Cracking in dissimilar Mg alloy friction stir spot welds M. Yamamoto*1, A. Gerlich2, T. H. North2 and K. Shinozaki1 Cracking during dissimilar friction stir spot welding of thixomoulded AM60 and AZ91 sheet materials is investigated. Liquation cracking occurs in the thermomechanically affected zone (TMAZ) region during friction stir spot welding when AZ91 material is the lower sheet in the dissimilar AM60/AZ91 sandwich. A TMAZ microstructure comprising elongated a-Mg grains and aligned Mg17Al12 is created during the tool penetration stage in spot welding and melted eutectic film formation is facilitated since the temperature in the stir zone (504uC) is much higher than the a-MgzMg17Al12 eutectic temperature (437uC). There is no evidence of liquation cracking in the stir zone or the TMAZ region of AM60 sheet when it is the lower sheet in the dissimilar sandwich. However, liquation cracking is observed in the upper sheet material (AZ91) in the location beneath the tool shoulder close to its periphery. Keywords: Friction stir spot welding, Mg alloy, Cracking, Dissimilar joint, Local melting
Introduction Lighter base materials provide significant reductions in weight compared to steel during automobile manufacture and this readily explains why there is so much interest in evaluating magnesium alloys for automotive applications.1,2 Weight reduction is important in the face of demands for stricter government standards and improved safety, performance and reliability in combination with reduced emissions and fuel consumption. The availability of satisfactory methods for fabricating Mg alloy components is a critical issue. Arc welding of Mg alloys such as AZ91 is problematic since fusion welded deposits are prone to hydrogen porosity formation and preheating temperatures as high as 400uC are required to prevent weld cracking in highly restrained joints.3,4 The high thermal expansion coefficients of the Mg alloys mean that there is increased distortion during fabrication5 and welded joints are susceptible to solidification cracking in the fusion zone and to liquation cracking in the heat affected zone (HAZ) region. When the AZ31B base material is fabricated using gas tungsten arc welding, for example, the solidification cracking susceptibility is determined by the welding parameters and the polarity used during fusion welding.6,7 Fusion welded joints are less susceptible to solidification cracking when alternating current is employed. Friction stir spot welding, a variant of friction stir seam welding, produces joints at particular locations on the component.8,9 The temperature in the stir zone 1 Department of Mechanical System Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1, Kagamiyama, HigashiHiroshima, Hiroshima 739-8527, Japan 2 University of Toronto, Department of Materials Science and Engineering, 184 College St, Rm 140, Toronto M5S3E4, Canada
*Corresponding author, email
[email protected]
ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 21 May 2007; accepted 8 August 2007 DOI 10.1179/174329308X349520
approaches the solidus temperature of the Al alloy or Mg alloy sections being fabricated since extremely high power densities (around 1010 W m23) and rapid heating rates (from 200 to 400uC s21) are involved.10,11 In this connection it is worth pointing out that the temperature attained in the stir zone during friction stir spot welding depends on the tool rotational speed and dwell time that is selected.12 In a similar manner to that during friction stir seam welding, the stir zone regions produced during spot welding comprise fine grained dynamically recrystallised microstructures.12–17 It is generally assumed that friction stir welding produces joints, which are free of many of the defect formation issues associated with fusion welding (porosity formation, cracking in the stir zone and in the thermomechanically affected zone (TMAZ) regions). However, the validity of this assumption has recently been brought into question when investigating friction stir spot welding of Mg alloy sections. Yamamoto et al.18,19 found evidence of liquation cracking in the TMAZ regions of AZ91, AM60 and AZ31 friction stir spot welds and liquid penetration induced (LPI) cracking in the stir zones of AZ91 spot welds. The temperature in the stir zone at the end of the dwell period in AZ91 friction stir spot welding (438uC) is remarkably close the (a-MgzMg17Al12) eutectic temperature in the Al–Mg binary equilibrium phase diagram (437uC).18 Much higher stir zone temperatures are generated in AM60 and AZ31 friction stir spot welds (500 and 550uC).19 It would therefore be expected that local melting might be observed in the TMAZ region immediately adjacent to the stir zone extremity in AZ91, AM60 and AZ31 friction stir spot welds. Yamamoto et al.18,19 found evidence of melted eutectic film formation early in the dwell period during AZ91, AM60 and AZ31 spot welding and suggested that dissolution occurred following their engulfment by the
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expanding stir zone. Cracking was only apparent very early in the dwell period in AM60 and AZ31 friction stir spot welds since high stir zone temperatures (500 and 550uC) promoted rapid dissolution of melted eutectic films. In contrast, AZ91 friction stir spot welds made using a dwell time of 4 s exhibited cracking since the stir zone temperature was close to the eutectic temperature (437uC) and the dissolution rate of melted eutectic films was lower.18 The present paper examines cracking during dissimilar friction stir spot welding of thixomoulded AZ91 and AM60 friction base materials. It is shown that liquation cracking occurs no matter which base material, AZ91 or AM60, is the upper or lower sheets in the dissimilar sandwich.
