Solid state welding of aluminum and its alloys

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Production engineering department Alexandria UniversityAssigned to Dr:Islam Elgaly

Table of ContentsIntroduction1Solid state welding1Cold welding1Explosion welding1Magnetic pulse welding (MPW)1Friction welding1Ultrasonic welding (UW)1Friction stir welding (FSW)1 list of references16

Table of figures

Figure 1(a) _Cross-sectional view of a copper to aluminium magnetic pulse weld...........6Figure 1(b),(c) copper to aluminium magnetic pulse weld .....7Figure 2 (a)(d) Schematic of the basic steps involved in friction welding..8Figure 3(a) Schematic illustration of an ultrasonic welding setup.......9Figure 2(b), (c) cross-sectional view of an ultrasonic weld, the wavy nature of the bond....10Figure 3- Schematic illustration of friction stir welding.....11Figure 4(a)Schematic illustration of friction stir spot welding 13Figure 5(b) cross-sectional views at various magnifications of FSSW..14Figure 5- cross-sectional view of a FSW.....14Figure 6 _schematic hardness profiles16

IntroductionMost manufactured products are assembled from parts or components. As such, joining and welding are key enabling technologies for manufacture. The quality and reliability of a manufactured product are often determined by the quality of its joints. the selection of the best process for a particular material in a given application is an important consideration. The general use of aluminium worldwide is expected to continue to increase, particularly in the transport sector. Increased use of lightweight materials, including aluminium alloys, in transport applications is well recognized as a strategy to reduce the vehicle mass, thereby reducing fuel consumption. The high strength-to-weight ratio, excellent formability and corrosion resistance of aluminium alloys make them materials of choice for the aerospace, marine and rail transport industries. This means that many of the joints will be between dissimilar materials which can compound the problems of quality and reliability in the final product or subassembly. To avoid joint failures, developing or selecting the appropriate joining technology is a critical priority, and has to become an integral part of product design. There are two fundamental approaches to joining a material to another of similar (or dissimilar) composition. The principal difference between the two approaches is in the nature of the bond created, which, in turn, influences the nature of the joint and its strength. In one approach, the parts are held to each other by mechanical means; in the other case, bonding between the materials or parts occurs at the atomic/molecular level. Mechanical joining uses the first approach while fusion welding, solid state welding, brazing and soldering, and adhesive bonding fall under the second category. Aluminium and its alloys can be joined by all of these different joining technologies, with varying degrees of success. My objective is to present an overview of the solid state processes used for joining aluminium and its alloys and highlight recent advances which have enabled the joining of these materials, both to themselves and to other materials, more economically, at higher productivity and with excellent quality and reliability.Solid state welding Solid state welding processes are a group of processes that accomplish joining at the faying surfaces of parts, through plastic deformation of the parent material(s) due to the application of pressure, at temperatures below their melting point, without the addition of a filler material that melts A metallurgical bond is produced, with assistance from solid state diffusion. Some heat is generated by the process or supplied externally, which enables plastic deformation to occur at lower stresses, without any melting occurring. The energy sourcesThe energy sources for solid state welding processes are generally mechanical in nature, either directly the pressure applied or derived from frictional forces. Heat and pressure, either individually or in combination, together with time.Advantages over fusion processesSolid state welding processes offer several advantages over fusion processes: No melting occurs generally The heat input is lower than in fusion processes There is less disruption to the microstructures of the materials being joined and hence less effect on their properties. Problems of hot cracking and porosity A cast solidification microstructure at the weld, are avoided. In the absence of fusion, intermixing of the materials involved tends to be minimal on a macroscopic scale in most of these processes, and so materials of dissimilar compositions can often be joined.

This group of welding processes includes cold welding, hot pressure welding, forge welding, roll welding, explosion welding, friction welding, ultrasonic welding, magnetic pulse welding and diffusion welding. It is interesting to note that some of the oldest welding processes ever used are solid state processes (forge welding) as also some of the newest (friction stir welding). In this report, I examine briefly the solid state welding processes that are commonly used in the joining of aluminium alloys.

