Elsevier

Materials & Design

Volume 31, Issue 9, October 2010, Pages 4567-4576
Materials & Design

Technical Report
Establishing relationship between the base metal properties and friction stir welding process parameters of cast aluminium alloys

https://doi.org/10.1016/j.matdes.2010.03.040Get rights and content

Abstract

In friction stir welding (FSW), the material under the rotating action of non-consumable tool has to be stirred properly to get defect free welds in turn it will improve the strength of the welded joints. The welding conditions and parameters are differing based on the mechanical properties of base materials such as tensile strength, ductility and hardness which control the plastic deformation during friction stir welding. The FSW process parameters such as tool rotation speed, welding speed and axial force, etc. play a major role in deciding the weld quality. FSW Joints of cast aluminium alloys A319, A356, and A413 were made by varying the FSW process parameters and the optimum values were obtained. In this investigation, empirical relationships are established and they can be effectively used to predict the optimum FSW process parameters to fabricate defect free joints with high tensile strength from the known base metal properties of cast aluminium alloys.

Introduction

The usage of cast aluminium alloys in automotive and aerospace applications is ever increasing due to their light weight and castability. It is necessary to weld aluminium castings to themselves or to wrought alloys for various applications. These alloys are not easily weldable by conventional fusion welding techniques because the quality of the welded joint is deteriorated due to the presence of porosity, hot cracking and distortion. Friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded, does not melt and recast [1]. FSW creates the weld joint without bulk melting. In addition, the extensive thermomechanical deformation induces dynamic recrystallization and recovery that refine the microstructure of the stir region. Therefore, welds made by FSW are shown to have much improved mechanical properties than the corresponding fusion welds. Since, FSW is a solid state process and the material under the rotating action of non-consumable tool has to be stirred properly to get good, defect free welds [2]. The material flow behaviour is predominantly influenced by the material properties such as yield strength, ductility and hardness of the base metal, tool design [3] and FSW process parameters.

There have been lot of efforts to understand the effect of process parameters on material flow behaviour, microstructure formation and mechanical properties of friction stir welded joints. Cavaliere et al. [4] studied the effect of welding parameters on mechanical properties of AA6056 and found that tensile strength and ductility of the joints are mainly influenced by rotation speed and welding speed. Ma et al. [5] studied the effect of friction stir processing (FSP) on microstructure of cast A356 aluminium alloy and found that FSP parameters had a significant effect on the macrostructure of the stirred zone. The welding speed had a significant effect on the tensile properties and fracture locations of the joints FSW joints [6]. In FSW joints of ADC12 aluminum die casting alloy it is found that the average diameters of the Si particles decrease with the increasing welding speed from 250 to 750 mm/min in all the regions. It is considered that the welding speed influences the heat input during the FSW, and the heat input affects the size of the Si particles [7]. The axial force on the tool decides the weld formation of FSW of aluminium alloy [8]. Most of the researchers focused on finding the effective parameters on properties of friction stir welds as well as realizing their influence on the weld properties [9], [10], [11], [12]. The influence of some of important parameters such as tool rotation speed (N), welding speed (S) and axial force (F) on weld properties have been investigated. In this investigation, an attempt has been made to relate the optimized friction stir welding process parameters and base material properties of cast aluminium–silicon alloys. This will pave way to predict the optimized friction stir welding process parameters to get defect free joints and to attain maximum tensile strength from the known base material properties [13].

Section snippets

Experimental work

Castings of aluminium–silicon based alloys of A319, A356, and A413 were made by sand casting method and they were machined to rectangular plates of size 175 mm × 75 mm × 6 mm. The chemical composition and mechanical properties of base metals are presented in Table 1, Table 2 respectively. As-cast base metals were used for the experimental work. The initial joint configuration was obtained by securing the plates in position using mechanical clamps. Single pass welding procedure was used to fabricate

Results

In fusion welding of aluminium alloys, the defects like porosity, slag inclusion, solidification on cracks, etc. deteriorates the weld quality and joint properties. Usually, friction stir welded joints are free from these defects since there is no melting takes place during welding and metals are joined in the solid state itself due to the heat generated by the friction and flow of metal by the stirring action. However FSW joints are prone to other defects like pin hole, tunnel defect, piping

Establishing relationships

From the macrostructural analysis, microstructural analysis and tensile strength of the FSW joints, it is found that the process parameters have profound effect. Most of the recent investigations, the FSW process parameters are being selected by trial and error to fix the working range to get defect free welds. The welding conditions and parameters are not same because of the change in the mechanical properties of base metals. The important mechanical properties which control the plastic

Discussion

From the relationships (1), (2), (3), (4), (5), (6), it is found that the FSW process parameters are greatly influenced by the properties of base metals which in turn due to the heat input generated during FSW. The heat input during FSW is calculated by the following expression [18].

  • q = (2π/3S) μ F N R η,

  • q = Heat input (kJ/mm),

  • S = Welding speed (mm/min),

  • F = Axial force (kN),

  • N = Tool rotation speed (rpm),

  • R = Tool pin radius (m),

  • μ = Coefficient of friction, and

  • η = Welding efficiency.

The heat input is directly

Conclusions

From this investigation, the following important conclusions are derived:

  • (i)

    The tensile strength and hardness of the cast aluminium alloys play a major role in deciding weld quality of FSW joints.

  • (ii)

    The empirical relationships established in this investigation can be effectively used to predict the optimum FSW process parameters to fabricate defect free joints with high tensile strength from the known base metal properties of cast aluminium alloys.

Acknowledgements

The authors are grateful to the Centre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalainagar, Tamil Nadu for extending the facilities of Metal Joining Laboratory and Materials Testing Laboratory to carryout this investigation.

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