Elsevier

Journal of Manufacturing Processes

Volume 25, January 2017, Pages 323-339
Journal of Manufacturing Processes

Transient model of heat transfer and material flow at different stages of friction stir welding process

https://doi.org/10.1016/j.jmapro.2016.11.008Get rights and content

Highlights

  • A transient model of the whole FSW process is developed and experimentally validated.

  • The model is used to analyze the thermal processes in the plunge stage, dwell stage, welding stage and cooling stage of FSW.

  • A detailed understanding of heat transfer and material flow in the whole FSW process is provided.

  • Effect of process parameters on the heat generation and material flow has been analyzed.

Abstract

A better understanding of heat transfer and material flow for different stages (including plunge stage, dwell stage, welding stage, and cooling stage) of friction stir welding process is critical for the tool design and selecting appropriate welding variables. In this study, a transient model is developed to quantitatively analyze the dynamic variations of the heat generation, temperature profile and material flow for different stages of friction stir welding process, and is used to investigate the dependence of these aspects on the process parameters such as welding speed and the tool rotation speed. It is found that the total heat generation increases persistently during the plunge stage and reaches its peak value when the FSW tool shoulder contacts with the top surface of the workpiece. From the peak value, the total heat generation decreases monotonically during the dwell stage before attaining the quasi-steady state in the welding stage. The materials flow analysis predicts that the plastically deformed material in the front of the tool flows in counter-clockwise direction, passes the tool on the retreating side and gets released behind the tool during the welding stage. The model is experimentally validated by comparing the measured tool torque and peak temperature values with the predicted results, which agree well with each other.

Introduction

Nowadays a large demand for lightweight, fuel-efficient and low emissive structures has been fulfilled by the widespread application of aluminum and magnesium alloys in manufacturing industries. As a solid-state joining process, friction stir welding (FSW) is considered as an energy efficient, environment friendly and versatile method of joining the lightweight materials than the fusion welding processes [1], [2], [3], [4]. The FSW process involves four main stages: plunge stage, dwell stage, welding stage and cooling stage, as shown in Fig. 1. The process is initiated with the plunge stage during which a rotating tool, comprising a shoulder and a pin, is gradually penetrated into the abutting edges of workpieces until the shoulder contacts with their top surfaces. Next is the dwell stage during which the plunged tool is continued to rotate for a while to soften the material near the tool. This is followed by the welding stage during which the rotating tool is translated along the abutting edges, resulting in a weld joint. When the weld distance is covered, the tool is immediately pulled out of the workpiece which results in a rapid decrease in the temperature of the joint. This is referred to as the cooling stage.

During the FSW process, the heat energy is generated by friction between the tool and the workpiece, and plastic deformation of the workpiece [5], [6], [7]. While the heat energy softens the material in the shear layer around the tool, the plastic material flow in the shear layer produces localized viscous dissipation heat energy. Combination of the tool rotation and translation leads the softened material to flow from the front of the tool (leading side) to the back of the tool (trailing side), where it is forged into a joint. In FSW process, both the heat generation and material flow have crucial effects on the metallurgical characteristics and mechanical properties of the weld joints [8], [9], [10]. Furthermore, the preheating effects of the plunge and dwell stages have significant effect on the welding force and tool wear [11]. Therefore, a complete understanding of both the heat generation and material flow at different stages of FSW process is imperative in optimizing the process, and controlling microstructures and properties of the joints.

Numerical modeling of the FSW phenomena is powerful for understanding different phenomenon [4], [12]. Several models have been developed to explore the heat generation and material flow phenomena in FSW process [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Some of them dealt only with the thermal conduction and ignored the material flow [13], [14], [15]. Others rightly emphasized the coupling of heat transfer and material flow in the FSW process but limited the analysis only to the quasi-steady state of the welding stage and paid no attention to the plunge and dwell stages [16], [17], [18], [19], [20], [21]. There are lots of situations where the steady-state conditions cannot be established, for example, at the start and the end of the welding process, or with varying process conditions/workpiece geometry [22]. Furthermore, the heat transfer and material flow during the plunge and dwell stages have significant effects on the properties of the joints and tool wear [11]. A thorough understanding of different stages of FSW process is important in the development of tools and processes for successfully welding of materials with high melting point [4]. Therefore, a transient numerical model is required for those common cases.