Cold welding Cold welding (CW) is performed without the addition of heat. An external pressure is applied to the two parts being joined, resulting in substantial plastic deformation. Accordingly, a fundamental requirement of CW is that at least one of the materials being joined is ductile and does not display significant work hardening. As such, commercially pure aluminium and some of its alloys are well suited to CW. CW can be conducted in both butt and lap configurations, In the butt mode, the metal expelled from the joint during welding has to be removed mechanically. The plastic deformation occurring in the butt mode breaks up the oxides on the surface and these are also expelled with the flash. Pre-cleaning of the faying surfaces conditionspre-cleaning of the faying surfaces is not as critical for butt joints, but in lap joints, surface cleaning is critical, usually degreasing followed by wire brushing is carried out. It is also important that there is little delay between the cleaning and CW. While CW does not produce a HAZ, it does produce a mechanically affected zone (MAZ) at the bond region, where the material has been subjected to strain hardening as a result of the imposed plastic deformation. Tensile testing of samples from a butt joint of commercially pure aluminium, designed to fail at various regions in the MAZ has shown that the tensile strength of the bond is higher than that of the base material, and that in a standard tensile test, failure occurs away from the bond line, in the unaffected base material . Aluminium alloys of the 2xxx and 7xxx series, which cannot be fusion welded due to their tendency for hot cracking, can be successfully cold welded in the butt configuration. Butt joints can be made in most aluminium alloy wire, rod, tubing and simple extruded shapes. An upset distance of approximately 1.5 times the material thickness is required for butt joints in soft annealed alloys; higher strength alloys require greater upset distance, about 45 times the material thickness. Welds in lap joints require a thickness reduction of about 70% at the weld location.Explosion weldingExplosion welding uses energy from the detonation of an explosive to produce a solid state weld.Explosion welding is limited to lap joints and to cladding of parts with a second metal. The process is used for cladding carbon steel, stainless steel, copper or titanium with aluminium. Conventional fusion welding is used to weld similar metal on each side of the segment. In effect, aluminium to steel joining is achieved, without the problems associated with joining these two metals directly to each other.Procedure The controlled detonation of a properly placed and shaped explosive charge causes the properly aligned workpieces to come together extremely rapidly, at a low contact angle. When this occurs, the air between the workpieces is squeezed out at supersonic velocities. The resulting jet cleans the metal surfaces of oxides and causes highly localised rapid heating to high temperatures. The high strain rate deformation occurring at the immediate vicinity of the impacting parts, caused additional heating. The clean surfaces are compressed together under high pressure from the explosion, which promotes the formation of a metallurgical bond. The bond zone of an explosion weld has a characteristic wavy appearance, reflecting the severe, highly localized plastic deformation occurring in the process.Magnetic pulse welding (MPW)Magnetic pulse welding (MPW) is analogous to explosion welding in that the high velocity impact and jet phenomenon occur, but the energy required is produced by magnetic pulses, rather than by controlled explosion.The process is ideally suited for metals with high conductivity, such as aluminium and copper. As a solenoid coil is used, the process is limited to tubular parts, but in a recent development either a double layer H-shaped coil or a single layer E-shaped coil is used for joining flat, overlapping sheets of aluminium and steel.Procedure A typical MPW system, shown in Figure (a) below, consists of a power supply which contains a bank of capacitors, a fast switching system and a coil. Figure 7(a) _Cross-sectional view of a copper to aluminium magnetic pulse weld

The parts to be joined are inserted into the coil, the capacitor bank is charged, and the low inductance switch is triggered by a pulse trigger system causing the current flows through the coil. When current is applied to the coil, a high-density magnetic flux is created around the coil, and as a result an eddy current is induced in the workpieces. The eddy currents oppose the magnetic field in the coil and a repulsive force is created. This force drives the workpieces together at an extremely high velocity and creates an explosive or impact type of weld. MPW displays the advantages of a solid state welding process: The ability to join dissimilar materials and no HAZ. The welding speed is very high as the complete weld is made in milliseconds. The power demand is low and the process is claimed to have excellent reproducibility.Potential applicationsPotential applications in the automotive industry include welding of aluminium fuel filters, tubular seat components involving both steel and aluminium materials, drive shafts and hydroformed parts. Figure 1(b),(c) copper to aluminium magnetic pulse weld

Figure 1(b) shows an aluminium tube that has been welded to a copper tube placed coaxially within it, by MPW.

