Science and Technology of Welding and joining

how many welding and joining processes are currently available commercially welding and joining of aerospace materials pdf welding and joining of advanced high strength steels
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Comment Page 1 Monday, December 10, 2007 4:15 PM Section 4 Processing and Manufacturing The cost-effective scaling-up of new processes and new manufacturing tech- niques for the mass production of automobiles is a huge technological and economic challenge. The supply chain in the mass production automotive sector is notoriously competitive and production margins are often small. Even in niche automotive manufacture, there are significant constraints and downward pressure on costs, so that new materials and their associated fabrication and assembly procedures must offer demonstrable economic ben- efit. In recent years in some automotive segments, cost of ownership and especially fuel costs, have increased in importance and are beginning to favor the adoption of some new manufacturing processes—for example the hydro- formed Al alloy sub-chassis—allowing more durable, more cost-effective and low life-cost materials to be used. This section discusses the drivers and opportunities for new processes and manufacturing technology including: •Welding and joining •Titanium alloys in harsh environments • Casting • Durable and high-performance composites • Surface treatments in autosport Page 2 Monday, December 10, 2007 4:15 PM Page 179 Monday, December 31, 2007 5:02 PM 15 Welding and Joining J. G. Wylde and J. M. Kell CONTENTS Introduction .........................................................................................................179 Friction Stir Welding ..........................................................................................180 Clearweld™ .........................................................................................................186 AdhFAST™ ..........................................................................................................189 Laser Welding of High-Strength Steels...........................................................193 Conclusions..........................................................................................................194 References.............................................................................................................195 Introduction Within the automotive sector there is a continual drive toward reducing costs, improving performance, and increasing sustainability. Inevitably, this leads to the search for new materials and structures that will offer improved performance and reduce cost. These efforts have resulted in developments and advances in materials across a spectrum of materials including metals, plastics, ceramics, and composites. However, all too often, one vital ingredient is ignored. This is the simple question, how will these materials be joined? Thus, with few exceptions, new materials can only be effectively used in engineering structures if they can be joined to themselves, and in many cases, to other materials. These joins must be capable of being made cheaply and reliably in a mass produc- tion environment, and furthermore, the properties of the joints must be sufficient to avoid premature failure in service. Consequently, materials joining technology is one of the key enabling technologies in almost every branch of manufacturing, and the automotive sector is no exception. Thus, automotive engineers are keenly interested in 179 Page 180 Monday, December 31, 2007 5:02 PM 180 Automotive Engineering: Lightweight, Functional, and Novel Materials developments in joining technologies. Reducing the cost of manufacturing can have a major impact on price. Increasing the strength and integrity of joints can have a major impact on design, and offer the potential for reducing material thickness, and thus, reducing weight. Dozens of different joining technologies are used in the manufacture of automotive components and structures. This chapter covers some of the recent developments in joining technology that provide opportunities for improved use of advanced materials, and improved aesthetic appearance and design. Friction Stir Welding In recent years, there has been an increasing interest in the use of lightweight materials in automotive fabrication. Aluminum alloys offer considerable potential for weight reduction because of their high strength-to-weight ratio. However, they are not generally as readily weldable, particularly in a mass production environment, as many steels. For this reason, friction stir welding (FSW) has gained increasing interest since its invention some ten years ago. 1 Friction stir welding is a novel joining process developed at TWI in 1991. Engineers have long recognized that frictional heating could be used to join materials. Conventionally, one round or tubular component is rotated, and pushed against another. The frictional heating that takes place causes both components to become hot, and one or both to become plasticized. The application of a forging force to push the components together can then be used to form a solid state or friction weld. Such welds have consistently