Canadian Welding Association
JOURNAL An official publication of the Canadian Welding Association • Spring 2008 • $7.95
Inside: Understand Understanding U d t d the Interaction of Titanium and Oxygen Welding Lightweight Alloys Precious Metals
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Overview of Friction Stir and Laser Welding Techniques for Lightweight
Alloys
By X. Cao and M. Jahazi
Abstract: Solid-state friction stir and high-energy density laser welding are two effective joining techniques for aluminum and magnesium alloys, since they are able to produce sound, high-quality welds. At the NRC Institute for Aerospace Research, extensive R&D programs on friction stir and laser welding for lightweight alloys have been undertaken. These investigations have been mainly concentrated on process/structure/property relationships and process optimization to reliably produce high-quality welds. Some of the Institute’s research progress is briefly summarized in this work.
Introduction Aluminum (Al) and magnesium (Mg) alloys are the two lightest metal structural materials. To date, aluminum alloys have been widely used in various industries. Use of magnesium alloys is rapidly expanding. Their utilization in automotive and aerospace industries can lower weight, reduce fuel consumption and meet stringent environmental regulations. Conventionally, arc welding processes have been their main joining methods [1]. However, low welding speeds, large heat-affected zones (HAZ) and fusion zones (FZ), high shrinkages, variations in microstructures and mechanical properties, high residual stresses and distortion of the arc-welded joints have caused attention to be drawn towards solid-state friction stir welding (FSW) and high-energy density laser welding (LW) techniques.
Friction Stir Welding Friction stir welding is a solid-state thermo-mechanical joining process
(a combination of extruding and forging) invented by The Welding Institute (TWI) in 1991 [2]. In this process, a cylindrical shoulder-type tool with a rotating profi led pin (probe) is inserted into the work pieces to be joined. Frictional heat is created between the shoulders/wear-resistant pins and the work pieces, which are assembled together and clamped onto a backing bar. The heat produced causes the materials to soften below their melting points, and allows the pin to continue rotating and to move forward along the joint. The frictional heat causes surrounding material to be plasticized at the front of the rotating pin and to be transported to the rear of the pin, and then the material consolidates and cools down to form a solid-state weld. Figure 1a schematically shows the principle of this process [3], and Figure 1b shows an MTS Systems Corp. friction stir welding machine at the Aerospace Manufacturing Technology Centre of the NRC Institute for Aerospace Research.
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Figure 1: (a) Schematic diagram indicating friction stir process [3] and (b) MTS FSW machine installed in Aerospace Manufacturing Technology Centre
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(c) Figure 2: Overviews of 4.95-mm friction stir welded AZ31B-H24 magnesium-alloy butt joints obtained at various tool-rotation rates and welding speeds: (a) 750 rpm and 2 mm/s, (b) 1,250 rpm and 4 mm/s, and (c) 1,750 rpm and 6 mm/s (AS = Advancing side, RS = Retreating side)
Friction stir welding has many inherent advantages, such as its lack of a molten pool, low tendency for the formation of oxides, porosities and cracks, low generated heat, low shrinkage, low
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residual stress and low distortion, for example [2]. Good surface fi nish, and clean and uniform joint surface and root quality can usually be obtained. Excellent mechanical properties have
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been proven by bend, tension and fatigue, among other tests. This method can join extruded, forged and cast materials with melting points from low to high temperature, such as Al, Pb, Zn, Mg, Ti, Cu, low-carbon ferritic steel, low-carbon chromium steel, stainless, plastics, ceramics and composites, for example. Some “difficult” and “impossible-to-weld” (“unweldable”) materials can also be joined. The formation of undesirable brittle intermetallics that is usually encountered in most fusion-welding processes for dissimilar material combinations can be avoided during FSW. Joint preparation and post-weld operation are simple. Prior to joining, no grinding, edge profi ling, brushing, pickling or oxide removal to prepare the material are required. In addition, this method is energy- and cost-effective. It is also an environmentally friendly process due to its lack of such requirements and by-products as arc glare, reflected laser beams, shielding gas, welding fume, spatter, noise, radiation, high voltage, liquid metal and a consumable.
