Deepak Kumar Yaduwanshi at the Indian Institute of Technology Guwahati for the award of the degree of Doctor of Philosophy was conducted under my supervision in the Department of Mechanical Engineering, Indian Institute of Technology Guwahati. In my opinion, the thesis has reached a standard that meets the conditions for obtaining a doctorate in philosophy in accordance with the Institute's regulations.
Abstract
Quantitative calculation through a sophisticated mathematical model of hybrid friction stir welding for different materials is a challenging task due to complex issues such as mixed property in the welding zone, flow mixing action and solid state phase transformation. Keywords: Friction stir welding, hybrid friction stir welding, plasma assisted friction stir welding, preheating, finite element method, heat transfer analysis, dissimilar material joining.
CONTENTS
No. Title Page No
Thermal history at the experimental condition of 815 rpm, welding speed 98 mm/min and tool displacement 2 mm - (a) without preheating and (b) with a preheating current of 55 A. Material flow pattern in the welding zone in the condition welding current of 55 A preheating current, 2 mm tool displacement, 815 rpm and welding speed 98 mm/min.
List of Tables
Nomenclature
𝑈Ɵ Component of the resultant velocity of the material in the h direction m/s 𝑈𝑟 Component of the resultant velocity of the material in the direction r m/s 𝑈𝑧 Component of the resultant velocity of the material in the z direction m/s.
Introduction
- General background
- FSW process
- Limitation of FSW process
- Hybrid FSW process
- Research Objectives
- Layout of thesis
Studying the feasibility of the P-FSW process during joining of similar and dissimilar materials is the sole objective of the present investigation. A detailed description of the FSW process heat generation model is included in this chapter.
Literature Survey
General background
FSW and HFSW cover a wide range of topics, it is not possible to review all topics in this chapter. The purpose of this chapter is to review to some extent experimental investigations of FSW and HFSW and issues related to numerical modeling approaches for heat transfer and material flow during the FSW process.
Heat transfer in friction stir welding
- Experimental investigation
- Mathematical model
- Representation of heat source
- Flow stress analysis
An attempt was also made to validate the analytical model of heat generation in the FSW process. Heat generation was determined to be the product of effective stress and effective strain rate.
Material flow behavior
To understand the friction stir welding process, it is very important to know the nature of the material flow in and around the tool. The flow fields created by the flow line plot give an idea of the material flow. Some researchers, such as Arbegast [125], have tried to develop the model of the deformation zone separated from the flow of material during FSW as shown in Fig.
They estimated the material flow around the tool and suggested that the material flow can be accelerated with the increase of translational speed and angular speed of the pin. A vortex exists on the advancing side, and the material flow in the vortex on the advancing side becomes faster as the translational speed increases.
Hybrid friction stir welding
- Electrically assisted FSW (EAFSW)
- Induction assisted FSW (IAFSW)
- Laser assisted FSW (LAFSW)
- Arc assisted FSW (AAFSW)
- Ultrasonic vibration assisted FSW (UAFSW)
- Other thermally assisted FSW
They applied an induction heating system just before the FSW tool to plasticize the workpiece. The heat source is used to heat the area directly in front of the rotating probe. The results indicated that the tensile strength was approximately 93% of the aluminum alloy base metal strength.
The ultrasonic energy reduces the yield point of the material and has a similar effect to thermal softening. Lotfi and Nourouzi [20] used the hot gas stream, such as nitrogen stream, to preheat the workpieces slightly before the FSW tool, as shown in Fig.
Design and performance of FSW tool
184] analyzed the influence of the welding speed, which decisively affects the degree of radial wear of the pin. They identified the mechanisms of tool degradation by studying the tool microstructures before and after welding. Metallographic and metrological techniques have shown that changes in tool dimensions are the result of rubbing and tool deformation.
They had not identified the FSW parameters that affected tool wear and the nature of the wear products. Tool wear not only reduces tool life, but is also likely to affect material flow and weld properties.
Dissimilar material joint
- Mechanical properties and process parameters
- Formation Mechanism of IMCs
The average UTS of the welds decreased as the welding speed increased due to low. The study showed that the thickness of the intermetallic compound layer is a function of temperature and dwell time. Different processing parameters affected the size and amount of IMCs formed in the granulation zone.
The thickness of the intermetallic layer and its composition at the weld interface is mainly caused by tool offset as shown in Fig. However, most welds failed at the aluminum/copper interface for very low bend angles.
Microstructural study
Adamowski and Szkodo [229] observed material softening in the weld grain and heat affected zone, i.e. They found that the grain size gradually increases locally from the base material area towards the center of the mixing area. They suggested that the formation of onion rings is the result of the reflection of the material flow from the cooler walls of the heat-affected area.
They observed that the base metal microstructure was replaced by equiaxed and fine grains in the mixing zone. The lamella structure at the base of the lump is more homogeneous and finer than in other areas.