been shown to possess exceptional mechanical properties. During the past thirty years or so, a number of developments of the friction welding process have been made to allow the process to be applied to a wider range of geometries and shapes. Orbital and linear friction welding enabled the process to be applied to a variety of non-round components. However, virtually all of these techniques involve relative motion between parts to generate the frictional heating. This naturally limits the application of friction welding to relatively modest sections and components, which can be held within a machine and moved relative to each other to develop the frictional heating. Furthermore, they involve the joining process taking place at, more or less, the same time across the entire joint area. Thus, as the size or length of the component grows, so the forces involved in the application of the process also increase dramatically. 2 In the 1980s, Thomas developed other variants of frictional heating of mate- rials and showed that friction could also be used to extrude metals—friction extrusion. It was from this development and the desire to extend friction 3 techniques to larger parts, that friction stir welding was born. Thomas et al. discovered that butt-welded seams could be produced using a rotating tool Page 181 Monday, December 31, 2007 5:02 PM Welding and Joining 181 Work piece Tool shoulder a b c d a Unaffected material c b Heat affected zone (HAZ) Backing bar b a c Thermomechanically affected zone (TMAZ) Profiled pin d Weld nugget (Part of thermomechanically affected zone) FIGURE 15.1 Principle of FSW. with a shoulder and pin. The design of the tool is key to the successful application of the process, and a number of different designs have been investigated. Essentially, the height of the pin is the same as the thickness of the material to be joined. The pin is pushed into the seam until the shoulder comes into contact with the top surface of the material. After a brief dwell period, the tool is then moved along the length of the seam. As the tool moves through the material, some material is taken from the edges of the parts being joined, mixed, and transported to the back of the pin where it extrudes into the area behind the pin to form a solid phase bond. This concept is illustrated in Figure 15.1. Being a friction process, there is no melting of the material, thus the weld produced is a solid phase joint. Consequently, there is no fume and no associated loss of alloying elements. There is no porosity, as there has been no solidification from molten material. Figure 15.2 shows a transverse section through a typical friction stir 4 weld. Threadgill has characterized the various regions of the weld as indicated in the figure. The weld “nugget” shows a fully recrystallized material with a fine grain structure. Next to the nugget lies a zone of material that has been subject to considerable mechanical and thermal effects, the thermomechanically affected zone. Here, the original structure has been distorted by the mechanical motion of the tool. Some recrystal- lization has taken place and the material has been subjected to a thermal cycle. Adjacent to this, is a thermally affected region that forms a transition to base material. Page 182 Monday, December 31, 2007 5:02 PM 182 Automotive Engineering: Lightweight, Functional, and Novel Materials FIGURE 15.2 Typical transverse section through friction stir weld. A typical hardness distribution taken across the center of a friction stir weld is shown in Figure 15.3. The distribution of hardness will depend on the kind of alloy being welded, e.g., a heat-treatable or work-hardened alloy. However, there will generally be a softening in the thermomechanically affected region, and a consequent reduction in strength for the heat treatable alloys. However, invariably, researchers have discovered that the mechanical properties of friction stir welds are at least equal to, and generally exceed those, of arc-welded joints. In Figure 15.3, 5083-0 refers to annealed alloy AA5083, whereas 5083-H321 is the same alloy after cold-working and vari- ous heat treatments. Industrial applications of friction stir welding were reported within five years of the invention of the process, driven by some advantages of the process: 100 95 90 85 80 75 70 5083-O 65 5083-H321 60 –40 –30 –20 –10 0 10 20 30 40 Distance from weld centerline, mm FIGURE 15.3 Typical hardness distribution through a friction weld. Hardness, HV 2.5 Page 183 Monday, December 31, 2007 5:02 PM Welding and Joining 183 • Very low distortion • Fully mechanized process • No fume, porosity, or spatter • No melting of the base material • Cost-effective for suitable applications Although reported applications cover many industrial sectors, by far the most reported uses come from the broad area of transportation. Thus, appli- cations have been reported for ships, railway vehicles, automotive compo- nents, space vehicles, and more recently, aircraft structures. New applications for the process are arriving on a regular basis. 