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The FSW process was originally developed for Al alloys and, hence, the major research and development efforts have been mainly concentrated on those used in automotive and aerospace industries. To date, most commercial Al-alloy systems have been well investigated, including the unweldable 2xxx and 7xxx series. The commercial applications of FSW to Al alloys are now rapidly increasing. Typical examples include 2014 Al propellant tanks of the Delta II and Delta IV space-launch vehicles by Boeing, 2195 Al-Li liquid hydrogen and liquid oxygen barrel segments of the external tanks for the space shuttle by Lockheed Martin, the longitudinal and circumferential internal stiffeners to the aft fuselage section by Eclipse Aviation, and lap joints of Al stamping to extrusions by Ford [2, 4, 5]. In contrast, FSW for Mg alloys has been mainly focused on Mg-Al-Zn (AZ31, AZ61, and AZ91) and Mg-Al-Mn (AM50 and AM60) alloys, especially for automotive applications. Most work has concentrated on characterizing weld
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Welding speed (mm/s) Figure 3: Effect of welding speed on tensile properties of 4.95-mm friction stir welded AZ31B-H24 magnesium alloy at a tool-rotation rate of 1,750 rpm
joints and investigating the influence of such processing parameters as toolrotation rate and welding speed [6-9]. Generally speaking, there is still a lack
of systematic scientific knowledge in process optimization (process parameters, pin-tool materials and designs), process modeling (e.g., thermal, force,
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Figure 4: (a) Overview of a lap joint, (b) hooking at advancing side, (c) hooking and kissing bond at retreating side, (d) and (e) kissing bond in stir zone
metal flow, residual stress, distortion), microstructural characterization, property characterization (re fatigue, fracture, creep and corrosion, for example) and non-destructive evaluation. During FSW, such defects as porosity, ‘kissing’ bond, lack of penetration, lack of bonding and hooking may occur, but sound and high-quality weld joints can be reliably obtained in a relatively wide operating window (Figure 2). Joint quality significantly depends on the combination of two main processing parameters: tool-rotation rate
and welding speed. Figure 3 shows the effect of welding speed on tensile properties. Tensile strength and plastic elongation increase with increasing welding speed. However, the tensile strength and ductility become lower at too-high welding speed, due to the formation of a kissing-bond defect caused by cold welding. The joint efficiency can reach 77% for 4.95-mm AZ31B-H24 Mgalloy welds. The highest percentage ratios of yield strength and elongation of the joint to the base metal are 69% and 54%, respectively. In lap joints,
the weld quality is mainly controlled by two main defects: hooking and kissing bond. In “cold” welding (i.e., high welding speed or low tool-rotation rate), kissing bond is formed due to weak stirring and mixing at relatively low temperature; in “hot” welding (i.e., low welding speed or high tool-rotation rate), hooking defects appear due to strong stirring and mixing at relatively high temperature. Joint quality results from the compromise and balance of the two defects, but hooking is more damaging. The highest shear strength
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Figure 5: Microstructures of friction stir welded AZ31B-H24 magnesium-alloy joint obtained at 1,750 rpm and 6 mm/s for (a) stir zone, (b) TMAZ, (c) HAZ and (d) base metal
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can be obtained when downwards hooking appears at the maximum stress location (i.e., advancing side in retreating side near edge of top sheet setup) as shown in Figure 4. The microstructure is well understood for friction stir welded joints. Recrystalization appears in the stir zone [2], the thermo-mechanically affected zone (TMAZ) and partially in the HAZ for hot-rolled AZ31B-H24 Mg alloy (Figure 5). Grains in the stir zone become equiaxed and refined for most alloys [2]. For hot-rolled AZ31B-H24 magnesium alloy, the grains in the stir zone can be coarsened or refined (Figure 6), depending mainly on the microstructures of the base metal, recrystalization and growth kinetics, and heat input. At low heat input (i.e., high welding speed or low tool-rotation rate), a lower processing temperature
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Laser Welding
Figure 7: Continuous-wave 4 kW Nd:YAG laser system with ABB 4400 industrial robot
obtained in the stir zone leads to less time available for grain growth. Therefore, the grains can be refined and the hardness increases at low heat input. In
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contrast, the grains in the stir zone can grow and the hardness may decrease at excessively high heat input (low welding speed or high tool-rotation rate).
