K Kumar

Influence of Tool Geometry in Friction Stir Welding

In this article we highlight the results of a recent study undertaken to understand the influence of tool geometry on friction stir welding (FSW) of an aluminum alloy with specific reference to microstructural development, defect formation, and mechanical response. The welding trials were made on 4.4 mm thick sheets using tools made of die steel and having different diameters of the shoulder and the pin, and the profile of the pin. Throughout the welding operation, the rotational speed, traverse speed, and tool axial tilt were held constant at 1400 rpm, 80 mm/minute, and 0 degrees, respectively. For a shoulder diameter of 20 mm and a pin diameter of 6 mm, the severity of defects in the weld was found to be the least and the resultant tensile strength of the weld was high. For the welds that were made using a tool having a shoulder diameter of 10 mm and a pin diameter of 3 mm the tensile strength of the weld was the least since the degree of defects observed were higher.

An Investigation of Friction During Friction Stir Welding of Metallic Materials

The technique of friction stir welding (FSW) puts effective use frictional heat for the purpose of joining metallic materials. In this research article, we present and discuss an experimental method to determine the coefficient of friction during FSW. The experiments were conducted to study the interaction between the FSW tool (a die steel) and the base metal (a high strength aluminum alloy) at various contact pressures (13 MPa, 26 MPa, and 39 MPa) and rotation speeds (200 rpm, 600 rpm, 1000 rpm, and 1400 rpm). The experimental results, the microstructure, and the process temperature reveal the experimental setup to be capable of simulating the conditions during FSW. The coefficient of friction was found to vary from 0.15 to 1.4, and the temperature increased to as high as 450°C. The coefficient of friction was found to increase with temperature. There exists a critical temperature at which point a steep increase in the coefficient of friction was observed. The critical temperature decreases from 250°C at a contact pressure of 26 MPa to 200°C at contact pressure of 34 MPa. Below the critical temperature at a specific contact pressure the maximum coefficient of friction is 0.6, and above the critical temperature it reaches a value as high as 1.4. The steep increase in the coefficient of friction is found to be due to the seizure phenomenon and the contact condition during FSW between the tool and the workpiece (base metal) is found to be sticking.

The Role of Tool Design in Influencing the Mechanism for the Formation of Friction Stir Welds in Aluminum Alloy 7020

Design of the required tool is a key and important parameter in the technique of friction stir welding (FSW). This is so because tool design does exert a close control over the quality of the weld. In an attempt to optimize tool design and its selection, it is essential and desirable to understand the mechanisms governing the formation of the weld. In this research study, few experiments were conducted to systematically analyze the intrinsic mechanisms governing the formation of the weld and to effectively utilize the analysis to establish a logical basis for design of the tool. For this purpose, the experiments were conducted using different geometries of the shoulder and pin of the rotating tool in such a way that only tool geometry had an intrinsic influence on formation of the weld. The results revealed that for a particular diameter of the pin there is an optimum diameter of the shoulder. Below this optimum shoulder diameter, the weld does not form while above the optimum diameter the overall symmetry of the weld is lost. Based on experimental results, a mechanism for the formation of friction stir weld is proposed. A synergism of the experimental results with the proposed mechanism is helpful in establishing the set of welding parameters for a given material.

Positional dependence of material flow in friction stir welding: analysis of joint line remnant and its relevance to dissimilar metal welding

Abstract Understanding material flow in friction stir welding is important for production of sound dissimilar metal welding that control the intermixing of two alloys being welded and consequent formation of new constituents which influences the weld properties. In the present experimental investigation material flow patterns are visualised using dissimilar and similar aluminium alloys using a simple innovative experiment. The experimental results reveal that only a portion of material transported from the leading edge undergoes chaotic flow and the remaining is deposited systematically in the trailing edge of the weld. Using this information it is shown that the formation of a friction stir welding defect, joint line remnant, does not occur only when the weld interface is on the advancing side. The material flow visualisation study has been utilised to analyse the mechanism of weld formation and its usefulness in improving fatigue properties and for dissimilar metal welds.

