USE OF LASER AND FRICTION STIR WELDING FOR AEROSPACE MAGNESIUM ALLOYS

Transcription

1 Proceedings of the Light Metals Technology Conference 27 USE OF LASER AND FRICTION STIR WELDING FOR AEROSPACE MAGNESIUM ALLOYS X. Cao, M. Jahazi Aerospace Manufacturing Technology Centre, Institute for Aerospace Research, National Research Council Canada, 5145 Decelles Avenue, Montreal, Quebec, H3T 2B2, Canada Abstract High energy-density laser and solid-state friction stir welding (FSW) have been identified as promising joining techniques for magnesium alloys since they can reduce or eliminate some typical defects such as underfill, sag, loss of alloying elements, pores, cracks, residual stresses and distortion encountered in conventional arc welding. At the NRC Institute for Aerospace Research, an extensive R&D program on laser welding and friction stir welding of aerospace magnesium alloys has been undertaken. These investigations have concentrated mainly on process window optimization as well as understanding the fundamental process-structureproperty relationships to reliably produce high quality welded joints. The research progresses made and the main issues encountered in laser welding and FSW of some aerospace magnesium alloys will be briefly summarized in this work. The reliability and reproducibility of the welding processes are also addressed. Introduction Among the lightest structural materials, magnesium alloys have the potential to replace steel and aluminum in some structural applications. The wider use of magnesium alloys needs reliable joining techniques. There has been a lack of effective and efficient welding methods for magnesium alloys [1, 2]. The conventional arc welding process is the main technique, especially for the repair of casting defects [3]. 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 high energy density laser welding and solid-state FSW techniques. In this work, some main issues encountered and progress obtained during the Nd:YAG laser welding of sand-cast Mg-4.2Zn- 1.2Ce-.7Zr (ZE41A-T5) and the FSW of hot rolled AZ31B-H24 magnesium alloys are discussed. Laser Welding In spite of high capital cost, strict safety requirements, low tolerance for clamping, fitting, and alignment, laser beam welding has many advantages: low and precise heat input, small HAZ, deep and narrow FZ, high welding speed and productivity, low residual stress and distortion, great process flexibility and reliability. Magnesium alloys have low heat capacity and latent heat of fusion, and therefore usually require relatively low heat input and allow high welding speeds. Using low heat input laser welding, the post-weld heat treatment for stress relief may even be eliminated for Mg alloys [3]. Although high power direct diode and fiber lasers have become commercially available in recent years, CO 2 and Nd:YAG lasers have proved their reliability and repeatability as welding techniques for magnesium alloys [1]. Better weld quality can usually be obtained by Nd:YAG than by CO 2 laser due to the shorter wavelength, which can improve welding efficiency, reduce threshold irradiance required for keyhole mode welding, produce more stable weld pool and thereby obtain better surface morphologies. The optic fiber delivery provides great process flexibility for the Nd:YAG laser beam. In addition, joint fit-up is of less concern for larger beam diameter Nd:YAG than for the CO 2 laser beam. Laser welding can be carried out in either conduction or keyhole mode as shown in Figure 1 [4]. In conduction welding, the surface of the material is heated above its melting point but below its vaporization temperature. Fusion occurs only by heat conduction through the melt pool. A weld-bead, hemispherical in cross section, with an aspect ratio of 1.2 or less is formed in a similar manner to conventional

