make the aluminium alloy steel joining challenging. The weld-braze joints are developed with controlled heat input, where filler wire welds with the aluminium alloy and is brazed with the steel sheets. The recent advancements in the GTAW power sources give the operator the flexibility to control the shape of the AC waveform to precisely control the heat input of the welding arc. Similarly, the advanced variants of the cold metal transfer (CMT) process have excellent gap bridging ability and can produce spatter-free joints with lower heat input and controlled deposition rates than other fusion welding processes. The cold wire gas tungsten arc welding (GTAW) process is used to join AA6061-T6 and galvannealed (GA) steel. The welding current distributed in sine, rectangle, square, and triangle AC waveforms are used to understand their influence on the arc behavior, temperature distribution, joint profile, and the intermetallic compound (IMC) layer thickness. The instantaneous welding current and voltage waveforms are recorded in synchronization with the high-speed arc images to understand the arc behavior and heat input. The rate of instantaneous welding current change between the polarities and the arc duration at the peak current is more in square and rectangle waveforms. The peak current in both the positive and negative pulses and the arc heat input of the triangle waveform is the highest among all the waveforms considered in the present work. The wetting length increases along with the reduction in the bead height and wetting angle sequentially with sine, rectangle, square, and triangle waveforms for a constant machine setting welding current. IMC layer thickness is minimum for sine waveform followed by square waveform and maximum for the triangle waveform. The shear tensile strength is maximum for the square waveform followed by sine waveform, and least for triangle waveform. Overall, the ii effect of the sine, rectangle, square, and triangle current waveforms on the arc behavior, bead dimensions, IMC layer thickness and morphology, tensile shear strength, and microhardness distribution are studied. Sine and the square waveforms are suitable for joining aluminium alloy and galvannealed steel because of minimum heat input, IMC layer thickness, and better mechanical properties. All the samples prepared with different AC waveforms failed in the root of the weld bead, and the porosity at the root is evident from the fractured samples and microhardness results. To solve the porosity problem, an approach of varying the electrode tip diameter in the sine and square waveform is followed to minimize the zinc vaporization and zinc entrapment in the weld pool. The change in electrode tip diameter results in change in effective arc diameter ultimately affecting the arc density and exposed area of zinc layer to the arc. The CT-scan images shows that the porosity in minimum and in acceptable limit with electrode tip diameter of 1.2 mm and square waveform. In the next step, using the optimized waveform (square waveform) and electrode tip diameter (1.2 mm), detailed parametric study is conducted to investigate the effect of process parameters on the heat input, bead profile dimensions, IMC layer thickness and morphology and the mechanical properties like microhardness and shear tensile strength of the joints under different joining conditions. The process parameters include welding current, welding speed, and wire feed speed. In the present study, three level full factorial design is followed to carry out the experiments, out of which 21 successful joints are discussed. The current and voltage waveforms are recorded to calculate the heat input. Its subsequent effect on bead profile dimensions, microstructure, and mechanical properties is studied to find the best-suited joining condition. The welding current of 50 to 55 A, wire feed speed of 1.2 m/min, and welding speed of 90 to 120 mm/min can be used to produce the Fe-Al joints with the required joint strength. The heat input range of 175 to 225 J/mm is optimum for Fe-Al joining in the GTAW process. The detailed parametric study shows that the IMC layer thickness seems to be more sensitive to the welding current than the overall heat input. This may be due to the narrow working range and minor variations in slow speed in the GTAW process. The weld bead of Fe-Al joints in the GTAW process has non-uniform wetting of the steel surface due to the free flow of molten filler wire under a constricted arc having high heat energy density. The weld bead width stabilizes after some distance from the start, and the mechanical properties of the joints suffer due to uneven wetting of steel surface and stress concentrators developed at the joint bead. In this study, the reduction of heat energy density iii and control of molten filler wire flow is made by oscillating the welding torch and filler wire to produce the weaving pattern. The Fe-Al joints are prepared under two conditions, i.e., with and without weaving, to compare the mechanical properties. Fe-Al joints were prepared with different heat inputs by varying the welding current and welding speed. The thermal profile and the intermetallic compound layer results indicate the decrease in heat energy density with arc oscillations. Further, the mechanical properties of the joints were compared with no weaving condition and found the enhancement in aesthetics, microhardness, and failure strength of the Fe-Al joints with arc oscillations. In the conventional gas metal arc welding (GMAW) process, increasing weld deposition increases the heat input, adversely affecting the steel-aluminium joint performance due to the brittle IMC layer growth. In the present work, AA6061-T6 and galvannealed steel sheets are joined, keeping weld deposition constant. All the joints are made using constant joining speed and wire feed rate in the CMT process variants like CMT-Standard, CMT-Advanced, CMT-Pulsed, and CMT-Pulsed-Advanced for joining of at a wide range of heat inputs. The influence of current waveforms of all CMT variants on thermal profile, joint bead profile, IMC layer, and mechanical properties was studied. The welding arc and molten droplet transfer phenomena were explained using recorded welding current, voltage waveforms, and synchronized welding arc images captured from a high-speed camera. The localized heating due to positive current pulses in CMT-Pulsed and CMT-Pulsed-Advanced processes led to the higher peak temperature at lower heat input than the CMT-Standard process. Also, positive current pulses affect the IMC layer thickness and induce cracks at the root of the Fe-Al joints, leading to low shear tensile strength. The crack-free joints were observed with the CMT-Standard and CMT-Advanced processes. 110 J/mm to 150 J/mm of heat input in the CMT-Advanced process was suggested to obtain good joints with better mechanical properties. The present study outlines the theoretical basis for the three-dimensional heat transfer analysis of the GTAW process using the finite element method, which is carried out in the present work. The governing equations and the boundary conditions are presented at the outset. A novel methodology is proposed to estimate bead profile dimensions in the lap joining of aluminium alloy and steel using the minimum energy principle. The methodology to analytically estimate the intermetallic layer thickness as a function of thermal cycles at the joint interface is also explained. The developed analytical model for weld bead profile uses welding current, voltage, and welding speed as input parameters to estimate the wetting length and bead height. The numerical model for heat transfer analysis is validated using the measured thermal iv cycles at the location TC2 in the experiments and the estimated thermal cycles at the same location from the model. Further, the numerically computed thermal cycles at the Fe-Al joint interface is used to estimate the growth and the final IMC layer thickness. The estimated results of IMC layer thickness at the joint interface are validated with a set of experimental results. An increase in heat input increases the interface peak temperature and the layer thickness.