5 In the automotive sector, current applications include wheel rims, sus- 6 7,8 pension arm struts, and a variety of body components. Figure 15.4 shows some typical examples. In terms of the mechanical properties of friction stir welds, it is important to note that far more data are required before any definitive conclusions can be drawn concerning their long-term engineering performance. The two properties that are generally looked for by designers of aluminum structures are tensile properties, i.e., proof and tensile strength, and fatigue strength. 200 mm (a) (b) FIGURE 15.4 Examples of friction stir welding applications. (a) Wheel rim—photograph courtesy of Simmonds Wheels P/L. (b) Suspension arm strut—photograph courtesy of Showa Denko. Page 184 Monday, December 31, 2007 5:02 PM 184 Automotive Engineering: Lightweight, Functional, and Novel Materials In terms of tensile strength, various researchers have carried out 180º face and root bend tests, and cross-weld tensile tests. The bend test is a useful guide to weld quality and ductility, and provided friction stir welds are made under optimum conditions, the bend test is invariably passed without evi- dence of cracking. In terms of the cross-weld tensile tests, many comparisons have been made with base material properties and with other welding meth- ods. The results vary according to the type of alloy being tested, but are generally very encouraging. In terms of fatigue performance, friction stir welds demonstrate good properties. In general terms, welded joints possess a much lower fatigue strength than that of the base material. There is a combination of reasons for this including the presence of sharp discontinuities on a microscopic scale at the edge of the weld, a stress concentration caused by the weld shape, and the presence of tensile residual stresses. This phenomenon is well understood by designers, and various sets of fatigue design curves have been developed for arc-welded joints. Thus, the design stress for a conventional-welded joint can be determined by establishing the appropriate joint classification and determining the design stress according to the required fatigue life. Remarkably, fatigue tests on friction stir welds have indicated that they can possess a higher fatigue strength than arc welds. In some cases there seems to be very little difference between the fatigue performance of the friction stir weld and that of the base material. The reasons for this are not fully understood, and more tests are required to verify this conclusion. Preliminary results suggest that the design stress for a friction stir weld might be some 50% higher than that for an arc weld. Clearly, if this increase is confirmed with additional data, it will have very significant consequences for designers of automotive structures. In general, in fatigue-sensitive structures it is the fatigue strength of the welds that determines the design stresses, and hence, the material thick- nesses. If it is confirmed that a higher design stress can be used for friction stir welding, then this may result in some structures being fabricated from thinner section materials, and this could have major significance for the automotive industry where the use of thinner materials will reduce weight, increase performance characteristics, and improve fuel economy. Conse- quently, it is not surprising that many designers and fabricators are investi- gating the potential for friction stir welding. The vast majority of research on friction stir welding, and virtually all the production applications of the process to date, relate to aluminum alloys. However, the process has also been shown to be applicable to a range of other materials, including magnesium, copper, and zinc. It has also been shown to be feasible for steels, but further work is required before it can be considered as a production process for joining steels. One final and particularly interesting feature of friction stir welding is its ability to make joints between some dissimilar materials. For example, joints between different aluminum alloys are feasible, as are joints between cast and wrought aluminum alloys. This flexibility is particularly relevant to the Page 185 Monday, December 31, 2007 5:02 PM Welding and Joining 185 FIGURE 15.5 Wrought aluminum welded to cast aluminum alloy. automotive sector, where there is an increasing interest in the use of cast components. Figure 15.5 shows an example of a friction stir weld between cast and wrought aluminum alloys. Friction stir welding has also been used with some success to join com- pletely dissimilar materials, e.g., aluminum and magnesium alloys. This is particularly relevant to the automotive sector as there is increasing interest in the use of magnesium to reduce weight. Figure 15.6 shows a joint between aluminum alloy 2219 and magnesium alloy AZ 91. FIGURE 15.6 Friction stir weld between aluminum and magnesium alloys. Page 186 Monday, December 31, 2007 5:02 PM 186 Automotive Engineering: Lightweight, Functional, and Novel Materials Clearweld™ Plastic materials are increasingly used in automotive components. TWI in con- junction with Gentex Corporation have developed a technique for laser weld- ing plastics with an infrared absorbing material, creating a joint that is almost invisible to the human eye. Typically, carbon black would be used as the absorb- ing medium for the laser light; although