2 fusion welding processes. Conduction welds, however, are limited to thin sections. By contrast, keyhole laser welding uses a higher power density to cause local vaporization. Thus, a narrow and deeply penetrating vapor cavity with aspect ratio higher than 1.2 is usually formed by multiple internal reflection of the laser beam. Keyhole mode welding results in better energy coupling, higher penetration depth and welding speed. Therefore, most applications of laser welding are centered around the keyhole process. The research into stable laser welding has been focused on the influence of process parameters on keyhole stability so as to reliably produce macroscopically defect-free welds at high welding speeds. As indicated in Figure 1b, sound weld joints without macroscopic pores and cracks can be obtained. Due to their inherent properties, however, magnesium alloys may frequently exhibit such weld defects as unstable weld pool, substantial spatter, strong tendency to drop-through, sag, undercut, porosity, liquation and solidification cracking, oxide inclusions and loss of alloying elements. The porosity defect (Figure 2) is another concern for magnesium alloys. The pores can be caused by hydrogen precipitation during solidification, entrapment of gases due to turbulent flow at the surface of the liquid metal, collapse of unstable keyholes, evaporation of high vapor pressure and low boiling point elements and shrinkage during cooling as well as expansion of the original gases in the alloys [2]. In some cases, the outgassing and evasion of the gases including metal vapors may cause surface porosity as shown in Figure 2a. In Zr-containing ZE41 magnesium alloy, hydrogen can react with Zr to form ZrH 2 and thus hydrogen pores can be reduced or even eliminated. The pores as indicated in Figure 1a are more probably due to the evaporation of Zn and Mg elements in ZE41A-T5 alloy because the evaporative rate is more intense in conduction welding due to a higher ratio of evaporative area to liquid volume compared with keyhole welding [2]. Magnesium alloys also have the tendency of liquation and solidification cracking in the HAZ and FZ (Figures 3 and 4) because of the presence of low melting-point intermetallics and relatively wide freezing interval for ZE41A-T5 alloy. Figure 1: Conduction and keyhole mode welding for sand-cast Mg alloy ZE41A-T5. Liquid magnesium has relatively low viscosity and surface tension, leading to the occurrence of underfill, sag, or even drop-through defects. The undercut defect frequently occurs in full penetration welding without the use of filler metal [5]. The low boiling point (about 19ºC) and high vaporization pressure (36 Pa) of molten magnesium can cause evaporation, substantial spatter, unstable weld pool, excessive weld metal at the face and evaporative loss of some elements, particularly in the presence of lower boiling point and higher vaporization pressure alloying elements such as zinc [3,6]. Thus laser welded joints of magnesium alloys are characterized by some irregularity and instability in surface quality. Figure 2: Porosity in laser welded ZE41A-T5. Figure 3: Liquation cracks in the heat-affected zone of laser welded ZE41A-T5 alloy.

3 Figure 4: Solidification cracks in the fusion zone of laser welded ZE41A-T5 alloy. Magnesium is easily oxidized due to its high affinity for oxygen. Thus, high purity shielding gas is needed during laser welding. The surface oxides, hydride layers, grease and releasing agents that are usually present at the surface of magnesium alloys can induce porosity, cracks and solid inclusions, and therefore should be cleaned or removed prior to laser welding. P Ln(Ln(1/(1-P))) Base metal As-welded Aged Base metal As-welded Aged y = 22.51x R 2 =.98 TS (MPa) y = 31.98x R 2 = Ln (TS) Figure 5: Cumulative probability and Weibull plot of tensile strength (TS) [7]. To investigate the process reliability and reproducibility of the laser welding technique, 8 butt joints of ZE41A-T5 magnesium alloy were welded using the optimized process setup [7]. Smooth, geometrically regular and macroscopically defect-free sound joints were obtained. Due to the inherent properties of magnesium, shape defects such as sag and underfill were observed in some locations. There is considerable scatter in the geometrical dimensions and tensile properties of the welded joints. Figure 5 shows the cumulative probability and Weibull plot of tensile strength (TS) data [7]. The tensile strength of the laser welded joints can be more accurately described by a Weibull distribution than by a Normal distribution. A higher Weibull modulus value in the aged condition indicates that the tensile strength becomes even more scattered after the artificial aging (T5). The welded joints in both the aswelded and aged conditions have joint efficiencies of approximately 92-95%. Friction Stir Welding Friction stir welding is a relatively new solidstate thermo-mechanical joining process (a combination of extrusion and forging) invented in This process has many advantages compared with fusion welding techniques such as lack of molten pool, low tendency for the formation of oxides, porosities and cracks, low heat generated, low shrinkage, low residual stress, low distortion, etc. Clean and uniform joint surfaces and root quality can be obtained. Dissimilar alloys, some difficult and "unweldable" materials can also be joined using friction stir welding. To date, however, the major research and development efforts associated with FSW have been mainly focused on aluminum alloys. Some preliminary investigations have been carried out for magnesium alloys, mainly on Mg-Al-Zn (AZ31, AZ61, AZ91) and Mg-Al- Mn (AM5, AM6) [8-11]. Little work has been reported for other magnesium alloys, and particularly for aerospace magnesium alloys. As indicated in Figure 6, sound welded joints can be obtained, with no macroscopic porosities and cracks, thus indicating the great potential of FSW for joining magnesium alloys. The different dimensions of the various regions in the advancing side (AS), and retreating side (RS), indicate the inhomogeneity of the FSWed joints. As shown in Figure 7, the shoulder plunge depth is rather sensitive to forge force indicating that

