phd project

Friction Stir Spot Welding of Advanced High Strength Steels

Start / End:

September 2012 to September 2016  

Contact

University of Twente

Faculty of Engineering Technology
Chair of Production Technology
P.O. Box 217 7500 AE Enschede
The Netherlands Room N131, “Horst” building
Phone : +31-53-489 2529
E-mail: shaojie.liu[a]utwente.nl

Motivation

New developments in the automotive industry are directed towards the use of stronger steel grades to enhance the passenger’s safety, to decrease the car’s weight and to improve its fuel efficiency. Application of such Advanced High Strength Steel (AHSS) with a yield strength above 600 to 800 MPa requires an efficient and economical way of bonding these types of steel sheets. The AHSSs are usually welded by Resistance Spot Welding (RSW). However, they exhibit poor weld-ability, and spatters, blowholes, and solidification cracks are formed easily in AHSSs during RSW. A possible new method of bonding AHSSs is Friction Stir Spot Welding (FSSW) that enables bonding of steel sheets in the solid state. In this way many of the problems observed during RSW of AHSS that are related to the solidification process during the cooling stage after welding can be avoided.

Welding Process

Friction Stir Spot Welding (FSSW) is a solid state welding technique for spot welding of thin metal sheets. It is a process variant of Friction Stir Welding (FSW) technique invented at The Welding Institute (TWI) in the UK in 1991. The welding techniques involve a rotating non-consumable tool with a specially designed pin and shoulder. The tool is plunged into the work piece materials to be joined until the tool shoulder touches the top of the work pieces and softens the material around the pin (Figure 1) [1]. The locally softened material moves along the pin and the shoulder under the applied stress which causes the formation of a strong metallic bond between the work piece sheets at temperatures below the melting point of the work piece materials. After a sufficiently long dwell time, the tool is retracted from the work piece materials.

Fig 1. schematic illustration of Friction Stir Spot Welding
[1]. W.Yuan, R. B. (2012). Material flow and microstructure evolution during friction stir spot welding of ZA31 magnesium alloy. Materials Science and Engineering A, 200-209.

Focus

The project concentrates on various types of AHSSs, such as DP steels, boron steels and twinning induced plasticity steels. The development of a validated, quantitative FSSW process model that describes the occurring phenomena during spot welding, the heat generation during the plunging and dwell stage, the work piece plastic deformation, the changes occurring in the material microstructures, the bonding processes at the interfaces and the design of the welding tool and selection of the tool materials will be studied in detail during both the simulation and experiments.

Current Work

At the start of the project easier to weld aluminium alloys will be considered first. Consequently, the attention is currently paid to doing some literature research and conducting some relatively simple experiments on Al and other similar alloy sheets. With this introductory work the typical phenomena occurring in the welding process can be studied, which lays a foundation for the following studies.