As seen in the optical micrograph in Figure 1(c), a wavy, metallurgical bond has been formed between the two dissimilar materials.Friction weldingFriction welding is a solid state joining process, which is based on the conversion of mechanical energy into heat at the faying surfaces of the parts being joined by causing relative movement between them while in intimate contact under a compressive force.The most common form of friction welding uses rotary motion and is best suited for joining circular parts in rod, bar, tube and pipe form.Linear friction welding, based on a linear reciprocating motion has also been successfully used to join aluminium parts without circular symmetry. However, the process is not capable of welding longitudinal seams in a flat plate. Most aluminium alloys can be joined by friction welding, including those belonging to the 7xxx series which are not fusion weldable because of their cracking susceptibility. Pre-weld cleaning is not as critical as for other welding processes as the rubbing action at the interface breaks up the oxide layer and exposes clean metal. Joint strength approaching that of the base material can be obtained, even for these high strength heat treatable alloys. The process also enables joining of aluminium to other metallic materials: two widely used combinations are aluminium to copper alloy joining, used in the electrical industry, and aluminium to stainless steel joining, used for producing transition couplings in piping systems and pressure vessels Procedure The relative movement may be achieved by rotation or by angular or linear reciprocation. Figure (2) displays schematically the basic steps involved. The process begins by rotating one workpiece while the other is held stationary, Figure. 2 (a). When the required rotational speed is attained, the two workpieces are brought in contact, as shown in Figure. 2 (b). Frictional heating occurs at the interface and softens the materials at its vicinity, and upsetting starts, Figure. 2 (c). A hot forging action occurs under the applied compressive force, forming a metallurgical bond at the interface, as shown in Figure. 2(d). The rotation is stopped and the upsetting is completed. Figure 2 (a)(d) Schematic of the basic steps involved in friction welding

Ultrasonic welding (UW)Ultrasonic welding (UW) accomplishes joining by the local application of high frequency, low amplitude vibratory motion to the workpieces. The self-cleaning nature of UW and its ability to form metallurgical bonds without melting are both important advantages of the process.As in other friction welding processes, metallurgical bonds can be obtained without melting at the bonding interface. The process is not suitable for joining thicker sections. All aluminium alloys can be ultrasonically welded, but the degree of weldability varies with the alloy and temper. In common with other solid state welding processes, dissimilar joining of aluminium to other metals is possible with this process.The process has been used to join foils and sheet gauges of aluminium alloys, as well as to join thin wires to sheet and foil. Figure 3(a) shows the basic process set up for making spot welds in metals.Procedure An electromechanical converter (such as a piezoelectric transducer) converts high-frequency electric current to mechanical vibrations. The mechanical vibration is then modulated and amplified by the booster/horn before it is applied to the workpiece through the sonotrode. A moderate clamping force is applied to ensure that the mechanical vibration is transferred to the sheet-to-sheet interface (the faying surface), where the weld is created. Typically, the mechanical vibration is at 2040 kHz with an amplitude range of 550 mm. The power delivered to the workpiece is in the range of several hundred to several thousand watts, although more powerful UW power sources are being developed.Figure 3(a) Schematic illustration of an ultrasonic welding setup

Figure 8(b), (c)Figure 3(a) Schematic illustration of an ultrasonic welding setup

Figure 9(b), (c) cross-sectional view of an ultrasonic weld, the wavy nature of the bond