4 magnesium alloys have a rather narrow processing window. Also indicated in Figure 7, steady welding can only be obtained after an initial transitional stage. Macroscopic defect-free joints were obtained but defects such as pores, kissing bond, lack of penetration, lack of bonding, etc. may occur. For example, cavity defects may be caused by excessive material loss and/or poor material flow and mixing (Figure 8). Comprehensive experimental and modeling work is essential to understand the formation of the porosity defects, particularly from the flow behavior of metal during FSW. Figure 6: FSWed AZ31B-H24 magnesium alloy butt joint (RS - Retreating Side; AS Advancing Side; WN Welding Nugget; TMAZ ThermalMechanically Affected Zone; HAZ Heat Affected Zone; BM Base Metal). Plunge depth (mm) kn kn kn kn Welding length (mm) Plunge depth (mm) 5 Shoulder plunge depth mm 2 2 mm Forge force (kn) 9. Figure 7: Effect of forge force on shoulder plunge depth for AZ31B-H24 magnesium alloy. Figure 8: Transverse and horizontal sections indicating pores in AZ31B-H24 joint. To date, most work on FSW has mainly concentrated on characterizing weld joints and investigating the influence of tool rotational rate and welding speed on joint quality. The evolution of the microstructure and hardness are also well documented. However, in some cases different results are reported by various authors. For AZ31B-H24 magnesium alloy, it is found that the grains became gradually larger from the BM to the HAZ, TMAZ and then WN, corresponding to the gradual decreases in the Vickers microindentation hardness [8,9]. However, grain refinement in the weld nugget has also been reported [1,11]. Figure 9 shows the variations of tensile properties with pin rotational rate and welding speed [12]. No significant influence of tool rotational rate on tensile strength (TS) is observed but yield strength (YS) decreases with tool rotational rate. Both tensile and yield strength increase with welding speed. Therefore, better tensile properties are obtained at lower heat input (lower tool rotational speed and higher welding speed). The FSWed AZ31B-H24 alloy has joint efficiencies of approximately 6%. Strain rate has little influence on tensile properties [12]. Fracture surfaces display mixed dimple-like and cleavage-like characteristics [12]. Almost all FSWed AZ31B-H24 butt joints were fractured at the interface between the WN and the TMAZ on the advancing side. It is found that subsurface porosity may occur at the upper half of the welding nugget on the advancing side, mainly due to volume deficiency caused by excessive metal loss during the welding [9]. In addition, this region is also the last area to be filled during the FSW, possibly causing internal porosity, weak bonding, and discontinuities. A typical discontinuity is magnesium oxide (MgO) observed at the fracture surface (Figure 1). Oxides may also lead to the formation of lack-ofbonding and crack defects, causing the decrease in mechanical properties. These oxides are most

5 probably entrapped from the original surfaces of the work-piece. Therefore, great care should be taken to clean the surface oxides prior to welding. The formation of oxides during FSW seems unlikely due to the close contact between the shoulder and the top surface of the workpiece. Even if the oxides are formed in the solid state during the FSW, they are rather thin, and are difficult to detect. Strength (MPa) Pin rotational rate (rpm) YS TS porosity, liquation and solidification cracks, and oxide inclusions. This area of research is important in order to obtain defect-free magnesium alloy joints on a reliable basis. Friction stir welding may have great potential to join magnesium alloys. To date, some preliminary work has been conducted on magnesium alloys used by automotive industry. Most of the work has concentrated on characterizing weld joints and investigating the influence of processing parameters such as tool rotational rate and welding speed. There is still a lack of systematic scientific knowledge in process optimization (process parameters, pin tool materials and designs), process modeling (thermal, force, flow, residual stress, distortion, etc.), microstructural characterization, property characterization (tensile, fatigue, crack growth behavior, fracture, creep, corrosion, etc.), nondestructive evaluation, etc. Therefore, systematic investigations into the process structure property relationships of the FSWed magnesium alloys are essential for their wider applications. 25 Strength (MPa) YS (5 rpm) TS (5 rpm) YS (1 rpm) TS (1 rpm) Welding speed (mm/s) Figure 9: Effect of pin rotational rate and welding speed on tensile properties [12]. Summary To date two main types of industrial lasers, including both CO 2 and Nd:YAG, have been used to investigate the weldability of magnesium alloys. Sound welded joints with lack of porosity and cracks and with good surface quality can be obtained. Due to the inherent properties of magnesium, some welding problems may occur such as: unstable weld pool, substantial spatter, strong tendency to drop-through, loss of some elements, sag, undercut, and the formation of Mg Al Figure 1: Oxides entrapped in FSWed AZ31B- H24 magnesium alloy butt joint [12]. O

