Magnesium Joining: Process Advances and Future Requirements

Magnesium Joining - Process Developments and Future Requirements

 

L Keith France and Richard Freeman 

Presented at International Magnesium Association World Magnesium Conference 2001, Brussels, Belgium 20-22 May 2001.

Introduction

Driven largely by the desire to reduce the weight of products, the magnesium supply industry is predicting significant growth in the use of the metal; estimates vary from 10 kg/vehicle to as high as 60 kg/vehicle in 10 years time ^[1] . As structures become more complex, so the necessity to join magnesium to itself will grow. However, with a few exceptions, magnesium components will constitute only a part of the total structure, and the need to join such components to dissimilar materials will assume increasing importance; indeed this could become a limiting factor in the increased use of magnesium. A wide variety of industries would benefit from improved magnesium joining methods, including the aerospace, automotive & road transport, food processing and electronics sectors. As part of the continuing research and development activities at TWI, several joining methods have been investigated for making joints of magnesium to itself and to aluminium.

This paper will focus on the options for making high performance joints for magnesium. The techniques covered will be laser and friction stir welding, and mechanical fastening techniques. The paper will conclude by reviewing past and present magnesium joining research programmes, and suggest areas for further collaborative research and development to ensure that the use of magnesium continues to thrive by meeting the ever more demanding needs of industry.

Challenges

The challenges facing the joining of magnesium alloys to themselves or to other metals are essentially the same as those encountered in most joining processes: joint integrity (notably - for welding processes - porosity, susceptibility to cracking and toughness) and, for joints to different materials, chemical incompatibility, whether within the joint (particularly for fusion processes) or as the origin of corrosion problems. Of course, all the technical possibilities have to be balanced against speed and cost.

Magnesium to magnesium joining

The spectrum of joining processes available for magnesium (not all of which are reported on further here) is similar to that for other metals:

• Seam joints can be performed using arc welding (GTAW, GMAW), laser welding, or friction stir welding.
• Point joints may be made using resistance spot welding or various mechanical fastening techniques.
• Bonding processes are also attractive, either alone or as hybrids with mechanical fastening.

Laser welding

Laser welding of magnesium offers the same potential advantages as it does with other metals, high speed, autogenous and neat, narrow welds, together with the same disadvantages of high capital cost and relatively poor tolerance to fit-up tolerances. In fact magnesium seems to be particularly suited to laser welding, with good coupling with the laser beam and a low threshold energy density for initial coupling; lower speeds may be necessary to help prevent cracking in susceptible alloys. Examples of laser welding are, for alloy AZ91, good weld profiles in cross section and good, neat bead profiles; see Figs 1, 2. Alloys AM50/AM60 also show very good weld ability with rough-smooth weld bead surfaces.  

Friction stir welding

The friction stir welding (FSW) process was invented by TWI in 1991. It is a non-fusion process whereby a rotating tool with a profiled pin is pressed into the material to be welded and is traversed through the interface between the two components ( Fig.3). It is characterised by a weld nugget having a very fine, dynamically re-crystallised structure surrounded by a thermo-mechanically affected zone. FSW was originally developed for aluminium alloys, but work at TWI has shown that it is well suited to magnesium. Fig.4 shows cross-sections of FSW welds in AZ91 and AM50 magnesium alloys. (It should be noted that the rather low speeds represent early work; tool developments in respect of aluminium indicate a good prospect of higher speeds in the future. This is an area where development work is specifically needed.)

Micrographic examination of these welds illustrates one of the main features of FSW in any material. Magnesium components are usually cast, and even good castings contain some porosity; however, the squeezing and mixing that the FSW process entails results in a fine-grained, highly homogeneous structure, with the porosity effectively eliminated.

Tensile properties - Welds in alloys AZ91 and AM50 show rather different behaviours. In AZ91 ( Fig.5), welds exhibit properties that are essentially the same as those of the parent material, with the majority of the failures being in the parent material, for weld speeds in the range 90 to 120 mm/min. In contrast, welds in AM50 ( Fig.6) had properties generally worse than for the parent material, with no preferred failure location, though it should be noted that the results in the parent material showed a considerable degree of scatter.

Fatigue properties - Given the extremely homogeneous micrographic structure of a typical friction stir weld, good fatigue behaviour might be expected, and this is in general found to be the case. In the case of AZ91, while there was considerable scatter, the results for FSW joints were very similar to those of the parent material. Alloy AM50 gave much more scattered results and generally inferior performance in comparison to that of the parent material. This is line with other work that has been done on non-vacuum electron beam welding.

Joining magnesium to other materials

While 'other materials' might cover a vast range of possibilities, in practice it is the joining of magnesium to aluminium that is seen as the most important area, and the present paper will focus mostly on this process, though some magnesium-steel results are mentioned.

Fusion welding

Numerous attempts have been made to join magnesium to aluminium using laser welding and resistance spot welding, but they have invariably led to failure. The two metals react to form brittle intermetallic compounds in the melted zone and the weld literally falls apart. While there are proposals to overcome this (for laser welding) by the use of special filler materials, it is difficult to believe, given a molten zone perhaps 3mm wide, how inter-diffusion of the two components can be prevented outside a highly controlled laboratory environment.

