Technical ReportPrediction of tensile strength of friction stir welded aluminium matrix TiCp particulate reinforced composite
Introduction
AMCs are continuously finding their application in the field of aerospace, marine, automotives, sports and recreation. Al–TiCp are of interest for structural applications, where weight saving is of primary concern. Titanium carbide (TiC) material is very interesting because it is thermodynamically stable and enhances hardness and lightness of the composite. Ceramic particles in the ductile matrix can lead to desirable properties such as: increased strength, higher elastic modulus, higher service temperature, improved wear resistance, decreased part weight, low thermal shock, high electrical and thermal conductivity, and low coefficient of thermal expansion compared to the conventional metals and alloys [1]. According to the type of reinforcement, the fabrication techniques can vary considerably.
Fusion welding of Al–TiCp is difficult and it has been posing a continuous challenge to fabricators so far. It is primarily due to the segregation of reinforced particles when it is in liquid state, and the deleterious reactions between reinforcing hard particles and liquid aluminium [2], [3]. The said problems can be overcome by employing solid state welding process. Friction stir welding process allows welding of several materials that are extremely difficult to fusion weld. It is predominantly used for welding low melting point materials like aluminium and its alloys with the reduced human skills. So FSW is chosen to make weld joint. It is a hot shear, an energy efficient, and environmentally friendly welding process. The FSW tool is a crucial part of this welding process. It consists of a shoulder and a pin. Pin profile plays a crucial role in material flow and in turn regulates the welding speed of the FSW process. Tool geometry such as probe length, probe shape and shoulder size is also a key parameter; because it would affect the heat generation and the plastic material flow. Oosterkamp et al. [4] emphasized the role of tool pin profile in FSW. The primary function of the non-consumable rotating tool pin is to shear and stir the material. The severe plastic deformation is due to the flow of the material around the rotating and advancing tool. The material flow depends on the welding parameters. Frictional heat is generated due to friction developed by the tool shoulder and weld material surface, and by deformation. The material softens and flows around the tool [5] as it is progressed. The heat input and temperature distribution during FSW is due to frictional heat generation between the rotating tool shoulder and surface of the plate to be welded and in turn depends on the co-efficient of friction. Apart from the properties of tool and plate material, the axial force decides the coefficient of friction. Hence, axial force plays a significant role in friction stir welding process. Ouyang and Kovacevic [5] observed that the plunge depth of tool pin inside the work piece is based on axial force. Kumar et al. [6], [7] studied the role of axial load on tensile strength of friction stir welded aluminium alloys. Forging occurs under the axial pressure applied by the tool shoulder. FSW joints avoid the formation of shrinkages and porosity which is more common in fusion welds, as well as segregation of the ceramic reinforcement. They significantly reduce the thermal stresses [8]. Since there is no melting during FSW, problems associated with liquid solid reactions are completely eliminated. Literature shows that [9] using FSW process, better tensile properties are obtained by using square tool pin profile on aluminium alloys irrespective of other welding parameters used. In this study, an attempt has been made to produce Al–TiCp in a cost effective way by indigenously developed stir casting process. FSW is used to join Al–TiCp plates autogenously. The effect of various friction stir weld parameters on UTS was studied using Design of Experiment (DoE).
Section snippets
Experimental work
Chemical composition and mechanical properties of both Aluminium and TiC are provided in Table 1, Table 2. AA6061 T6 material in extruded form is in heat treated and solution hardened condition. TiC having particle size less than 2 μm was supplied by M/s. Alfa Aesar (India). AMC reinforced with TiC is produced by Modified Stir Casting Process with bottom pouring arrangement in an argon atmosphere. TiC addition varied from 3% to 7%. In the above process, problems associated with the conventional
Microstructure
The welds are characterized by Optical Microscopy (OM) to investigate the micro structural modifications induced by the FSW process on the aluminium alloy matrix and TiC reinforcement particles. Optical micrographs of the cross-section of the FSW joints are observed in bright field after polishing the specimen initially with fine emery up to 1200 grade and finally with fine finishing using diamond paste. Microstructure of parent material is seen in Fig. 4a. It shows the effect of Ti, which act
Results and discussion
It has been found that the fracture locations of the joints are at or nearer to the interface between the weld nugget and the TMAZ on the advancing side. Fig. 5 shows that the elongated grains in the TMAZ are transformed into fine equiaxed grains in the weld nugget. The weld nugget is composed of fine-equiaxed recrystallized grains, while the TMAZ is composed of coarse-bent recovered grains [15], [16]. Therefore, the interface between the weld nugget and the TMAZ is clearly visible and becomes
Conclusion
Problems associated with the fusion welding of aluminium matrix TiCp particulate reinforced composite were solved by using a solid state process known as FSW process. Joint efficiency obtained in most of the weld trails are more than 90%. A mathematical model has been designed to predict the tensile strength of the friction stir welded aluminium matrix TiCp particulate reinforced composite. It is found that the tool pin profile has maximum effect on tensile strength of friction stir welded
Acknowledgements
The authors wish to place their sincere thanks to Naval Research Board, DRDO, Govt. of India for funding to purchase Friction Stir Welding Machine, (vide funded project; Ref No. DNRD/05/4003/NRB/85 Dated 30.10.2006,) and All India Council for Technical Education (AICTE), New Delhi for funding the material purchase (vide funded Project; File No. 8023/BOR/RID/RPS-195/2007-08.Dated 05.03.2008). Authors are grateful to Department of Mechanical Engineering, Coimbatore Institute of Technology,
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