6 Acknowledgements The authors would like to thank M. Xiao and M. Guerin for the preparations of the welded joints, and Y.L. Lin for the metallurgical analyses of the laser welded joints. Some results were also quoted from master students H. Al-Kazzaz from Concordia University and N. Afrin from Ryerson University. Thanks are also due to them and their supervisors Profs. M. Medraj at Concordia University and D.L. Chen at Ryerson University. References [1] X. Cao, M. Jahazi, J.P. Immarigeon, W. Wallace, A review of laser welding techniques for magnesium alloys, J Mater Process Tech 171 (26) [2] X. Cao, M. Xiao, M. Jahazi, J.P. Immarigeon, Continuous wave Nd: YAG laser welding of sand-cast ZE41A-T5 magnesium alloys, Mater Manuf Processes, 2 (25) [3] W.R. Oates, Welding Handbook, American Welding Society, 8th Ed, Miami, Florida, 1996, [4] X. Cao, M. Xiao, M., Jahazi, L.Y. Ling, Continuous wave Nd: YAG laser welding of sand-cast ZE41A-T5 magnesium alloys: conduction or keyhole mode, in: P.C. Patnaik, M. Elboujdaini, M. Jahazi, J. Luo (Eds.), 43rd Conf. of Metallurgists of CIM, Hamilton, Ontario, Canada, August 24, pp [8] X. Cao, M. Jahazi, M. Mehta, Friction stir welding of AZ31B-H24 magnesium alloy butt joints, 6th Int. Symp. on Friction Stir Welding, Montreal, Canada, 1-12 Oct. 26, paper #75, pp [9] X. Cao, M. Jahazi, M. Guerin, Friction stir welding of an aerospace magnesium alloy, in: R.S. Mishra, M.W. Mahoney, T.J. Lienert, K.V. Jata (Eds.), Friction Stir Welding IV TMS, 27, pp [1] K. Katoh, H. Tokisue, T. Kitahara, J. Microstructures and mechanical properties of friction stir welded AZ31 magnesium alloy, Light Met Weld Constr 42 (3) (24), [11] T. Nagasawa, M. Otsuka, T. Yokota, T. Ueki, Structure and mechanical properties of friction stir weld joints of magnesium alloy AZ31, in: H.I. Kaplan, J. Hryn, B. Clow, Magnesium Tech 2, TMS, 2, pp [12] N. Afrin, D.L. Chen, X. Cao, M. Jahazi, Microstructural characterization and tensile properties of friction stir welded AZ31B- H24 magnesium alloy, Mater Sci Eng A (27) in press [5] A. Weisheit, R. Galun, B.L. Mordike, Laser welding of various magnesium alloysmicrostructure and mechanical properties, in: Proc. Magnesium Alloys and Their Applications, Wolfsburg, Germany, 28-3 April 1998, pp [6] H. Haferkamp, M. Goede, A. Bormann, P. Cordini, Laser beam welding of magnesium alloys-new possibilities using filler wire and arc welding, in: Proc. Laser Assisted Net Shape Engineering 3 (21) [7] H. Al-Kazzaz, X. Cao, M. Jahazi, M. Medraj, Reliability of laser welding process for ZE41A-T5 magnesium alloy sand castings, Metall Mater Trans A (27) submitted