Process heat flow model for temperature and hardness prediction during friction taper stud welding of AISI 4140

Friction Taper Stud Welding (FTSW) is a relatively new solid state welding process, developed from the concepts of friction welding, which theoretically operates below the melting temperatures of the material being welded. During friction welding, heat is generated by conversion of mechanical energy into thermal energy at the interface of the work pieces, during rotation under pressure. Quality welds are dependant on the correct selection of welding process parameters, which are currently chosen empirically, and the FTSW evaluated by mechanical testing. This method is time consuming, uneconomical and could cause that optimised conditions are overlooked. A proposed solution would be to numerically model the process, but reference to successful computational modelling of the FTSW process is currently not available and data regarding the responses during the process are limited. The ultimate aim of the present study is to develop a finite element model to simulate the FTSW process using AISI 4140 medium carbon low alloy steel, delivering temperature profiles and hardness predictions through the Heat Affected Zone (HAZ) – using a combined experimental and numerical study. To achieve the objectives of this study a systematic approach was adopted and conducted in several phases. A weld matrix was configured with ranging weld input parameters to determine the affect of weld input parameters on real-time responses. To provide a relationship between these factors, welding was conducted using a portable friction taper stud welding platform linked to a control and data logging system for measuring the real time axial forces, spindle speed, material displacement, torque and temperature responses as a function of time. The input process parameters applied being motor speed, axial forces, displacement and forging time. The temperature distribution through the weld, by direct measurement, as a function of weld time and position is investigated. During the experimental welds temperature responses, as influenced by welding parameters, were recorded using embedded N-Type thermocouples at various locations in the near vicinity of the weld interface. The main hot spots during welding were identified to be close to the top surface just before weld completion and at the bottom centre surface of the plug weld at the interface line. All the welds showed similar trends and a maximum temperature of 1078°C at the bottom of the weld was reached for a rotational tool speed of 5160rpm, axial friction force of 15kN and displacement of 6.5mm, due to the heat generated by friction between the tool and weld coupon. The weld torque increase rapidly at the start of the weld and reached a peak value shortly after the start of the weld, while a peak temperature of 1366°C, for a rotational tool speed of 5160rpm, axial friction force of 10kN and displacement of 8mm was reached at the top edge of the plug weld. This position of anticipated peak temperature value is due to the heat transferred during the FTSW process together with the accumulation of expelled material forming on the surface of the weld coupon. Statistical methods were applied to obtain knowledge of the trends and relationship between weld input parameters for various weld responses, including energy input, temperature, friction time, torque and displacement rate. Although it was shown that no single parameter solely controls the temperature gradients in the weld, the dominant influence of the rotational speed at the bottom of the weld and that of the displacement, at the top of the weld, were evident. The peak temperatures during the weld are of interest as these temperatures, together with the subsequent cooling rates, determine the Vickers hardness, of the material, through the weld. Spindle speed was found to have the dominant effect on temperature in the bottom half of the weld with displacement having a contributive effect closer to the top of the weld. Friction force dominate the effect on friction time, displacement rate and total energy input with friction force and spindle speed having an equal effect on torque. The multiple regression analysis resulted in valid models with varied, but acceptable accuracy with the equation for friction time resulting in an R predict value of 93.34%. These models provided a clear insight to the influence of weld input parameters on the weld responses and the model for friction time was used as an input parameter to the FTS welding simulation. The accurate prediction of the interface temperature is fundamental for process optimisation which will allow for producing consistent, reliable plug welds. A fully coupled transient two-dimensional axi-symmetrical analysis of heat flow during the FTSW process of AISI 4140 steel and subsequent Vickers hardness profiles through the HAZ, making use of numeric simulation applied in the commercially available FEA software, COMSOL Multiphysics®, is developed and reported on. Process optimisation hinges on a better understanding of the heat distribution during welding, making a major contribution to the resultant hardness. The thermal-plastic flow coupling of the model is such that temperature values are resolved together with that of the velocity field. The simulation utilises a Computational Fluid Dynamics (CFD) two phase laminar flow and Heat Transfer physics, applied in an Eulerian mesh-based scheme. The viscosity of the fluid is based on a constitutive law of the flow stress using the Zener-Hollomon parameter with a flow model based on the Navier-Stokes’ equations to simulate the plastic deformation. Temperature dependant thermo-physical material properties and coefficient of friction are applied, and the application of viscous heating is controlled by a material state variable. The heat source model, required for material softening, is applied as two components, frictional and shear, with the heat source moving along the z-axis delivering sufficient energy to soften the metal, causing flow. The Navier-Stokes approach is applied with solid-state material transport during the weld based on laminar, viscous flow of a non-Newtonian fluid, dependant on temperature and strain rate. Numerically calculated values for temperature profiles and peak temperatures through to the weld as well as subsequent Vickers hardness profiles at points through the HAZ, obtained from the Finite Element model, were found to be in close agreement with values from trial welds. The largest variance was 19% for the peak temperature of weld E4W2, applying an axial friction force of 7.5kN, 6.5mm displacement and a tool rotational speed of 4080rpm – resulting in a friction time of 330 seconds. Predictions of hardness are found to be between 0% and 19% (mean 3%) of experimentally determined values with the biggest variance at the positions of peak temperatures due to the friction interfaces. The heat applied as a result of plastic deformation was found to be 5.4% of the total heat. The FTSW model predicts the temperatures at the friction interface, during the welding process, to be within the range, and frequently exceeding the solidus temperature of AISI 4140 steel. Results show that the models applied in the FTSW simulation show good agreement when compared to experimental values. The main contribution of this thesis, towards knowledge of the FTSW process, is: The relationships between weld input parameters and responses; Temperature dependant models of thermo-physical properties for AISI 4140 in the high temperature region (ranging from ambient to the solidus temperature); Successful application of the Navier-Stokes approach to simulate the plastic flow during FTSW and A numerical finite element model for the prediction of temperature gradients and hardness profiles through a FTSW