Wear of Polycrystalline Boron Nitride Tool During the Friction Stir Welding of Steel

M Almoussawi,M Faraji,A J Smith

doi:10.1007/s13632-018-0439-0

Abstract

The wear issue of a polycrystalline boron nitride (PCBN) tools during the friction stir welding of two grades of steel, DH36 and EH46, was studied. Two welding traverse and tool rotational speeds were used when welding the DH36 steel. A low tool speed (200RPM, 100 mm/min) and a high tool speed (550RPM, 400 mm/min) were denoted by W1D and W2D, respectively. Nine welding conditions were applied to the welding of EH46 steel plate including seven plunge/dwell trials (W1E–W7E) and two steady-state trials (W8E and W9E). SEM–EDS and XRD tests were applied in order to reveal the boronitride (BN) particles inside the welded joints, and the percentage (%) of BN was calculated according to the standard quantitative metallographic technique. The findings showed that tool wear increases when the tool rotational speed increases as a result of binder softening which is a function of the peak temperature (exceeds 1250 °C) at the tool/workpiece interface. When considering the EH46 steel trials, it was found that an increase in the tool traverse speed in friction stir welding caused a significant tool wear with 4.4% of BN in the top of the stirred zone of W9E compared to 1.1% volume fraction of BN in W8E which was attributed to the higher thermomechanical action on the PCBN tool surface. Tool wear was also found to increase with an increase in tool plunge depth as a result of the higher contact between the surface of friction stir welding tool and the workpiece.

Highlights

Friction stir welding (FSW) is a solid-state welding technique invented at The Welding Institute (TWI) in 1991 [1]
When considering the EH46 steel trials, it was found that an increase in the tool traverse speed in friction stir welding caused a significant tool wear with 4.4% of BN in the top of the stirred zone of W9E compared to 1.1% volume fraction of BN in W8E which was attributed to the higher thermomechanical action on the polycrystalline boron nitride (PCBN) tool surface
PCBN FSW tool wear has been found to increase with an increasing tool rotational speed as a result of W-Re binder softening

Summary

Introduction

Despite the high strength and thermal properties, the PCBN tool has low fracture toughness and is susceptible to wear problems, especially during the plunge period of the weld due to the higher temperature generated which can cause a softening of the binder [3]. The PowerStir FSW machine includes a cooling system applied to the tool shank in order to reduce the temperature resulting from the friction stir welding of steel which is readily transmitted due to the high thermal conductivity of the PCBN tool. Previous work on PCBN tool wear during the friction stir welding of steel did not investigate the possibility of W-Re binder softening as a result of high temperature which may reach the local melting of steel alloys during the process, especially when the tool rotational speed reaches specific limits. The aims are to identify the causes of PCBN tool wear and to highlight the different suitable welding parameters that can increase the tool longevity

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Result

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Conclusion