Friction stir welding

FSW is known to be one of the joining process most tolerant to different materials, and the possibility of using the process for magnesium to aluminium was immediately attractive. A small amount of work has been done to demonstrate the basic feasibility of the process, and initial results are encouraging ( Fig.7). The crucial feature of the process that makes this success possible is that the 2 materials are plasticised but do not melt. The joint is, in effect, a complex mechanical interlock and there is no evidence for the formation of intermetallic compounds. However, this work is in its infancy, and the characterisation of these joints has yet to be done. Apart from mechanical and fatigue data, it will be necessary to ensure that the materials can withstand high manufacturing temperatures (e.g. during painting) and high in-service temperatures for long periods; corrosion issues are also likely to be significant.

Mechanical fastening

Until recently the only mechanical fastening system for magnesium to other materials has been by bolting. While this method performs well, it is unlikely to be suitable for mass production vehicles for weight, cost and speed considerations. In recent years there has been a great deal of development for aluminium of self-piercing riveting (SPR) and clinching, and it was natural to try to extend these processes to magnesium-aluminium. This has been done with considerable success ( Fig.8); in addition, SPR has been demonstrated for Mg-steel joints.

The principal difficulty is caused by the poor ambient temperature ductility of magnesium, necessitating heating of at least the magnesium component prior to making the joint. While this is easy under laboratory conditions, the successful industrialisation of the process will require the development of machine tools capable of providing the right temperature conditions in an acceptably short time; it is believed that, by using a highly localised mechanism applied to the magnesium component only, a cycle time of 5s is achievable. This is an important area for further work.

Joint performance - To give an indication of joint performance, Fig.9 illustrates the shear strength of SPR and clinched joints in relation to an M8 steel-bolted joint - a severe comparison. Not surprisingly they are considerably weaker. However, a more useful comparison is against the broken line, which indicates the strength of a comparable resistance spot welded (RSW) joint for aluminium-aluminium: a recognised technique that is regarded as having acceptable properties; in every case the SPR and clinched joints are superior. In the case of tensile loads, SPR joints were found to perform significantly better than clinched joints for a wide range of alloy combinations. All the joints performed considerably better than equivalent RSW joints in aluminium-aluminium. Some hybrid (SPR + adhesive) joints were also tested, and it was interesting to note that, in some cases, these were weaker than the equivalent SPR-only joint; this effect can be attributed to the entrainment of adhesive into the SPR joint, adversely affecting the details of the expansion of the rivet.

Corrosion - Some corrosion testing has been performed on a number of joints. While the number and nature of the tests mean that sound statistical conclusions would be premature, it is possible to make some tentative remarks:

• Both the ASTM B117 salt spray test and the GM9450P cyclic salt spray test caused corrosion in the joined components, as would be expected from the materials involved. Most important, in the case of SPR, there was no attack of therivet.
• The presence of adhesive is beneficial.
• Clinched joints were generally inferior to SPR joints due to the geometry (causing water to collect) and thinning of the section.

New work needed

TWI has done a significant amount of work on the joining of magnesium over the past 3 years, including:

• laser welding (Nd:YAG, CO ₂) of AM50, AM60, AZ91, RZ5, WE43, Z5Z, ZE41, ZK51.
• a joint industry project on the joining of magnesium to steel and aluminum (SPR and clinching);
• resistance and fusion welding of magnesium MMCs;
• the EU project MAGJOIN - Laser welding and friction stir welding;
• fatigue performance of Mg FSW joints;

and the foregoing sections have given a summary of the main results achieved. However, it has to be recognised that, in comparison to the joining of well-established materials, magnesium joining is still in its infancy.

The work that needs to be done if the expansion of the use of magnesium is to proceed as predicted can be divided into two parts, process development and joint characterisation. The current situation may be summarised as follows:

• Conventional processes for magnesium-magnesium joints are now relatively well understood, and a reasonable amount of design data are now widely available
• Although SPR and clinching (for lower performance applications) are now clearly serious contenders for joining, both for magnesium-magnesium and magnesium-aluminium/steel, the development of a tool that can provide the necessary heating cycle at a sufficiently high speed is seen as crucial; the same consideration applies to clinching. The characterisation of these joints needs a great deal of work.
• FSW shows considerable promise for producing high quality joints but, compared with its use for aluminium, much more work needs to be done. Most importantly this will be to do with increasing the speed of the process, where the current status of FSW for magnesium (~150 mm/min) compares unfavourably with that for aluminium (2 m/min). Joint characterisation is an urgent requirement.
• In the case of magnesium-aluminium joints, corrosion aspects need more work; this is likely to be particularly the case for FSW, where the intimate contact between the two components will require special attention.

Conclusions

It is concluded that:

• Laser welding is a very capable process for magnesium-magnesium joining, but is unlikely to be successful for magnesium-aluminium.
• FSW offers great promise for magnesium-magnesium and magnesium-aluminium seam joints.
• SPR and clinching, particularly the former, are viable processes for magnesium-aluminium point joints.
• A wide spectrum of processes is now available for making high performance seam and point joints in magnesium-magnesium and magnesium-aluminium.
• However, a great deal of detailed development work remains to be done before the promise of these techniques can be realised in practical applications.

Acknowledgements

The invaluable assistance of the authors' colleagues Lee Smith and Scott Lockyer is gratefully acknowledged.

References

1. Diem W: 'Magnesium in different applications'. Auto Technology 2001, 1 (1) 40-41.