CHAPTER 1 INTRODUCTION

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1 1 CHAPTER 1 INTRODUCTION 1.1 Resistance Spot Welding Resistance Spot Welding (RSW) is an important welding process, used commonly in automobile and railcar manufacturing industries [30] [70]. It is less expensive, faster and can be automated easily compared to other welding processes [24]. In this process, two or more metal sheets are joined by fusion at discrete spots, at the interface. Both electric current and mechanical force are applied together to make joints in RSW. When an electric current is passed through the metal sheets, heat is generated at their interface, by resistance to the flow of current. The temperature rises at the interface of the worksheets and when the melting point of the metal is reached, the metal will begin to melt, and a nugget starts to form out of the fused metal. Subsequently, the current supply is cut off, with electrode force still acting, and the weld gets solidified [21] [98]. The formation and enlargement of the weld nugget is, determined by the variables, such as welding current, weld time, electrode force and electrode tip size [15] [92]. A modern vehicle typically contains 2000 to 5000 spot welds [13]. In contrast to other welding processes, no filler metal or fluxes are used. The welding current and force are applied to the work piece via copper alloy electrodes [96]. Ferritic Stainless Steel (FSS) account to nearly half of the AISI 400 series stainless steels [43]. They do not contain nickel and hence are considered as cheaper substitutes to austenitic stainless steels. Nowadays, ferritic stainless steels are widely used for structural applications in automobile and rail coaches [65] [56]. These steels contain 10% to 30% Cr along with other alloying elements, notably molybdenum. Ferritic stainless steels show excellent resistance to stress corrosion cracking. They are also

2 2 highly resistant to pitting and crevice corrosion [43] [84]. However, the toughness of ferritic stainless steel welds is reported to be low due to the large grain size of the fusion zone [43] [90]. Among FSS itself, low chromium ferritic stainless steels, such as AISI 409 fill the gap between austenitic stainless steels and low carbon steels, as they have the advantages of both [90]. In this study, the effects of resistance spot welding on mechanical performance and microstructure in resistance spot welded joints of ferritic stainless steel AISI 409M were investigated in similar and dissimilar configurations and also the optimization of input process variables was done to achieve maximum weld strength, with a view of enhancing the crashworthiness of the vehicle. 1.2 Need for the Study Vehicle crashworthiness is defined as the capacity of a vehicle structure to provide enough safety to its passengers, in the event of an accident in the form of a crash. It largely depends on the integrity and the mechanical performance of the spot welds, as RSW process is one of the main welding processes used in the manufacturing of vehicle structures [55] [101]. Moreover, the joint failure, e.g. resistance spot weld joint failure, has been identified as one of the key fault types when a vehicle crashes [60]. Failure of spot welds may affect the vehicle s noise, vibration, harshness (NVH) performance on a global level as well as stiffness [16] [54]. Therefore, the quality, performance and the failure characteristics of resistance spot welds are necessary for the determination of durability and safety design of the vehicles [89]. There are three measures for quality evaluation of resistance spot welds, such as weld attributes, mechanical properties and failure mode [58]. An in-depth understanding of the effect of resistance spot welding

3 3 process on the mechanical and metallurgical properties of the weld joint is a prerequisite for quality evaluation and further improvement. Any improvement in the mechanical performance of the spot welded joints in a vehicle or rail coach will certainly contribute to the enhancement of its crashworthiness and ultimately to passenger safety. The need for the current study was to help to enhance the crashworthiness of the rail coach manufactured by RSW, thereby to improve passenger safety. 1.3 Problem Statement Nowadays, ferritic stainless steel finds more and more applications in railcar manufacturing, especially for body paneling of railcars. RSW is a prominent welding process in this area. Quality, performance and failure characteristics of resistance spot welds are essential for the safety design of the vehicles. Evaluating spot weld quality requires studying welding process variables and weld performance interrelations. It is hard to predict weld properties in RSW due to its diverse nature [29]. Hence it is vital to have a thorough understanding of the nature of the spot welds, for ensuring crashworthiness of the vehicle. Evaluation of spot welding quality is also crucial, in improving the economics of vehicle production. Enormous research works have been reported on RSW. However, most of the existing literature on RSW deals with materials, such as low carbon steel, advanced high strength steel and stainless steels, especially austenitic stainless steel. Reported literature on mechanical behavior and microstructure of spot welded joints of various grades of ferritic stainless steel are limited. Ferritic stainless steel grade AISI 409M is a widely used material in railcar manufacturing. In rail coach manufacturing, it is quite common that ferritic stainless steel is often required to be resistance spot welded with same material as well as with different materials, such as other types of stainless steel, low carbon steels, etc.