Figure 3(b) shows the cross-sectional view of an ultrasonic weld made between overlapping 1 mm thick sheets of AA 2024 and AA 6061 aluminium alloys. Figure 3(c) shows the wavy nature of the bond formed is visible in the higher magnification image of the weld interface.Friction stir welding (FSW)Friction stir welding (FSW) is undoubtedly the most significant new development for the joining aluminium and its alloys.FSW was first applied for joining aluminium alloys, and subsequently developments are ongoing to apply it to other materials such as copper alloys, magnesium alloys, steels, nickel alloys and titanium alloys.The basic concept of FSW is remarkably simple. Figure (4) illustrates the main features of the process and also defines the terminology used to describe these features. Procedure A non-consumable, rotating tool with a cylindrical shoulder and a specially profiled pin is plunged into the joint line between two abutting or overlapping workpieces. The tool serves three primary functions:1. heating of the workpieces, 2. movement of material to produce the joint3. Containment of the hot metal beneath the tool shoulder. Heat is generated at the workpieces, both due to the friction between the workpieces and the rotating tool pin, and by the severe plastic deformation of the workpieces. The localized heating softens the material around the pin, without reaching the melting point. The tool rotation and translation along the joint line causes the softened material to move from the front to the back of the pin, thus filling the hole left by the pin and forming a solid state joint.Figure 10- Schematic illustration of friction stir welding

As shown in Fig4, the side of the weld for which the tool moves in the same direction as the traversing direction is called the advancing side; the other side where tool rotation opposes the traverse direction is referred to as the retreating side. The process is thus asymmetrical, as most of the deformed material is extruded past the retreating side of the tool. Joining is accomplished by the combined action of heat and deformation, but there is no bulk fusion; thermocouple measurements and microstructural evidence indicate that during FSW of aluminium, temperature stays below 500. Tool design has a critical effect on the microstructure and mechanical properties of friction stir welds. The main process parameters are the tool rotational speed and the traverse speed.

The main advantages of FSWThe main advantages of FSW are summarized in Table below.AdvantagesDisadvantages

FSW, as a solid state process, can be applied to all the major aluminium alloys including hard-to-fusion weld alloys of 2xxx, 7xxx and 8xxx series.

Avoids problems of hot cracking, porosity, etc. common to fusion welding. Mechanized process, no specialized welding skills required. No shielding gas or filler wire required. Remarkably tolerant to poor quality edge preparation: gaps up to 20% of plate thickness can be tolerated. Absence of fusion leads to significant reduction in distortion. Welding in any position; both butt and lap geometries. Excellent mechanical properties. Energy requirements fall between those for laser welding and GMAW. High welding speeds and high joint completion rates: single pass welding for a wide range of sheet thickness (0.550 mm or more). Absence of filler wire means process cannot be easily used for fillet welds. Lacks flexibility of manual processes when access is difficult or complex weld shape is required. Presence of a hole at the end of weld may be a disadvantage, but solutions have been devised. Workpieces need to be restrained in well-designed support tooling to react to the applied forces and to prevent the probe from pushing the workpieces apart. Efficient power consumption is dependent on matching machine size with the size of the weld, but this is not always practical.

Table 1_comparison between advantages and dis advantages of FSW

Applications The process is now mature and robust and is becoming increasingly well established in the fabrication of critical components made from aluminium alloys in the marine, aerospace, rail and automotive industries. The first major application of FSW in the aerospace sector was in the welding of fuel tanks for the Delta II and later Delta IV rockets. The process has also been used in the fabrication of the large fuel tank for the space shuttle

