A thermomechanical hot channel approach for friction stir welding

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Abstract

This paper provides a theoretical framework for developing a thermomechanical hot channel (THC) approach for augmenting the friction stir welding (FSW) process. The model follows from the Rosenthal solution for moving point sources where heat input from the tool shoulder acts as a warm source while plasmas or laser heat sources provide higher energy input. The THC approach aims at decreasing tool wear by reducing the demand for frictional heat from the tool shoulder and pin. In the proposed model, the THC processing temperature is approximately 1000 °C, which is close to the temperature required for stir welding steel.

Introduction

Friction stir welding (FSW) has emerged as an innovative method for joining low melting temperature alloys like aluminum (Al) and magnesium (Mg). The process combines frictional heating with intense plastic deformation to produce cost efficient joints with better mechanical properties than conventional fusion welding techniques. FSW has already been applied to the construction of aluminum structures in both the transportation [1] and aerospace [1] industries, and it is being thoroughly investigated for application to steel structures in the shipbuilding industry.

Although most of the research on FSW has been on aluminum, investigations for steel are still relatively new. Laboratory trials carried out on low carbon steel by Thomas [1] from TWI showed that the heat required to induce plastic deformation similar to aluminum is much higher. FSW for steels require the tool to spin at much faster speeds resulting in a far higher tool wear rates for steel joints compared to softer aluminum joints. This factor is highly pronounced for long welds. Thomas [1], [5] and Lienert et al. [6] reported significant tool wear during the friction stir welding of steel. In their experiments, Thomas minimized tool wear by using pre-drilled holes, less than the diameter of the tool pin as a means to reduce the high tool wear during plunging. In their research, Lienert et al. [6] suggested that increased wear during plunging may be due to the high load spikes resulting from the greater flow stress of the cold workpiece. Besides pre-drilling holes to minimize wear, allowing the tool to spin in one position for a sufficient time plasticizes additional workpiece material and creates additional heat input through plastic work.

These ongoing experiments reflect significant effort towards the development of more robust FSW techniques for steel that offsets problems with tool wear. Frequent replacement of worn out tools is expensive and it carries additional cost through delays and reduced production rate. Also, the presence of inclusions from worn out tools, in the weld would significantly reduce the quality of the joint.

Given the significance of tool wear during the friction stir welding of steel and other harder materials, this study investigates thermomechanical hot channels as an approach for taking FSW beyond the limits of aluminum and other softer materials. Here, the goal is to combine FSW with pre-heating sources that create a thermomechanical hot channel (THC) ahead of the FSW tool. The idea is to pre-heat the workpiece, and reduce the amount of frictional heat and subsequently the tool wear rate (Fig. 1).

Section snippets

Theory

The analytical model presented in this paper is a two-dimensional model based on Rosenthal's model of thermal distribution [7] during welding. Three point heat sources are assumed to be moving on a semi-infinite medium. Here, we consider distributed point sources; however, solutions are available for line source configurations. To simplify the problem, heat transfer by radiation and convection on the surface of the medium is neglected.

Results for multiple point heat sources

For FSW combined with THC, the welding speed and tool rotation speed are fixed at 4 mm/s and 400 rpm, respectively. The other parameters used to calculate temperature distribution in a normal FSW are also retained. In this case pre-heating sources are placed ahead of the tool. In a previous section, Eq. (7) was developed to calculate temperature distribution on a workpiece when n number of pre-heating sources is added to a normal FSW process. For the purpose of the current evaluation, two

Conclusions

The concept of a thermomechanical hot channel for pre-heating during FSW of high hardness materials like steel is analyzed with respect to its effects on tool life improvement. The results indicate that the presence of pre-heating sources ahead of the stir tool, significantly reduce the temperature gradient, as compared to a conventional FSW. This would lead to lesser amount of work done by the tool, which translates to lesser tool wear and longer tool life.

In summary it should be noted that

Acknowledgement

This work was supported by the National Science Foundation's Division of Design, Manufacturing & Industrial Innovation under Award #0343646.

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