Some developments in the joining of light alloys (Nov.2001)

Paper presented at 7th International Symposium of the Japan Welding Society, 20-22 November 2001, Kobe, Japan.

In the last decade or so there have been big advances in welding technology for light alloys (especially those based on aluminium, magnesium and titanium). Friction technology has expanded at an amazing pace, and the paper covers recent tool developments and applications for friction stir welding of light alloys. Linear friction welding of light alloys is also reviewed and this process shows promise once machine costs can be reduced. The use of self-piercing rivets and clinching is discussed in the context of joining aluminium and magnesium and finally some advances in the development of laser welding of light alloys are summarised.

1. Introduction

The application of light alloys in engineering applications poses many challenges to designers and fabricators who are more familiar with ferrous construction. Even those who are already light alloy fabricators are faced with the need for increased production and/or quality. Indeed, the greater application of light alloys in new sectors is exposing more and more fabricators to aluminium, titanium and magnesium. Many, but by no means all of the alloys of these metals can be welded using conventional arc welding processes, but these can be restrictive and expensive to apply. The replacement of conventional parts with a lightweight alloy alternative is by no means straightforward. The changein material will almost certainly require a complete redesign to take into account differences in materials performance; Al and Mg alloys exhibit lower strengths than many steels and all have lower fatigue performance. Furthermore, the higher price of the basic materials often dictates that costs be reduced elsewhere. These drivers increase the demand for high productivity and quality joining processes that enable greater productivity and the use of thinner sections.

Aluminium alloys are undoubtedly the most common light alloys, with extensive application in a broad range of industrial sectors. However, until the advent of laser welding, only mechanical fastening, GTA and GMA welding were commonly applied. The advent of modern welding processes has had a dramatic impact on the joint quality and cost-effectiveness of these alloys. Most notably, friction stir welding has enhanced the economics of aluminium alloys by enabling higher quality, stronger joints to be achieved at reduced cost. Friction stir welding is not suitable for all aluminium applications, but recent advances in laser welding of these materials are improving the capabilities of this process, whilst retaining the high productivity and low distortion that are its hallmarks.

Magnesium alloy die-castings are finding increasing application in the automotive and electronics sectors. Here, one of the key enabling factors is that magnesium can be die-cast into thinner and more complex shapes than aluminium alloy equivalents, reducing material usage and enabling some large parts to be cast in one piece. However, die-castings exhibit quite high levels of trapped gas, making fusion welds susceptible to gross weld metal porosity. Solid state processes, such as FSW and mechanical fastening, are not influenced by the trapped gas and offer particular advantages for die-cast magnesium alloy joints, especially since they can be employed for Mg to Mg and Mg to Al dissimilar alloy joints.

Of all the lightweight metals, titanium currently has the most stagnant growth and suffers most from price fluctuations. However, its potential for weight savings is great due to the very high strength of the alloys allied to superb corrosion resistance. In its traditional aerospace sector, laser welding is proving an effective alternative to the more routinely applied electron beam welding process. Similarly, linear friction welding is helping to reduce the weight of key aeroengine components.

The last 10 years has been an exciting period in the development of innovative processes for joining light alloys. Whilst friction welding, mechanical fastening and laser welding can be said to be relatively mature processes, there has been more innovation in these processes in the last 10 years than in most previous periods in their history. The purpose of this paper is to summarise several of these developments and their applications in industry sectors such as land transport and aerospace.

The technology changes to be described are contributing not only to reducing manufacturing costs, but also to opportunities to manufacture totally new products. Whilst some of the developments require substantial investment, lower cost equipment will undoubtedly become available in the next 10 years and benefit the smaller fabricator.

2. Friction stir welding

2.1 Introduction

The process was invented and patented by TWI in 1991 ^[1] . The concept is simple and now well known. A rotating tool with a central probe is passed into the components to be welded and transversed along the joint line. The joint created is a solid phase weld, with no melting involved. The initial work was done on Al alloy sheet and plate, and this is where there has been rapid commercial exploitation driven by the clear merits of the process. Conventional arc welding is difficult or impossible for many of the higher strength alloys and, even for those alloys that can be arc welded, filler wire is nearly always required regardless of sheet thickness, and porosity, liquation, distortion and trapped oxides are often difficult to avoid. Friction stir welding (FSW) suffers from none of these problems and is ultimately a cheaper process to apply. Improvements to the process have continued to build upon these benefits, meeting the strong industrial drivers for the wider use of FSW in the welding of Al and its alloys, for example, how to weld faster, weld thicker, weld very thin material, and weld difficult alloys.

As a consequence of the immense success that FSW has enjoyed with aluminium, there is an increasing requirement to weld other alloy systems. Recent process developments have demonstrated that other metal systems, including Pb, Zn,Cu and steel, can also be welded by FSW. However, of more relevance to the present publication, extensive work has also been performed in developing FSW for titanium and magnesium alloys.