Structural characterisation of reaction zone for friction stir welded aluminium-stainless steel joint

Abstract In the present investigation, commercially pure Al has been joined with 304 stainless steel (SS) by friction stir welding. The assembly finds widespread application in the field of cryogenics, nuclear, structural industries and domestic appliances. Microstructural characterisation was carried out using scanning and transmission electron microscopes. It has been found that diffusion of Fe, Cr and Ni is substantial within Al; however, diffusion of Al within 304SS is limited. Owing to interdiffusion of chemical species across the bondline, discrete islands of Fe3Al intermetallic form within the reaction zone. The rubbing action of tool over the butting edge of 304SS removed fine particles from 304SS, which were embedded in the stirring zone of Al matrix. Subsequently, austenite underwent phase transformation to ferrite due to large strain within this grain. Fracture path mainly moves through stirring zone of Al alloy under tensile loading; however, in some places, presence of Fe3Al compound has been also found.

Influence of reinforcement and processing on the wear response of two magnesium alloys

ABSTRACT Reinforcement of magnesium alloys with ceramic particulates has engineered a new family of materials that are marketed under the trade name metal-matrix composites. Rapid strides in the processing of these materials during the last two decades have provided the necessary impetus for their emergence and use in structure and automotive-related components. In this paper are reported the results of a study aimed at understanding the role of the reinforcing phase on the wear behavior of two magnesium alloys discontinuously reinforced with silicon carbide (SiC) particulates and saffil alumina short fibers. The wear rate of the reinforced magnesium alloy metal matrices is lower than that of the unreinforced counterpart (AM60-T5 and AZ92-T6). The improved wear resistance of the composite microstructures is attributed to the presence and distribution of the ceramic reinforcement phase, which minimizes the tendency for material flow or plasticity at the surface during sliding. Wear rate is influenced by sliding speed, nature, and volume fraction of the reinforcing phase. For the unreinforced magnesium alloys, an increase in sliding speed results in a marginal increase in wear rate. For a given reinforcement (particulate and saffil fiber) in the magnesium alloy metal matrix, an increase in sliding speed had a negligible influence on wear rate. An increase in volume fraction of the reinforcing phase in the magnesium alloy metal matrix resulted in a noticeable drop in wear rate. Coefficient of friction of the unreinforced magnesium alloys AM60 and AZ92 decreased with an increase in sliding speed regardless of the amount of applied load. Addition of reinforcement, i.e., particulates and short fibers, to the magnesium alloy metal matrix resulted in a significant drop in coefficient of friction. However, increase in volume fraction of the reinforcing base in the magnesium alloy metal matrix had negligible influence on coefficient of friction. The wear characteristics of the reinforced metal matrix are discussed in light of the mutually interactive influences of intrinsic microstructural effects, strength of the microstructure, sliding speed, and local stress state.

Influence of processing and reinforcement on microstructure and impact behaviour of magnesium alloy AM100

Reinforcing magnesium alloys with a discontinuously dispersed ceramic phase has engineered a new family of materials that are marketed under the trade name “metal-matrix composites”. Continuous research efforts in the processing of these materials have provided the necessary impetus for their emergence and use in structural, automotive and even aerospace-related components. In this paper we report the results of a study aimed at understanding the role of short-fibre reinforcements (discontinuously dispersed through the metal-matrix of magnesium alloy AM 100) on impact deformation and fracture behaviour. In particular, the role of volume fraction of the reinforcing phase on impact energy and fracture behaviour is presented and discussed. An increase in short-fibre reinforcement content in the magnesium alloy metal-matrix is observed to have a detrimental influence on impact energy when compared to the unreinforced counterpart. Micro cracking in the metal-matrix coupled with failure of the reinforcing fibres, both independently dispersed and in clusters, dominates the fracture sequence at the microscopic level. The final fracture behaviour of the composite material is discussed in the light of the concurrent and mutually interactive influences of nature of loading, local stress state, intrinsic microstructural effects and deformation characteristics of the composite constituents.

Damage Tolerant Magnesium Metal Matrix Composites: Influence of Reinforcement and Processing

A growing impetus to enhance our understanding of the behavior of magnesium-based alloys for use in weight critical applications resulted as a consequence of the low density of magnesium. In an attempt to enhance the applicability of magnesium for a wide spectrum of performance-critical applications, the addition of reinforcement to the alloy was considered as an economically affordable and potentially viable scientific alternative. In this paper are reported the results of a study aimed at understanding the influence of saffil alumina short fiber reinforcement on microstructural development of a squeeze-cast magnesium alloy. Preliminary results confirm promise of the reinforced alloy, which retains hardness, strength, and stiffness better at elevated temperatures compared to the unreinforced counterpart. However, impact strength and toughness of the reinforced alloy are inferior. The importance of the matrix alloy in governing the overall mechanical response of the composite microstructure is discussed based on fractographic observations. The importance of volume fraction of the reinforcing phase on properties of the composite microstructure is highlighted.