the new approach enables two similar clear (or colored) plastics to be joined, with a minimal marking at weld line. The development of the laser as an industrial heat source has resulted in a range of applications that utilize the precise, controllable energy it delivers. Early developments in welding plastics with lasers showed that thin films 9 could be joined. However, at that time, CO lasers were the principle power 2 source. The nature of the interaction between the 10.6 μm wavelength beam from the CO laser and thermoplastic materials meant an analogue to the 2 deep-penetration process used to weld metals could not be developed. The CO laser beam is absorbed at the surface of the plastic, relying on conduction 2 of heat through the thickness of the material, which results in decomposition, vaporization, and charring, before any significant depth of material is melted. Nonetheless, thin polyolefin films, of the order 0.1 mm thick, have 10 been successfully welded with a CO laser at speeds up to 500 m/min. 2 The increasing use of Nd:YAG solid-state lasers, and the advent of diode lasers (both producing beams with a near infrared wavelength), has made available lasers with different beam/material interaction characteristics with thermoplastics. In transmission welding, the laser beam passes through the top (transmitting) layer and is absorbed by the filler in the lower layer, producing sufficient heat to make a weld at the interface between the two parts. This process was first described for welding automotive components 11 in 1985. An example of the transmission-welding technique, utilizing a visually transmissive plastic material for the upper section and a carbon black loaded plastic for the lower layer, can be seen in Figure 15.7. FIGURE 15.7 Laser transmission weld in 4 mm thick polypropylene using a 100W Nd:YAG laser at a speed of 1.6 m/min. The weld is at the interface between the light and dark materials. Page 187 Monday, December 31, 2007 5:02 PM Welding and Joining 187 FIGURE 15.8 Laser overlap weld in clear 3mm thick PMMA made with an infrared dye impregnated film at the interface. An extension of the transmission laser-welding process that allows com- pletely clear or similarly colored components to be welded has also been 12 described. This technique uses an infrared absorbing material, clear in the visible range of the spectrum, but tailored to absorb strongly the specific wavelength of the laser beam at the interface of the materials to be joined. Infrared absorbing pigments are also available as an alternative. The nature of the absorbing material means the laser wavelength is absorbed with high efficiency, thus requiring relatively small amounts at the interface between the two components to be joined. Initial development work on the process was mainly carried out using polymethylmethacrylate (PMMA) test speci- mens, and an example of an overlap weld made by applying a painted layer of absorbent material at the joint region between two transparent sheets of 3 mm thick polymethyl methacrylate is shown in Figure 15.8. Although the example in Figure 15.8 is shown with two visibly clear sheets of polymethyl methacrylate, the process can be used to join several other materials, colored or otherwise. The absorbing material at the interface between the materials acts as the site where the light from the laser is absorbed and converted into heat in a well-defined area. The area of heating, and hence joining, may be defined by either the size of the laser beam, or the coverage of the absorbing material used. In the experiments reported here, both Nd:YAG and diode laser light have been used. Both these laser wavelengths are easily transmitted via optical fibers, which enhances the flexibility of the process in industrial terms. The process has also been applied to fabrics. Nd:YAG lasers are usually employed in a de-focus position to produce a spot of laser energy some few mm in diameter. This energy profile is almost ideal for the fabric-welding process. The welding occurs as the heat generated in the absorbing material is sufficient to melt, of the order ≈ 0.1 mm of the polymer fabric. The heat Page 188 Monday, December 31, 2007 5:02 PM 188 Automotive Engineering: Lightweight, Functional, and Novel Materials FIGURE 15.9 TM Continuous overlap welds made using infrared absorbing dye in the fabric Goretex . generation at the interface is controlled by the absorption coefficient of the layer and the processing parameters. The main parameters for a given width of weld are laser power, energy distribution in the focus, and the welding speed. For these experiments, an Nd:YAG laser with a 7 mm diameter focal spot was used at powers between 50 and 100 watts, and welding speeds in the range 500–1000 mm/min. Figure 15.9 shows continuous, hermetic overlap welds made in the water- TM proof fabric Goretex using an Nd:YAG laser beam of approximately 100 W in power, and a welding speed of 500 mm/min. Peel and lap/shear tests were performed on 25 mm wide samples of joined material at a test rate of 5 mm/min, and the results are shown in Table 15.1. The test results are quoted as the maximum applied force per mm of seam. As a percentage of the strength of the parent materials, 25% to 