The above process improvements have mainly been enabled by the development of novel tools. Similarly, tool and parameter development have led to an expansion in the metal systems that can be successfully welded. These developments are described in the following sections.

2.2 Tool developments

The ability to weld faster at given thicknesses relates crucially to the FSW tool design and at TWI, a new development has been the introduction of a scroll profile on the tool shoulder. The scroll channel captures most of the material extruded while plunging the tool pin into the work piece, and when the tool pin is travelling along the joint, a radially inward mechanical advantage is provided by the rotation of the scroll, increasing the compression about the upper threads of the tool pin.

This development was tested on a 5083 Al alloy of 6mm thickness and high quality welds were achieved at double the speed made using FSW tools with no machined scroll. This development has also allowed tools to be used in 0°tilt position, eliminating the normal 1-3° backward tilt. This simplifies the set up and, in principle, facilitates welding in the x and y directions.

In the initial development phase with industrial clients, Al alloy thicknesses were in the range 1.5-12mm and the tools used contained threaded surfaces on the pin. Heat is generated on the surface by friction between the rotating shoulder and the workpiece surface, and this is the main source of heat for the welding of thin sheets. More heat must be supplied as the sheet/plate thickness increases and, in addition, there are requirements for the probe to create sufficient working of material around the joint line and efficient flow of the material around the tool as the weld proceeds. Using these principles, Thomas and Gittos have developed two new types of tool at TWI known as Whorl ^TM and Triflute ^TM . The latter is shown in Fig.1 and involves a frustum-shaped probe with auger type threads, with the presence of helical flutes.

Fig.1: TWI's MX Triflute ^TM friction stir tool which gives increased welding speed and quality in Al alloys

Experiments with these new tools have shown that for Al alloy plates of 25mm or greater, one pass welds can be made with excellent quality as shown in Fig.2. For example, with 6082 alloy, 25mm thick welds have been made at up to 250mm/min and 7075 alloy, 25mm thick welds at up to 60mm/min. In contrast, when using the same materials and thicknesses along with conventional threaded tools, welds were of very poor appearance and quality.

Fig.2: Friction stir weld in 25mm thick 7075 Al alloy

One issue common to all FSW techniques at present is the requirement to react the downforce against the back of the weld. Fixed backing bars are the normal method of supplying the reaction force, but this limits the flexibility of the process. In the original patent, a bobbin tool was conceived which eliminates the need for backing bars. The tool, is a conventional tool with an extended pin and an opposing tool shoulder. This second shoulder is the integral backing bar. In recent TWI experiments the bobbin tool design was used with scroll shoulders and a threaded profile which changed direction at the mid-point. This approach produced good welds in 6mm 6081-T6 alloy and appears promising. It is shortly to be used in the manufacture of circumferential welds of Delta rocket casings by Boeing, USA.

2.3 Other light alloys

Magnesium alloys have been the subject of much interest in recent years, following the increased demand for Mg die-castings by the automotive sector. Friction stir welding is an obvious candidate process for seam joints in these materials since, unlike fusion welding, FSW is broadly immune to the presence of any trapped gas in the die cast material. Good quality welds have been made in wrought AZ31 and die-cast AZ91, AM50 and AM60 grades in thicknesses of around 3mm ^[2] . The work is in the developmental stages, but it is apparent that the process window for Mg alloys is probably more narrow than that for Al alloys. Thus, maximum welding speeds will probably be lower. However, welding speeds are still favourable compared with other processes. Furthermore, maximum welding speeds should be less of an issue since long lengths of weld, such as required for aluminium decking on high speed ferries, will not be required fordie-cast components.

Whilst impossible for fusion welds, dissimilar metal joints between aluminium and magnesium are feasible. Such joints are at a very early stage of development and work is required to determine desirable levels of mixing. No evidence of embrittlement (the key danger for Al to Mg joints) has been found, but this may become more likely if mixing of the two parent materials becomes more intimate. This particular material combination will undoubtedly be a challenge for FSW, but the potential benefits are great since no other seam process can achieve this type of joint.

The greatest challenge now is to weld other light metals such as titanium alloys. Here the tool material is crucial and the search is on for high melting temperature materials of adequate hot strength from which to manufacture appropriate FSW-tools. After only a few years work it is now feasible to weld metre lengths of Ti-6Al-4V alloy.

In summary, exciting progress has been made in FSW tool design for light alloy fabrication. Welding speeds have been more than doubled compared to the originally developed tools. One pass welds can now be made in up to 50mmthickness in 6000 series Al alloys using the new tool technology. In addition, high quality seam welds can be achieved in die-cast magnesium alloys, an issue which has defeated other processes and will aid the utilisation of Mgdie-castings for the automotive sector (instrument panels, cross beams, door frames, etc.). Industrial exploitation is gathering pace and current applications for Al alloys are numerous in high speed ships (Al superstructures, bulkheads, floors), railways (Shinkansen and other trains in Japan), aerospace (Delta II rocket fuel tanks), and various automotive components.

