New steels set challenging problems

Most ordinary cars are likely to continue to be made of steel, but the requirement for higher strengths is setting a whole host of challenges, especially because car manufacturers in the UK particularly, are still firmly wedded to using resistance spot welding as much as possible, because of its low cost and high speed. This was the general opinion of Andrew Woloszyn, a principal project leader with TWI, who formerly worked for Jaguar Land Rover. Jaguar Land Rover, of course, use a lot of aluminium in its bodywork in order to save weight, but they are and always have been upmarket quality vehicles, where customers do not mind paying a bit extra for quality. Where cost is the primary consideration, this leaves steel, but pressures to reduce weight and improve crash protection mean that designers have had to find ways of designing body in white structures based on increasingly strong steels – dual phase (DP), TRIP, martensitic, air quenched and now press hardening manganese boron steels with yield stresses up to 1800 MPa. These steels, which are now widely available, are so called because they are heated to 900 to 950°C and then rapidly cooled in the press. These and other advanced steels are not easy to spot weld, because the welding process also heat treats the area around the weld in a way that may be different from that used to produce optimum properties in the parent metal. It is usually possible, Woloszyn claims, to find a way to resistance spot weld almost any steel, although the welding parameters – current, time and pressure, may need to be controlled very precisely. Techniques such as applying subsequent current pulses to temper welds do not find favour with automotive manufacturers if they lengthen the time a body in white has to spend in a work cell, lowering production throughput. There is no shortage of alternatives to spot welding, of course. Audi, for example, uses a combination of resistance welding, laser welding, riveting and hybrid laser-arc welding in the manufacture of some of their cars. One of the main drawbacks of laser welding remains the cost of the equipment. A 10kW, Yb diode pumped disc and fibre laser is capable of immensely rapid and accurate welding, but costs around $1 million. New materials, such as 'TriBond' made by ThyssenKrupp, which has hard, outer, wear resistant layers hot roll bonded to an inner formable core, poses new problems that have to be solved. Development of spot welding processes is now a little easier than it was, thanks to software for the simulation and optimisation of resistance spot welding developed by Swantec, which Woloszyn described as, "Getting very close to reality." We asked if it was possible to spot weld the 'Super Bainite' steels described in Eureka's October 2010 edition and Woloszyn replies that nobody had asked him yet, but he would be happy to have a go if somebody cared to request that he do so. Aluminium can be welded by various means, including spot welding, but Jaguar Land Rover uses a lot of self-piercing rivets and clinching. He revealed that there are $120 worth of rivets in each Jaguar XJ. Clinching uses no rivets but the joints are not as strong. An alternative is TWI's friction spot welding process, the latest version of which leaves no holes, and which will join almost any metal to almost any other. Development continues. Design Pointers • Almost all steels used in automotive manufacture can be resistance spot welded, but parameters sometimes have to be controlled very precisely • Other viable rapid methods of joining automotive high strength steels include laser welding, hybrid laser and arc welding, laser brazing and riveting, All are used, often in combination, especially in Germany • TWI is still developing its friction spot welding process which will join almost any metal to almost any other