Brazing often used to be viewed as a poor alternative to welding, but newer steels cannot be welded using conventional processes because of the resultant metallurgical changes that weaken the parent metal. Paul Stevens reports on how Mig brazing is growing in popularity for automotive repairs and other applications.
Brazing is a process that has been popular in the past but which started to fall out of favour as alternative joining methods improved. Strength, durability and aesthetics have been the traditional advantages offered by brazing, together with the fact that the process takes place at temperatures much lower than those required for welding - hence problems such as distortion and embrittlement in the heat-affected zone are avoided.
With the development of modern adhesives and better welding techniques - such as Mig (metal-inert gas), Tig (tungsten inert gas), laser welding and electron-beam welding - some applications moved away from brazing. Nevertheless, there are many products that still benefit from brazed joints, such as spectacle frames and automotive air conditioning components. Of course, one of the advantages of brazing is that the braze material forms a strong metallurgical bond with the parent metals, and there is no requirement for the materials being joined to be the same grade or even the same material.
More recently there has been renewed interest in brazing as a high-strength joining technique for use with the latest generation of steels for automotive body structures. In order to save weight and achieve better protection for vehicle occupants, automotive manufacturers have started to use thinner grades of higher-strength steel. Various advanced high-strength steel (AHSS) and ultra-high-strength steel (UHSS) grades are available for applications within the body structure. Dual-phase (DP) steels are advantageous for energy-absorption structures, while TRIP (transformation induced plasticity) steel is used for energy-absorption elements and sections such as front floor panels, where there is also a requirement for high formability. HSLA (high-strength, low-alloy) steel offers a combination of high strength and good weldability, complex-phase steels can be used for floor members and other areas where reinforcement is required, and martensitic steels have extremely high strength and are used for reinforcement of areas such as the B-pillar and tunnel. Other terms encountered in this field are boron steel and manganese-boron steel.
The use of high-strength and ultra-high strength steels is growing rapidly; the second-generation GM's Opel/Vauxhall Zafira, for example, uses these steels for 51 per cent of the body structure, and the latest Saab 9-3 Convertible uses them for 60 per cent of the body structure. WorldAutoSteel, the automotive group of the World Steel Association, has completed the Ultralight Steel Auto Body (ULSAB) Programme to demonstrate steel's capability to reduce substantially the weight of a vehicle's body structure (see panel).
The excellent physical material characteristics of these latest-generation steels are derived from heat treatment processes. It follows that the steels are more easily affected by heat, hence they cannot be readily Mig welded, though parameterised spot welding is still possible. Furthermore, the tendency for modern automotive steels to be zinc-coated means that the elevated temperatures encountered with welding burn off the zinc coating and adversely affect both the joint strength and the subsequent corrosion resistance of the steel.
For these reasons, there is a resurgence in interest in brazing. Whereas welding requires the temperature to be raised to the melting point of the parent metal, brazing takes place at lower temperatures that minimise damage to the zinc coating and do not change the metallurgy of high- and ultra-high-strength steels.
A popular way to apply the braze is by using Mig equipment. Generally very similar if not identical to the Mig power source, controller and torch that would be used for Mig welding, Mig brazing sets are available in formats for use in both automated manufacturing environments and manual operations. Manual Mig brazing is widespread in automotive body repair shops, where it is important to avoid using traditional repair techniques on high-strength and ultra-high-strength steels.
Typically the brazing wire will be a copper-silicon alloy, and the process has the added advantage of not needing a flux. Special features recommended for the welding equipment include grooved wire feed rollers and a soft lining in the cable between the feeder and the torch. Pure argon is often used as the shielding gas to maintain a stable arc. The best results tend to be achieved with a synergic welding machine with a facility for pulsed Mig brazing, giving a combination of good speed, high strength and a finish that requires little or no dressing (Fig. 1). Unlike with welding, it is necessary to have a small gap between the components being joined so that the molten braze can penetrate by means of a capillary action.
Manufacturers of welding equipment are now actively marketing equipment as being capable of Mig brazing. For example, the Murex Tradesmig 140-1 unit weighs just 25kg and is suitable for light-duty tasks in workshops or on site (Fig. 2). Operating from a single-phase 230V supply, the machine can be used for brazing and also welding with conventional solid wires and self-shielding cored wires.
Mig brazing for future vehicles
Aluminium and composite materials may appear to be the obvious choice for lightweight vehicles, but the world's steel producers are arguing that steel offers good combination of strength, stiffness, weight and cost, as well as being easy to recycle. The WorldAutoSteel organisation also claims that steel produces five to 15 times less emissions than other materials used for reducing weight in vehicles.
WorldAutoSteel has undertaken a number of projects to examine the potential for steel to be used in lightweight vehicles, including the Ultralight Steel Auto Suspensions (ULSAS) Programme, Ultralight Steel Auto Closure (ULSAC) Programme, Ultralight Steel Auto Body (ULSAB) Programme and Future Steel Vehicle (FSV) Programme. By August 2008 the FSV Programme had produced styling sketches based on the packaging studies initiated in the Phase I research. The Programme is considering four technical specification options for the proposed year 2015-2020 vehicle: electric (EV) and plug-in hybrid electric (PHEV) vehicles for four or more passengers; and plug-in hybrid electric (PHEV) and fuel cell (FCV) vehicles for five-passengers. It is intended that the flexibility of powertrain component packaging, coupled with the qualities of AHSS (advanced high-strength steel), will enable these vehicles to maintain current and future consumer expectations for interior comfort, while significantly reducing greenhouse gas emissions throughout the vehicle's life cycle.
The FSV's architecture will feature the latest AHSS grades from around the world, enabling the total mass of the vehicle to be reduced without sacrificing safety or packaging requirements. As the FSV builds on the results of the ULSAB Programme, it is worth noting that more than 90 per cent of the ULSAB structure is manufactured from high-strength and ultra-high-strength steels (Fig. 3). Spot welding was used to join the majority of the parts, with the hydroformed side roof rails laser-welded to the adjacent structures and the rails in the front end structure also laser-welded.
Given the extensive use of high-strength and ultra-high-strength steels, as well as the use of laser welding - which cannot be readily reproduced in repair facilities - it seems likely that Mig brazing would be needed to carry out repairs to vehicles adopting the principles promoted by the ULSAB and FSV Programmes