The overall conclusion of this group is that joining and fastening should not be an afterthought. Rather, consideration of possible options and complications should be a part of initial material selection, manufacturing, and part design strategies. There are a variety of joining processes and options, including mechanical fastening that can be potentially applied to join Mg alloys. Multi-material solutions have great promise from a mechanical design perspective. However, in addition to concerns over galvanic corrosion, mentioned earlier, there are also significant challenges associated with joining Mg to other metals and/or polymer composites in a cost effective manner. The cost penalty associated with joining potentially challenging material combinations needs to be considered up front. Joint efficiency (i.e. the ratio of joint strength to the base-metal strength) also needs to be considered, as it can exceed 100% or be significantly lowered by a variety of metallurgical effects discussed below.
Fusion welding processes, such as gas metal arc welding (GMAW), resistance spot welding (RSW), and laser welding are attractive because of all the existing industrial knowhow and infrastructure. However, Mg weld quality can be poor. GMAW is a widely used mass production process for Al alloys, steels and stainless steels. It could also be widely useful for Mg alloys if the following fundamental issues can be solved: (1) spattering caused by high Mg vapor pressure, (2) gas porosity caused mainly by hydrogen dissolved into the weld pool, and (3) cracking caused by liquation (liquid formation and hence weakening along grain boundaries), which can occur easily because of the very low eutectic temperature (e.g., ~435oC) of many Mg alloys. Liquation cracking has been reported in fusion welding, resistance spot welding and even friction stir welding of Mg alloys. High vapor pressure, hydrogen porosity and liquation are problems that potentially can be solved by welding metallurgy approaches.
Dissimilar material joining between Mg alloys and other metals or polymer composites presents the greatest challenge. Solid state/cold processes such as friction stir welding (FSW), ultrasonic welding (UW), friction bit joining (FBJ), self-piecing riveting (SPR) are attractive, but require considerable research and development before they can be applied by the industry. For example, friction stir welding is not presently widely practiced in the automotive industry and it is difficult for certain applications. Most Mg alloys do not have sufficient high-strain rate ductility at ambient temperature required for SPR. Adhesive bonding does not appear to have been significantly investigated, but does have interest for multi-material joining where it may provide some electrical isolation to protect against galvanic corrosion.
There is also interest in developing hybrid solutions such as weld bonding and friction bit joining. There are questions regarding the compatibility of FSW with adhesives in weld bonding, and friction bit joining is only in its infancy of research and development. Different processes have different advantages and disadvantages depending on the application and materials. The optimal joining process for large-scale application of Mg is unknown. Current applications rely exclusively on mechanical fastening (bolting) coupled with galvanic isolation techniques. Science-based, systematic development and fundamental understanding of materials behavior before/during/after joining is critically needed to evaluate the other options. Although research publications on Mg joining have increased sharply recently, the main focus has been on the evaluation of various joining processes on Mg alloys. Much less has been done to understand and overcome the fundamental metallurgical issues. A widely useful process for joining Mg alloys is still not available, and this will hinder more widespread use of Mg alloys.
The effect of heat/deformation from joining processes on defects, microstructure evolution far from equilibrium, and related degradation of weld properties relative to base metal properties need to be determined. The sensitivity of the weld performance to base metal and weld crystallographic texture must be determined. There are indications that the strong texture developed during friction stir welding could render the weld vulnerable to shear loads. Finally, the interest in novel alloys, such as those containing rare earth elements raises questions regarding the effect these additions may have on alloy weldability and weld performance. Finally, there are not established protocols for inspection of Mg weld quality.
In the spirit of Integrated Computational Materials Science (other ICME topics are detailed below), it is suggested that advanced computer aided engineering (CAE) model tools must be developed and matured to accelerate the use of Mg alloys for automobile light-weighting. Such modeling tools are essential for body structure performance prediction (durability, crashworthiness, and rigidity), and body structure assembly dimensional tolerance control. These modeling tools must be able to capture the microstructure changes and inhomogeneity in the joints caused by different joining processes. Linking the desired joint properties with the underlying microstructure features by integration of joining process models with the structure performance CAE models would allow for intelligent design and optimization of the joint and joining processes for light-weighting, performance and cost-effectiveness. It is recommended that ICME needs to include material joining as an essential manufacturing technology in its future development.
Flammability and Aerospace Issues
The drive for light weighting is even greater in aerospace than in automotive. Fuel accounts for 35-40% of the cost in aerospace applications. A 20% weight reduction saves 10% fuel. A 30% weight reduction would save 10% of the entire operating cost. Aircraft manufacturers currently employ Mg castings in helicopter transmission housings, jet engine auxiliary gearboxes, thrust reversers, and a number of cockpit and cabin door fittings. However, other applications are currently under consideration, including fuselage interior, particularly seat applications, which have been considered banned by the FAA under Paragraph 3.3.3 of SAE Standard AS8049. At this point, Mg alloys meet Federal Aviation Regulations (FAR) requirements as well as Joint Aviation Authorities Europe Regulations (JAR). There has been NO known case of aircraft or helicopter accident due to Mg ignition. Nevertheless, flammability is clearly a general concern for all of the payload materials and structures, e.g. polymers, composites, Al, and (potentially) Mg alloys.
Recent full-scale flammability tests of aircraft seat structures (leg assemblies, cross tubes, spreaders, seat back frames and baggage bars) by the FAA Technical Center reveal that Mg alloy WE43 performs better than Mg alloy AZ31, but even the latter performs similarly to Al alloy 2024. Other recently introduced Mg-based materials, such as the Korean ECO-Mg (which contains CaO), also appear to have good flammability resistance. Outstanding gaps are quantitative explanations for alloy chemistry effects on flammability resistance, including rare earth element and CaO effects. Standardized seat frame testing methodologies for the FAA to use in the qualification of materials are being developed. A report describing a new test method will be submitted by the FAA project researchers to the Transport Airplane Directorate by March 2012. The review process will take several months, but it is conceivable that a path to certification of magnesium in aircraft seats could be available by the middle of 2012.
The aerospace industry is also interested in formable wrought products (sheet and extrusion) which could compete with current Al alloys in an effort to reduce weight. A goal of any such alloy design strategy must be to remain cost competitive. Due to currently escalating costs of rare earths (RE) from China, minimizing RE alloying elements could be an important approach for future applications. Of course, the interplay with the new RE mining undertakings in the U.S., Malaysia and the newly discovered undersea RE resources near Hawaii has the potential to stabilize RE pricing at lower levels than today. Moreover, there may be applications where the improvement in creep strength, texture modification, and increased flammability resistance would merit the increased cost associated with rare earth alloying.