40% strengths were obtained for the welded joints in a simple lap configuration. The work has shown that polymer fabric materials can now be laser welded using near infrared absorbing material as a mechanism to produce heat and localized melting. The welds produced are cosmetically appealing, and the upper and lower surfaces of the material are unaffected by the process. TABLE 15.1 Results of Mechanical Testing on a Range of Woven Fabrics Peel Lap/Shear Parent Material Thickness Strength Strength Strength Color (mm) (N/mm) (N/mm) (N/mm) Brown 0.19 0.70 2.08 8.47 Orange 0.23 2.16 5.22 13.95 Bronze 0.16 2.07 2.76 9.38 Yellow 0.41 4.40 6.79 16.12 Page 189 Monday, December 31, 2007 5:02 PM Welding and Joining 189 In mechanical testing, joint strengths of between 20% and 40% of the parent material strength have been achieved in a simple lap joint. The welding process is efficiently achieved using compact diode laser sources now avail- able commercially, and lends itself easily to high levels of automation. The process of laser welding using an infrared absorbing material has been TM given the trademark ClearWeld . In addition, patent protection has been initiated by TWI on this process. Gentex Corporation is licensed by TWI to TM exploit the ClearWeld technique. AdhFAST™ Despite widespread skepticism, adhesives are playing an increasingly impor- tant role throughout the engineering world, and will continue to find new applications in volume applications. The primary drivers behind the growth in the use of adhesives are the increasing interest in combining different materials in structures to maximize performance, e.g., plastics, metals, ceram- ics, composites, etc., and the advantages adhesives offer over traditional point joining techniques. These advantages can be broadly defined as: • Ability to join almost any material combination. • Superior fatigue properties. • Elimination/reduction of stress concentration points by bonding the whole joint area. • Ability to have mechanical properties tailored to joint function, i.e., rigidity, elasticity, toughness, coefficient of thermal expansion, etc. • Ability to have physical properties tailored to requirements, i.e., electrical and thermal conduction/insulation, and cure initiated by radiation (blue or UV light, electron beams). • Sealing ability. • Elimination/minimization of thermal distortion. However, despite these advantages, some engineers are concerned about the use of adhesives because of the perception that they are: • A poor or weaker substitute for welding or mechanical fasteners. • Messy to use. • Perceived to have significant health and safety risks. • Difficult to inspect and to assess the significance of any defects. • In need of complex pre- and post-processing. It is well established that adhesives can fulfill a structural function both reliably and effectively. For example, the brake shoes in most cars are bonded; Page 190 Monday, December 31, 2007 5:02 PM 190 Automotive Engineering: Lightweight, Functional, and Novel Materials aircraft rely upon adhesive in conjunction with rivets to bond and seal the fuselage; and composite-bonded drive shafts are used in lorries and cars. Although it is often true that an adhesive bond may never be as strong as the parent material, this can frequently be compensated for in the design of the joint to significantly enhance the overall performance of the structure. It is also accepted that the quality control (QC) behind such bonding operations can be difficult and complex, especially when the structural integrity of the joint is brought into question. These concerns necessitate the need for skilled personnel in both the design and the implementation stages of production. Without the appropriate skills, quality control confidence will be limited and there is a greater chance of failure, either though poor joint design/material selection, or during the assembly stages. Adhesives that cure at room temperature usually consist of two compo- nents that need to be mixed prior to application, and the process of applying the adhesive to the surfaces of the substrates can be messy and time con- suming. For many structures, it is common, and indeed best practice, to dry- assemble the joint first, to check for tolerance. The adhesive is then applied before reassembly in conjunction with jigging, which is used to hold the joint together while the adhesive cures and hardens. In basic terms, the process can be defined as follows: Surface preparation on the materials to be joined. This may be a simple degrease operation, but usually more complex processes are employed. For example, abrasion (hand papers, grit blast, or shot blast), chemical etching, anodizing, priming, use of coupling agents, flame plasma, or corona discharge are all used. Mixing and application of the adhesive. This is often a manual operation where beads of adhesive are applied to one or both of the surfaces with an adhesive dispenser combined with a mixing unit. To ensure complete wetting of the surfaces to be bonded, the adhesive can then be spread out evenly over the surfaces. Assembly of components. The components then have to be assem- bled and aligned correctly. This process can be messy if excess adhesive has been