4 4. In this context, the need to investigate the mechanical properties and microstructure of spot welded joints of the same, in similar and dissimilar metal configurations, assumes much significance. Furthermore, it is also essential to develop suitable welding conditions by optimizing the process parameters to obtain quality welds, for enhancing the crashworthiness of the vehicle. In a modern vehicle, out of a total of spot welds, around 20% to 30% of these spot welds are due to the uncertainty of the quality of spot welds [53]. The significant cost associated with over welding underlines the importance of optimization of spot welding process. Presently, literature dealing with the optimization of process variables in RSW of various grades of ferritic stainless steel to achieve quality weld are limited. Without a doubt, the most important quality characteristic of a spot weld is its load bearing capacity or weld strength. Indentation caused on the surface of the spot weld as a result of spot welding is an undesirable characteristic which needs to be minimized to improve the aesthetic value of a vehicle body fabricated by RSW. Therefore, this study also focuses on optimization of spot welding variables of AISI 409M ferritic stainless steel, by multi-response Taguchi approach, considering multiple quality characteristics, such as weld strength and electrode indentation simultaneously. Improvement in load bearing and energy absorption capacities of the spot weld joint will certainly enhance the crashworthiness of the vehicle and will ultimately contribute towards passenger safety. Use of double pulse current to improve the mechanical performance of spot weld is a relatively untested option for many of the steels. This study also aims to explore the use of the same to improve the mechanical performance of resistance spot welded joint of AISI 409M steel.

5 5 1.4 Objectives of the Study The purpose of this study is to investigate the effect of RSW on AISI 409M ferritic stainless steel in terms of mechanical properties, microstructure and failure mode. The core aim is to enhance the crashworthiness of the rail coach, fabricated by RSW of ferritic stainless steel AISI 409M, by improving the mechanical properties of the spot welded joints, such as weld strength and energy absorption capacity. To achieve this larger aim, the individual objectives are listed below. [1] To study the mechanical behavior, microstructure and failure modes of resistance spot welded joints of ferritic stainless steel AISI 409M, in both similar and dissimilar metal combinations. [2] To explore the effect of the double pulse current in improving the mechanical performance of the spot welded joint. [3] To optimize various input parameters to maximize the peak load and minimize the electrode indentation, by suitable design of experiments (DOE) approach. 1.5 Methodology of the Study [1] Investigate the mechanical properties and microstructure of the resistance spot welded joint of AISI 409M. [2] Investigate the effect of input parameters on the mechanical performance parameters. [3] Investigate the effects of RSW on mechanical behavior and microstructure of spot welds in a dissimilar welding configuration, when AISI 409M is welded with some of the other commonly used materials, such as high strength low alloy steel (HSLA) and austenitic stainless steel.

6 6 METHODOLOGY OF THE STUDY Figure 1.1 Flow Chart Showing Methodology of the Study

7 7 [5] Investigate the effect of double pulse welding current on the mechanical performance of RSW joints of AISI 409M. [6] Process parameter optimization in RSW of AISI 409M, considering multiple quality characteristics, such as weld strength and electrode indentation simultaneously, by multi response optimization approach in order to obtain a set of control parameters corresponding to the optimum output. 1.6 Limitations of the Study Mechanical properties and microstructure of resistance spot welded joints of ferritic stainless steel AISI 409M sheets are investigated in this study, with the focus mainly on its load bearing capacity and the energy absorption capacity. The major limitations of this study are as given below. 1. The mechanical behavior of the resistance spot welded joint of AISI 409M ferritic stainless steel under fatigue load condition is not discussed in this study. 2. Corrosion behavior of the resistance spot welded joint of AISI 409M ferritic stainless steel also is not discussed in this study. 3. In this study, an attempt is made to improve the mechanical performance of the resistance spot welded joint of AISI 409M ferritic stainless steel by using double pulse current. However the optimum welding conditions to achieve maximum mechanical performance parameters is not discussed in this study. 4. Ductile to brittle transformation of the joint due to drop in temperature also is not discussed in this study.

8 8 1.7 Organization of the Thesis The thesis is organized as given below. Chapter 1 gives an introduction to resistance spot welding process, ferritic stainless steel, motivation, problem statement, methodology, objectives of the study, benefits of the study, and thesis organization. Chapter 2 presents a detailed literature survey pertaining to the research work done in the area of resistance spot welding of various materials, especially on stainless steels. Chapter 3 provides details about materials, welding equipments and methods used in this investigation. Chapter 4 presents the details of experimental set-up including welding conditions for the whole study, such as RSW of AISI 409M in both similar and dissimilar mode, investigation on the effect of double pulse current on mechanical performance, optimization of process parameters with conditions for welding and performance testing. Chapter 5 deals with the results and discussion. Chapter 6 contains conclusion of the study and scope for further study.