3. Linear friction welding

3.1 The process

Whilst the idea of linear motion was patented by Caterpillar Tractor in 1969, the development of suitable machines for welding engineering size components only started just over a decade ago. A consortium of four British companies, led by TWI, was formed in mid '80s. It designed and built prototype machines with a linear reciprocating mechanism, the principle being shown in Fig.3. Based on an electromechanical system, the machines provided a reciprocating frequency of 5-75Hz and a reciprocating amplitude of 0-±3mm, with a maximum axial welding force of 150kN. The process opens up new design and manufacturing possibilities for metals, and is capable of welding square and rectangular components in one shot with accurate alignment of parts. With appropriate tooling it can be used for more 'irregular' components such as turbine blades.

Fig.3: Principle of linear friction welding

Early trials commenced with a range of both light and 'heavier' alloys, including Ti-6Al-4V (Grade 5 titanium), 6063 Al alloy, 304-type stainless steel and C-Mn steel, and were successful in producing sound joints ^[3] . Since then, tooling has improved and section sizes of 20mm x 100mm are now possible. The process also seems well suited to the joining of higher temperature materials, including light weight intermetallics such as Tialuminides and high strength nickel alloys.

3.2 Applications

To date, only the aircraft engine industry has used the process due to the very high machine costs involved. Figure 4 shows a linear friction welded blisk produced by MTU München for the Eurofighter using titanium alloy blades. In this instance the elimination of the 'Christmas tree' mechanical linkage between the individual blades and the disc, prevalent in conventional designs, yields very significant weight savings. For some years there has also been strong interest in the process from non-aero sectors, in particular mass production industries such as the automotive sector. It is expected that applications will increase as the equipment cost drops. TWI is involved with a consortium of European SMEs to design and build a low cost machine, and this is now in operation. This machine uses hydraulic actuation, allowing the use of novel stored energy approaches. It is expected that the cost will be less than half of an electromechanical machine of similar capacity and could become attractive to the smaller fabricator.

Fig.4: Linear friction welded blisk (prior to machining) produced by MTU München for Eurofighter

4. Mechanical fastening

Aluminium alloys can be difficult to resistance weld and thus require alternative methods for the formation of point joints. Blind riveting has long been employed in the aerospace sector, but self-piercing riveting (SPR), which eliminates the requirement for predrilled holes, has become an established method for the automotive sector. Similarly, clinching, which requires no consumables, provides a very low cost solution for lower strength joints. The capital outlay required for the equipment is low and productivity is high, making SPR and clinching attractive propositions for small to medium enterprises.

One of the key advantages of the above processes is their ability to join dissimilar metal alloys combinations that would be impossible to fusion weld. TWI has been active in the development of both clinching and SPR for sheet applications. The increasing use of magnesium die-castings in the automotive sector is leading to a requirement for magnesium to aluminium and magnesium to steel point joints. Currently, such combinations must be bolted, adding weight to the components and complexity to the die-castings through the need for large bosses.

TWI has been developing clinching and SPR for these applications and has succeeded in proving the concept of these processes ^[2] . The main obstacle that needed to be overcome was the poor ambient temperature ductility of the die-castings. However, by using a short time elevated temperature cycle during welding, successful joints have been made with good mechanical performance ( Fig.5). The elevated temperature SPR process (ETSPR) proved particularly suitable for joining magnesium to dissimilar metals and the Mg-Al joints exceeded the performance requirements for aluminium-to-aluminium resistance spot welds of similar total thickness, with reasonably low cost equipment.

a) self-piercing rivet joint using a coated steel rivet

b) clinched joint

Fig.5: Mechanically fastened joints between 1mm Al 6111 and 3mm Mg AM60 alloys

The ETSPR process for joining Mg to Al presents a powerful tool and, complemented by FSW, could provide a complete solution for making high quality point and seam joints for an alloy group of increasing importance to several sectors. TWI is currently seeking partners to industrialise the ETSPR process so that it can be employed in volume production.

5. Laser welding

The demands for laser welding of light alloys have increased with that for light weighting in the automotive sector and the need to improve upon conventional processes in the aerospace sector. Laser welding could soon replace riveting for some airframe aluminium structures and laser welded aluminium structures could replace spot welded steel pressed structures. Similarly, laser welding of titanium alloys offers many of the perceived benefits of electron beam welding (the conventional aerospace process for Ti), but with greater flexibility.