Summary
Murr et al [258] also investigated the microstructures of Cu-6061Al FSW joints, and found that a complex intercalation microstructure consisting of vortex-like and vortex-like features was formed in the joint. Some established numerical modeling techniques can explain and predict heat generation and material flow involved in the FSW. The numerical modeling of the FSW process can help to achieve optimal process parameters with less effort and with economic benefits.
Although commercial implementation of FSW on materials beyond aluminum alloys has been limited to date, and such implementation in the near future continues to require higher levels of FSW technology, model-based research, and better design and materials for FSW tool manufacturing would require. Furthermore, current studies on these variants of FSW are largely limited to aluminum alloys.
Scope of the present work
Therefore, the feasibility of P-FSW for joining different and high-strength materials can be realized. In dissimilar friction and stir welding, two dissimilar materials are plasticized and mixed together, which can form intermetallic compounds during welding. Therefore, designing the material properties of the weld zone, including the effect of intermetallic compounds, is important in thermal analysis.
The distribution and resulting properties of intermetallic compounds in the weld zone are difficult to predict due to the dependence on many parameters, such as the flow pattern. Researchers have studied the formation of intermetallic compounds and the flow pattern in weld nuggets, which resemble functionally graded material in the weld zone.
Theoretical Background
Introduction
The primary heat source is the frictional heat from the tool arm and the secondary heat source is the deformation heat from the tool pin [259]. While the shoulder is the primary source of heat generated during the process, it acts as the primary restriction to material ejection and the primary driver for material flow around the tool. The pin is the primary source for material deformation and the secondary source for heat generation in the nugget zone.
In hybrid friction stir welding, the auxiliary heat source is applied to provide additional heat to the workpiece immediately before the welding zone, so that the tool is imposed with a smaller amount of mechanical energy. However, temperature control of the weld zone in the hybrid FSW system is critical and several hybrid technologies have been developed to preheat the workpiece which can improve the plasticization condition, reduce tool forces and indirectly affect tool life.
Heat transfer model
- Governing equations and boundary conditions
In P-FSW, heat generation in the solution domain is due to the plasma arc and friction and plastic deformation by the FSW tool. The plasma arc preheats the workpiece before the FSW tool to affect material plasticization and improve material flow during welding. When the FSW tool moves in the positive y direction, the applicable heat transfer equation is expressed as.
The boundary condition for the thermal FSW model is specified as the surface interaction of the solution domain. On the free surface of the workpiece, natural convective boundary conditions with radiation are taken into account.
- Heat generation model by different tool geometries
- Hybrid heat source model
- Finite element discretization
- Analytical model of strain and strain rate in weld zone
- Material property for dissimilar materials
- Computational aspects
- Summary
- Introduction
- Friction stir welding
- Experimental set-up
- Non-symmetry in thermal history during FSW
- Effect of tool pin profile during FSW
- Hybrid FSW
- Experimental set-up
- Effect of preheating on thermal and mechanical properties of similar joint (Al-Al)
- Dissimilar welding
- Experimental data taken from literature
- Summary
However, the temperature on the advancing side close to the center line of the weld is higher than on the receding side. It is observed from the experiments that the temperature in the advancing side close to the weld centerline is higher than in the receding side. However, the temperature on the advancing side close to the weld centerline is higher than on the receding side.
Hardness is measured on the cross-section both in the transverse direction of the weld and in the thickness direction. The tensile strength of the P-FSW joint is significantly increased due to the application of preheating. The ratio of the oxygen content at the fracture surface is high due to the effect of preheating.
Then the plasma assisted friction stir welding is used to join aluminum alloy AA1100 to pure copper. The copper side is preheated to 500 K with optimum location of plasma arc and less mechanical energy is delivered by the tool which is converted into heat.
Results and Discussion
Introduction
Calculated thermal cycles in and around the vehicle are used to estimate the evolution of the cooling rate. Further validation of the cooling rate and its influence on the microstructure of the weld joint is reported in this paper.
Observation from experimental investigation
The temperature is ~20-50 K higher in case of plasma arc preheating for joining similar materials. The location and intensity of the preheat source in a relatively harder material affects the variation of the thermal history in the case of the P-FSW process for different materials (Cu-Al). It is beneficial to increase the temperature of the harder workpiece before the tool pin when joining dissimilar material, making the material easy to weld and possibly reducing tool wear due to the difference in stress value of flow.
The flow stress difference between copper and aluminum ~ 60 MPa without any preheating is reduced to ~ 10 MPa with optimal preheating conditions and tool compensation. The grain size in the nugget zone is determined by the combined effect of peak temperature and strain rate compared to the low cooling rate in FSW.
Numerical model and material properties
- Model geometry
- Calibration of numerical model
- Material properties
Heat transfer analysis of the FSW and P-FSW process is performed using the commercial software ABAQUS. The size of the weld plate for the simulation is considered the same as the experimental condition. The contact conductance value is assumed in most previous thermal models as a fixed uniform value.
In the present work, a convection heat transfer coefficient of 300 W/m2-K at the base of the workpiece is assumed [316]. At AA 1100, the value of thermal conductivity and specific heat increases almost linearly with temperature.