Several transient numerical models have recently been proposed to analyze the process mechanism at different stages of FSW [22], [23], [24], [25]. Song and Kovacevic [23] and Zhang et al. [24], [25] developed 3D transient thermal models which did not account for the convective heat transfer and heat generation from plastic deformation. Recently, Yu et al. [22] proposed a 3D transient model and investigated the heat transfer and material flow in friction stir processing of magnesium alloys. However, this model considered the heat generation only from the plastic deformation by assuming a full sticking condition at the tool-workpiece contact interfaces. In FSW process, the heat generation, the heat transfer and the plastic material flow pattern are fully coupled [26], [27], [28], and both friction heat and plastic deformation heat generate in the FSW process [27], [28], [29], [30]. However, the transient numerical models mentioned above either only consider friction heat or only consider plastic deformation heat. None of these models adequately accounted for both heat transfer and material flow at different stages of FSW process. In addition, the tool torque is much higher at the start of the process when the tool comes into contact and is inserted into the workpiece at the plunge stage [31], [32], [33]. The detailed dynamic variation of tool torque at different stages of FSW is still unrevealed, and the available transient model which could be used to analyze the dynamic variation of tool torque at different stages of FSW is still limited. Therefore, a quantitative analysis of the tool torque, heat generation and material flow at different stages of FSW process is still needed.

In this study, a transient model is developed to quantitatively analyze the heat generation, heat transfer, and material flow during the four stages of the FSW process. Both the friction heat and the plastic deformation heat are considered to determine the heat flux distribution at the tool-workpiece contact interfaces. The effects of process parameters on the contact condition and the friction coefficient between tool-workpiece contact interfaces are examined. The model is validated by comparing the measured tool torque and peak temperature with the predicted results.

Section snippets

Governing equations

Fig. 2 shows the geometric model for the simulation of FSW process. A moving coordinate system is established on the plate. During the plunge stage, the origin of the coordinate system is located at the intersection of the bottom surface of the workpiece and the axis of the tool. The welding direction is parallel to the positive x-axis, and the z-axis is along the plate thickness (upward). For simplification, the shoulder surface is assumed to be flat, and the thread on the pin side surface is

Experimental description

2024 aluminum alloy plates (300 mm in length, 300 mm in width, and 6 mm in thickness) were used to perform experimental and numerical investigations. The average tool torque at quasi-steady state welding stage was monitored and determined as reported in Ref. [44]. Meanwhile, K-type thermocouples were used to measure the peak temperature during the FSW process. In order to measure the temperature at certain locations, suitable holes were drilled from the top surface of the workpiece, and the

Transient heat generation

The new transient model is used to perform numerical simulation for different stages of FSW process. Fig. 5 shows the total heat generation during the FSW process and its constitution as a function of time, i.e., the heat generation from the shoulder, pin side, pin bottom and viscous dissipation. In this figure, the variation in heat generation during the different stages of the FSW process exhibits different trends. During the plunge stage, from t = 0 to tp = 28.5 s, the total heat generation

Summary and conclusions

A transient model is developed to analyze the heat generation, heat transfer and material flow for different stages of friction stir welding (FSW) process. It considers the interfacial heat generation due to both friction and plastic deformation at tool-workpiece contact interfaces and volumetric heat generation due to viscous dissipation near the tool. The model is confirmed to be reliable and valid from the good agreement between the predicted and measured values of tool torque and peak

Acknowledgment

This work is supported by the National Natural Science Foundation of China (Grant No. 51475272).

References (46)

  • H.B. Schmidt et al.

    Thermal modelling of friction stir welding

    Scr Mater

    (2008)
  • R. Nandan et al.

    Three-dimensional heat and material flow during friction stir welding of mild steel

    Acta Mater

    (2007)
  • H.H. Cho et al.

    Three-dimensional numerical and experimental investigation on friction stir welding processes of ferritic stainless steel

    Acta Mater

    (2013)
  • H. Su et al.

    Thermal energy generation and distribution in friction stir welding of aluminum alloys

    Energy

    (2014)
  • A. Arora et al.

    Toward optimum friction stir welding tool shoulder diameter

    Scr Mater

    (2011)
  • L. Fratini et al.

    CDRX modelling in friction stir welding of aluminium alloys

    Int J Machine Tools Manuf

    (2005)
  • C. Hamilton et al.

    A thermal model of friction stir welding in aluminum alloys

    Int J Machine Tools Manuf

    (2008)
  • M. Song et al.

    Thermal modeling of friction stir welding in a moving coordinate system and its validation

    Int J Machine Tools Manuf

    (2003)
  • G. Chen et al.

    Computational fluid dynamics studies on heat generation during friction stir welding of aluminum alloy

    Comput Mater Sci

    (2013)
  • A. Arora et al.

    Torque, power requirement and stir zone geometry in friction stir welding through modeling and experiments

    Scr Mater

    (2009)
  • L. Shi et al.

    Modified constitutive equation for use in modeling the ultrasonic vibration enhanced friction stir welding process

    Scr Mater

    (2016)
  • S. Mandal et al.

    Experimental and numerical investigation of the plunge stage in friction stir welding

    J Mater Process Technol

    (2008)
  • F. Gemme et al.

    Numerical analysis of the dwell phase in friction stir welding and comparison with experimental data

    Mater Sci Eng A

    (2010)
  • Cited by (88)

    View all citing articles on Scopus
    View full text