Japanese jewellery creation, and the somewhat less ancient practice of friction stir welding (FSW) came together recently in a project involving the art and design sector of Sheffield Hallam university and TWI.
The work involves a novel method for producing mixed metal multi-coloured layered materials using FSW and compares the results with the ancient Far Eastern art of Mokume Gane.
Mokume Gane is a unique process involving sheets of different coloured metal alloys bonded together into a laminated billet before being carved or milled to expose interior layers. The material is then hammered or rolled into a flat sheet which is used to form jewellery or hollow ware.
The visual effect is stunning, whether it be with the noble metals of silver and gold, or the industrially popular materials of brass and copper. Early on in the development of friction stir the process was identified as ideally suited for joining dissimilar materials.
Using these multi-coloured layers in the process has given TWI a unique insight into how the material flows around the friction stir tool and has thereby enhanced TWI's tool design capability.
- It is clear that it would be feasible to use FSW to produce Mokume Gane materials from a number of different metals including gold, silver, platinum, palladium, copper and brass.
- FSW, used in the correct manner, can produce well-bonded materials in laminate form in which the materials have been both bonded and mixed, forming an attractive and repeating pattern that can be reproduced.
- The fact that mixing occurs at the same time as the bonding provides a reduction in the amount of subsequent work required to form the patterns that give Mokume Gane its appeal.
- The method does not require high-temperature furnaces or the need to avoid oxidation of the metals, and very little cleaning or sample preparation is required. It is also a relatively low-energy process since the whole sample does not require heating.
- The patterns formed are unique to FSW and have the potential to be widely varied by changing the lay-up of the materials to be bonded and the friction stir conditions. After reaching steady-state conditions the patterns formed are stable and repeat in a regular manner but with a small natural and random variation, making each piece unique.
- The bonds formed in the friction stir zone are a combination of the intimate physical contact between the metals resulting in metal-to-metal bonds plus thermally enhanced mechanical alloying, producing microstructures not possible by conventional processing.
- The nature of FSW means that large ingots of bonded material can be produced relatively easily using multiple passes of the friction stir tool through laminate layers. Other possible material lay-ups are possible, giving many possible variations in pattern, for example, bars of materials side by side or materials with regions inset with a second material.
- On the smaller machines, such as the TTI, the maximum ingot size is ~ 300mm x 150mm, with thickness determined by the number and thickness of the layers to be bonded but in the region of 10mm to 30mm. Such large sizes are impossible using any other Mokume Gane production technique. In theory the larger PowerStir machine could make much larger sheets measured in metres.
- With the constant development of FSW and the increasing availability of machines and tooling capable of carrying out FSW, there is a great potential for both large companies and smaller individual makers to begin to experiment and develop new Mokume Gane materials.
In time this approach could also lead to the production of visually stunning architectural panels. To find out more about friction stir welding click here, or contact Stephen Cater.