Friction stir spot weldingWithin recent years, a variant of FSW has emerged as a technology for rivet replacement, primarily in the automotive industry: friction stir spot welding (FSSW), also referred to as friction spot joining.The major difference between FSSW and FSW is that there is no translation of the tool in the former process.The process was implemented in the automotive industry on the Mazda RX-8 aluminium rear door in 2003; in 2005, Mazda went on to use the process for joining the aluminium alloy trunk lid to galvanized steel bolt retainer in its MX5 (Mazda Motor Corporation-News Release, 2005; Gendo et al., 2007). Procedure As shown in Figure 5(a) a rotating tool with a pin at its tip is plunged into the overlapping sheets. The frictional heat generated between the rotating tool and the work piece causes plastic flow of material. A strong compressive forging pressure is generated when the tool shoulder contacts the sheet surface and then moves down further into the overlapping sheets. The tool is retracted after an optional dwell period. At this point, a solid state bond is formed at the interface between the two sheets. Figure 11(a)Schematic illustration of friction stir spot welding

Figure 5(b) cross-sectional views at various magnifications of FSSW

Figure 5(b) displays the cross-sectional view of a friction stir spot weld made between overlapping sheets of an Al-Mg-Si alloy (AA 6060-T5), 1.7 mm in thickness; the higher magnification images in Figure. 5(b) show the nature of the annular solid state bond formed during FSSW.

Microstructural features of friction stir weldsThe wide range of strain, strain rates and thermal cycles to which different regions of a friction stir weld are exposed means that a wide variety of microstructures form at the joint.Figure 12- cross-sectional view of a FSW

Figure 6 shows the cross-sectional view of a friction stir weld in which the regions with the different microstructures and the nomenclature used to describe them, are highlighted.

As we have seen, in a fusion weld, the highest temperature experienced by the solid parent material is its melting point and that at the fusion line, the temperature attained is the solidus temperature of the alloy. As in the case of fusion welds, the material remote from the joint line, which is not deformed or has experienced a thermal cycle that caused detectable changes in the microstructure and mechanical properties, is called the unaffected material, base or parent material The region close enough to the weld for thermal cycles to have modified the microstructure and/or properties, but no apparent plastic deformation is detected, is referred to as the HAZ, analogous to the HAZ in fusion welds. The HAZ is detected under an optical microscope by a change in etching response and by a change in hardness. In contrast, the highest temperature experienced by the material in the joint area of a friction stir weld is significantly lower than the bulk alloy melting temperature.

In heat treatable aluminium alloys, it is widely accepted that some coarsening of precipitates occurs here, and possibly, precipitate dissolution. In strain hardened non-heat treatable alloys, dislocation networks may recover, causing low angle grain boundaries to form. As the weld centre is approached, clear evidence of plastic deformation is discernable in the grain structure. In the thermomechanically affected zone (TMAZ), the material has been plastically deformed by the FSW tool and heat from the processing has also affected the material. In the outer part of the TMAZ, the original grains are identifiable in the deformed structure, but closer to the weld centre line, in the region referred to as the nugget or stirred zone, the strains, temperature and time at elevated temperature are such that dynamic recrystallization occurs, resulting in a fine, equiaxed grain structure. The processes occurring in the nugget (apart from recrystallization) differ depending on the type of alloy being considered. For non-heat treatable alloys, annealing is the only process that can occur If the base metal is in a strain hardened condition, then the nugget displays a substantial decrease in hardness relative to the base metal; if the starting temper is fully annealed , then the properties of the nugget is virtually similar to that of the base material, with a small strengthening increment due to microstructural refinement. In heat treatable alloys, the processes occurring within the nugget are more complex; depending upon the alloy and the combination of process parameters used, the nugget may become overaged, partially solution heat treated or a single phase solid solution. Transverse micro hardness profiles can provide insights into the effect of these changes occurring within the various regions on of a friction stir weld. Figure (7) presents schematic hardness profiles which summarize the effect of the microstructural changes occurring in friction stir welds made in heat treatable and non-heat treatable aluminium alloys.list of refrences Fundamentals of aluminium metallurgy Production, processing and applications Friction stir welding: from basics to applications Joining of aluminium and its alloys 607 S. LatHabaI, CSIRO Process Science and Engineering, Australia Messler, R.W., Jr. (1999) Principles of welding, New York, John Wiley & Sons.

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