Laser welding of aluminium and its alloys is more difficult than laser welding of steel, but well within the capabilities of the process. When compared to steel, aluminium alloys have a higher thermal conductivity and higher reflectivity to the laser wavelength, which means that a higher power density is required to establish keyhole welding conditions than for steel. Porosity and cracking are also weld quality problems that can occur to a greater extent in aluminium alloys, but this is true of all fusion processes. The advent of Nd:YAG lasers with a power output of 4kW and fibre optic beam delivery expands opportunities in this area ^[4] . Nd:YAG lasers have the advantage of a shorter wavelength of light, which reduces the threshold of power density for keyhole laser welding of aluminium alloys compared to CO₂ lasers.

Considerable progress has been made in the last year or so in improving Nd:YAG procedures for aluminium alloys in thickness ranges from 2 - 10mm. Studies into shielding gas compositions show that helium increases penetration in highpower Nd:YAG welding compared to nitrogen or helium/nitrogen mixtures. Shielding gas coverage of the weld area needs to be very effective and rear shields and plasma control jet arrangements are essential. The cleanliness on the top, underside and edges of the weld area also needs special attention in avoiding porosity in butt welds. Porosity can be minimised using twin spot Nd:YAG laser beams, which elongate the keyhole, providing a mechanism for gas to escape, although this approach needs more exploration. Efforts have also been made to expand the lower end of the thickness range that can be successfully laser welded. In particular, pulsed Nd:YAG lasers, with high peak powers have been effective for welding thin section aluminium alloys down to approximately 1mm thickness. Another key to successful laser welding of thin sheet aluminium alloys is efficient clamping to minimise misalignment. However, a mismatch in sheet thickness can be tolerated as seen in Fig.6, which is a prototype laser welded tailored blank.

Fig.6: Nd-YAG laser weld between 0.8mm and 1.6mm 5000 series Al alloy sheets, made at 4kW and 5m/min

The most recent development in laser welding technology has been the advent of hybrid welding (combination of laser and arc welding). Such methods increase potential welding speeds, penetration and, most importantly, tolerance to joint gaps.

In the case of magnesium alloys, laser welding is an appropriate method for fabricating common wrought and gravity cast alloys ^[2] . There is good beam coupling and the main issue is avoidance of hot cracking in the susceptible alloys. However, the majority of magnesium alloy applications utilise high-pressure die-castings rather than wrought or gravity cast products. Whilst laser welding produces crack-free joints in the most common AM and AZ die-cast alloys, weld metal porosity can be gross as a consequence of trapped gas in the castings. There is no ready solution to avoid this porosity, but all fusion welding processes are equally susceptible. However, it is expected that wrought magnesium alloys will become more common and it is likely that laser welding will play a key role in the fabrication of future sheet components.

Laser welding has also been successfully applied to titanium alloys in section thickness up to 20mm ( Fig.7). Porosity is again a potential problem and gas shielding must be very effective, as in the case of aluminium alloys. Laser welding of titanium alloys is certainly not new; research in this field spans a period of over20 years, and laser welding is used in production for seam welding thin wall tubes ^[5] . However, continuous improvements in laser powers and beam quality have increased the attraction of laser welding for a broader range of military and civilian structural applications.

  Fig.7: CO₂ laser weld between 11mm thick titanium plates of Ti-6Al-4V and pure Ti

6.Closing remarks

The need to reduce manufacturing costs means that fabricators must be continuously aware of welding process developments taking place worldwide. In light alloy manufacture, there has been excellent progress in the development of the non-traditional processes, such as friction welding, laser welding and mechanical fastening. Whilst the capital investment is initially high (except for the latter process), the cost benefits can be substantial. There will be lower price friction machines and laser welding and cutting machines on the market in the next decade and small and medium size fabricators should keep abreast of these developments. In particular, it is likely that many small precision components in light alloys could be readily fabricated using small scale friction stir machines and low power lasers and this is seen as a certain trend in the next 10 years.

References

1. W.M. Thomas et al 'Friction Stir Butt Welding', International Patent Application PCT/GB92, Patent Application GB 91259788, 6 December 1991.
2. L.K. France, R. Freeman, 'Welding and Joining of Magnesium', Society of Automotive Engineers Paper 01ATT-150, 2000
3. E.D. Nicholas, 'An introduction to linear friction welding', Proc. EuroJoin 1 pp 423-432, Strasbourg, November 1991.
4. M. Naeem and S.T. Riches, 'Optimisation of high power Nd:YAG laser welding procedures for automotive application', 32 ^nd ISATA conference Advances in Automotive and Transportation Technology and Practice for the 21st Century, 14-18 June 1999, Vienna, Austria, paper 99NM062.
5. L.S Smith, M.F. Gittos and P.L. Theadgill, 'High quality and productivity joining processes and procedures for Titanium Risers and Flowlines', presented at Titanium Risers and Flowlines Conference, SINTEF, Trondheim, Norway, 17 February 1999.