used. It is also often difficult to achieve accurate alignment of the components without special jigging and guides. Additional jigging and clamping. Such jigging/clamping is often required once the structure has been assembled, to apply an even pressure in the joint area while the adhesive cures. Curing of the adhesive. Many adhesives have been formulated to cure at room temperature through reactive chemistry, but there are others that require heat to react, therefore necessitating the use of an oven or heating equipment. Disassembly and checking of the structure. Once cured, time has to be spent removing the structure from the jigging, and checking that the adhesive has fully filled the joints (visual inspection). Page 191 Monday, December 31, 2007 5:02 PM Welding and Joining 191 For an adhesively bonded joint to be reliable, the bonding process must follow strict procedures by trained operators. Many companies do not appre- ciate the need for skilled staff, and this can result in failure. The arising unreliability can reinforce the perception that the adhesive is at fault, rather than the process. One way in which such problems are overcome is by combining a mechan- ical fastening system such as riveting, bolting, clinching, or spot welding with the adhesive to form a hybrid joint. The point-fastening system enables a safety back-up to be built into the joint (and some visible confidence), while retaining the superiority of the adhesive joint, especially in terms of fatigue, and sealing capacity. 13 It is from this background that the concept of AdhFAST™ arose. Adh- FAST™ is a hybrid joining system that brings together the advantages of both adhesives and fasteners, and in addition, offers a high degree of quality control with a minimum of additional operator training. In essence, Adh- FAST™ takes the form of a four-function fastener that: • Locates—enables positional accuracy between components to be defined. • Fastens—traditional function, plus acts as a jigging aid during the adhesive cure stage. • Spaces—controls the spacing between the materials to be joined, thereby enabling adhesive to be easily injected, and defining the final thickness of adhesive in the joint. • Allows adhesive to be injected—accomplished either though a cen- tral hole or down features on the sides of the fastener. The fastener, which can take a range of forms (nut and bolt, screw, rivet, etc.), fulfills its function as a fastener in that it locates and fastens the mate- rials to be bonded together. The fastener is positioned such that it sits within the prospective joint away from edges and high-stress areas. In addition to its normal function, the fastener contains a spacer element (a shaped washer or similar), which contains grooves or features that will allow a gap to be maintained between the two spacers. In turn, the fastener is designed in such a way as to allow liquid or paste adhesive to be injected through or past it, and around the spacer element into the bond cavity. The adhesive can, therefore, be pumped into the bondline to fully fill the joint from the inside out. Provided appropriate surface preparation has been carried out on both surfaces to be bonded, the joint will be fully wetted by the adhesive prior to curing. By using fasteners, the assembly and bonding process is simplified: Surface preparation, the materials to be joined. This part of the operation cannot be changed as the type of pre-treatment defines the level of adhesion attainable to the surface of each substrate. Page 192 Monday, December 31, 2007 5:02 PM 192 Automotive Engineering: Lightweight, Functional, and Novel Materials Assembly of components. The components are aligned, assembled, and held together using fasteners. The structure and associated joints can be quite complex in shape in that adhesive injection allows more than one component to be bonded at any one time. A structure using fasteners will not require additional external jig- ging. The edges of the joint may need to be sealed, which can be done in a number of ways including using adhesive release tape, using inflatable bellows, or a simple gasket. Adhesive injection. The adhesive is then mixed as normal and inject- ed into the joint cavity through the fasteners. As the adhesive fills the joint, its progress can be monitored by its appearance out of the hole in the next fastener. The injection process is then continued through that fastener after sealing the previous one. The amount of adhesive that the operator is exposed to is minimal. Curing of the adhesive. As described previously, the adhesive cures either on its own, or with the application of heat or some other energy source. Disassembly and checking of the structure. The only disassembly needed may be the peeling away of sealing tape, as no additional jigging is required. Visual inspection is as usual. Employing fasteners enables the following further benefits: • No external jigging • Simplified dry assembly with accurate location and checks of tolerance • Protection of pre-treated surfaces prior to bonding from excessive atmospheric exposure and operator contamination • Minimal operator exposure to uncured adhesive • Simplified adhesive application process • Accurate metering of adhesive