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In spite of high capital cost, strict safety requirements, low tolerance for clamping, fitting and alignment, high-energy density laser welding is a potential joining technique for Al and Mg alloys due to its low and precise heat input, small HAZ, deep and narrow fusion zone, high welding speed and productivity, low residual stress and distortion, possible elimination of pre- and post-weld heat treatments, high accuracy, great process fl exibility and reliability [10-12]. It was reported that, for the fi rst time in the aircraft industry, laser welding was used instead of rivets for Al-alloy lower-fuselage shells in A318 and A380 [10]. Highpower direct diode and fibre lasers have recently become commercially available, but CO2 and Nd:YAG lasers have been the two main lasers used. Better weld quality can usually be obtained by Nd:YAG than by CO2 lasers, due to the former’s shorter wavelength, which can improve welding effi ciency, reduce threshold irradiance required
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Figure 8: Aluminum-alloy aircraft panel welded using continuous-wave 4 kW Nd:YAG laser
for keyhole mode welding, produce a more-stable weld pool and, thereby, obtain better surface morphologies. Optic-fibre delivery provides great process fl exibility for the Nd:YAG
laser beam. In addition, joint fit-up is of less concern for larger-spot-size Nd:YAG than for CO2 laser. Figure 7 shows a continuous-wave 4 kW Haas Nd:YAG laser system equipped with
an ABB 4400 industrial robot. Figure 8 shows a typical Al 6013 fuselage structure welded by a continuous-wave 4 kW Nd:YAG laser. Due to their similar physical properties, Al and Mg alloys have some similar welding characteristics. They are both easily oxidized due to their high affi nity for oxygen. Thus, highpurity shielding inert gas is needed. The surface oxides, hydride layers, grease and releasing agents that are usually present on the surface of a work piece and fi ller material can induce porosity, lack of bonding (cracks) and solid inclusions and, hence, should be cleaned or removed prior to laser welding. The low viscosity and low surface tension of molten Al and Mg may cause sag, particularly for large weld pools and Mg alloys, and thereby lead to underfi ll defect [6, 13-15]. The relatively low modulus of elasticity and high heat-expansion coeffi cient may result in high residual stresses and distortion. Thus, rigid clamping is needed for Al- and Mg-alloy welding.
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Figure 9: Laser-welded butt joints of 2- and 6-mm ZE41A-T5 Mg-alloy sand castings [16]
The high residual stresses will also promote stress-corrosion cracking for some alloys. They also have a tendency for liquation and solidifi cation cracking, because of the presence of low melting-point intermetallics and relatively wide freezing intervals. Preheating prior to welding can greatly reduce susceptibility to weld cracking, as a result of decreased temperature gradients in the weld zones and the generation of lower thermal stresses during cooling. However, preheating is not recommended due to the energy waste. In Al and Mg alloys, hydrogen
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is the only dissoluble gas, possibly leading to the formation of gas porosity. For the welding and repair of castings, the presence of porosity in original castings is also a challenge. Compared with Al alloys, Mg alloys have a relatively low boiling point and high vaporization pressure, leading to substantial spatter, loss of chemical elements, unstable weld pool, and
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poor surface quality. However, many Al alloys have similar issues due to the presence of alloying element Mg. In spite of these challenges, Al and Mg alloys can be successfully welded through careful process optimization and control [13-16]. Sound weld joints without macroscopic pores and cracks can be well obtained, as shown in Figure 9. Joint effi ciency can reach 96% in as-welded condition for ZE41A-T5 Mg alloy [16]. The percentage ratios of the weld to the base metal for yield strength and elongation are 93% and 73%, respectively. The weld quality heavily relies on the weld defects and refi ned microstructure (Figure 11).
Summary
60 40 20 0 Figure 10: Percentage ratios of tensile properties of the weld joint to the base metal for 2-mm ZE41A-T5 Mg-alloy sand castings welded using 4 kW Nd:YAG laser [16]
Since it was invented in 1991, friction stir welding has been rapidly developed, and significant commercial success has been obtained for Al alloys and is expected for Mg alloys. Both CO2 and Nd:YAG lasers have significant potential to join Al and Mg alloys. Sound-macroscopically porosity- and crack-free welds can be reliably obtained, although some processing problems and weld defects can be encountered, such as inconsistent welding performance, loss of some elements, sag, undercut, porosity, liquation and solidification cracks, and solid oxide
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Figure 11: Microstructures of sand-cast ZE41A-T5 Mg alloy welded by 4 kW Nd:YAG laser; (a) fusion zone, (b) HAZ and (c) base metal.
inclusions due to the inherent properties of Al and Mg alloys. Recently, the introduction of fibre lasers has brought significant improvements in laser-beam technology and will increase potential applications, since fibre lasers are essentially maintenance-free during their entire lifetime and provide for improved optical performance, better systems flexibility, long uptime and improved reliability. Fibre lasers produce high beam quality and small spot size, and their quality becomes predictable and consistent at all power levels across all pulse sequences during the entire life of the laser, a critical feature to improve process reliability. Fibre lasers, therefore, can weld faster at lower power levels because of the smaller spot size and higher beam quality. The versatile fibre lasers offer the ultimate for solid-state laser systems and, hence, will replace conventional CO2 and Nd:YAG lasers in the near future. Clearly, much R&D for fibre lasers is needed for lightweight alloys.
Authors: Dr. Xinjin Cao is Associate Research Officer at the Aerospace Manufacturing Technology Centre (AMTC), NRC Institute for Aerospace Research, Montreal, Quebec (Xinjin.cao@cnrc-nrc. gc.ca); Dr. Mohammad Jahazi is Senior Research Officer and Manager of Metallic Products Group at the AMTC (
[email protected]). References can be found on the association’s website at www.cwa-acs.org. 30
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