within the joint • Accurate bondline control • Saving in time due to elimination of jigging assembly/disassembly In addition to the above benefits, fasteners offer a change in the manufac- turing approach to bonding by breaking the linearity of the process, i.e., there can now be a dry assembly stage followed by a separate injection stage. In reality, this means that these operations could be done in different geo- graphical locations, or at different times, depending upon production and manpower resources. With the correct selection of surface pre-treatment where a bonding window of days or weeks was possible, the storage of dry assembled parts ready for bonding, with the possibility of disassembly and re-use should an order be changed or amended, is made possible. Page 193 Monday, December 31, 2007 5:02 PM Welding and Joining 193 FIGURE 15.10 Typical AdhFAST™ joint. Industries likely to benefit from a hybrid fastener/adhesive approach include consumer goods, aerospace, automotive, railway, and shipbuilding. A typical AdhFAST™ joint is shown in Figure 15.10. Laser Welding of High-Strength Steels Despite the considerable increase in the use of aluminum and magnesium alloys and other advanced materials, it is almost certain that steels will be continued to be used for vehicle production for many years to come. High- 2 strength steels (UTS 600 N/mm ) are increasingly used to meet the severe requirements imposed by the automotive industry in terms of safety, reli- 14 ability, and reduction in gauge for energy saving. TRIP (transformation induced plasticity) steels have become of considerable interest in recent years because of their exceptional combination of high strength and ductility. Resistance spot welding is the main joining method for these steels but other methods such as laser welding are increasingly being investigated in the automotive industry, where there remains a need for weldability and joint performance data for these steels. The high carbon equivalent in TRIP steels, coupled with fast weld-cooling rates associated with the welding process, leads to high hardness levels Page 194 Monday, December 31, 2007 5:02 PM 194 Automotive Engineering: Lightweight, Functional, and Novel Materials (up to 580 HV) in the weldment. Typically, resistance spot welding is achieved using conventional procedures, which give interface (brittle) frac- ture on testing, and make the welds unable to meet current automotive welding standards. The restricted performance is linked to the high hard- ness/low toughness levels within the weld nugget and heat-affected zone (HAZ). Modified welding procedures, such as long weld times and post- 15 weld tempering, have been suggested to reduce this weld brittleness. However, these are not always feasible or practical because of the increased cycle time, and their effects on the static and dynamic properties are not yet clear. Given the continued move toward the implementation of higher strength steels in the automotive industry, the benefit of achieving plug weld fracture modes associated with high-quality welds for TRIP steels is clearly evident. Laser welding is increasingly used in the automotive industry as an alternative to resistance spot welding. It is generally recognized that a continuously welded joint can provide increased stiffness compared to 16 an equivalent resistance spot-welded joint. Further development of laser welding techniques, such as twin-spot beam and laser-arc hybrid, also 17 has the potential to reduce the susceptibility to cracking, and to maxi- mize the benefits of TRIP steels. There is little information presently available related to the laser weldability and weld performance of TRIP steels. This serves to demonstrate the importance of information relating to mate- rials weldability before any material can successfully be used in production. TWI and others are researching many of these issues. Conclusions The strive to reduce cost, and improve performance and sustainability con- tinue to interest automotive engineers in the new materials. However, all too frequently, engineers ignore the question of how such materials can be joined until far too late in the design and manufacturing process. Thus, the ability to weld or join materials safely and cost effectively in a production environment is vital to the successful application of new materials to engi- neering structures. Advances in materials joining technology continue to meet the chal- lenges provided by new materials development, and offer new opportu- nities for designers and manufacturers of automotive products. This chapter has briefly looked at a number of recent developments in joining technology that offer potential for joining a number of similar and dissimilar material combinations, and introduced some of the areas in joining tech- nology that will receive the attention of the automotive industry in coming years. Page 195 Monday, December 31, 2007 5:02 PM Welding and Joining 195 References 1. Thomas, W. M., Nicholas, E. D., Needham, J. C., Murch, M. G., Temple-Smith, P., and Dawes, C. J., Improvements relating to friction welding, International patent application PCT/GB92, 6 December 1991. 2. Thomas, W. 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