Cracking in dissimilar Mg alloy friction stir spot welds
1 Measured temperatures during AM60/AZ91 spot welding using tool rotational speed of 2500 rev min21, plunge rate of 1?0 mm s21 and dwell time of 4 s: AZ91 was lower sheet in dissimilar sandwich
Experimental Friction stir spot welding of Mg alloy sections is carried out using 1?6 mm thick sections of a thixomoulded semisolid AZ91 (Mg–9Al–0?7Zn (wt-%)) and 1?6 mm sections of thixomoulded AM60 (Mg–6Al–1Zn (wt-%)) sheet. The key differences during friction stir spot welding of these Mg alloy base materials are the reported stir zone temperatures and the differences between these measured stir zone temperatures and the a-MgzMg17Al12 eutectic temperature in the Al–Mg binary equilibrium phase diagram (437uC). The solidus temperatures of AZ91 and AM60 are determined by the Al content in the as received Mg alloy. Higher temperatures have been reported in the stir zones of AM60 friction stir spot welds. Also, it has been calculated that the dissolution tendency of melted eutectic material is much faster when higher stir zone temperatures are produced during friction stir spot welding.19 The microstructures of the alloys used in the study (both of the thixomoulded AZ91 and AM60 sections) comprise islands of primary a-Mg embedded in a matrix of a-Mg plus (a-MgzMg17Al12) eutectic. In both AZ91 and in AM60 the microstructural phases comprise a-Mg plus (a-MgzMg17Al12) eutectic. The a-MgzMg17Al12 phase is present at interdendritic boundaries in as received thixomoulded AZ91 and AM60 base materials. During friction stir spot welding material in the TMAZ region is deformed as a result of deformation at a high temperature. a-Mg grains are extended and the Mg17Al12 phase is aligned as a result of this deformation process. It is worth noting that throughout the present paper, the first mentioned alloy designates the material used as the upper sheet in the dissimilar sandwich. The spot welding equipment employed in the present investigation has a rotational speed capability up to 3000 rev min21 while a servomotor provides axial loads up to 12 kN. The amount of tool penetration (displacement) during spot welding is measured using a linear transducer with an accuracy of ¡0?01 mm. The axial load and torque are measured using a JR3 six axes load cell, which is coupled to a data acquisition system. All key parameters (axial force, torque, rotational speed, pin displacement, shoulder and pin temperature) are logged during friction stir spot welding operations. The tool design used in the present study has been described in detail elsewhere10 and comprises a shoulder diameter of 10 mm, a pin diameter of 4 mm and a pin length of 2?2 mm. The rotating pin has a simple
threaded geometry. Plunge rates as high as 25 mm s21 are selectable during friction stir spot welding, with the penetration depth being attained with an accuracy of ¡0?1 mm. The thermal cycle during friction stir spot welding is measured by embedding 0?25 mm diameter K type thermocouples at the locations 0?20 mm from the tip of the rotating pin and 1?3 mm from the outer periphery of the tool shoulder. During all temperature measurements the thermocouple junction is always in direct contact with dynamically recrystallised material formed during the spot welding operation. A detailed description of the temperature measurement set-up is provided elsewhere.10 The variation in temperature during the 4 s dwell period is ,7?4uC during repeat testing. The microstructural changes during the tool penetration stage and the subsequent dwell period in dissimilar AM60/AZ91 spot welding are examined in a series of tests where the depth of penetration of the rotating tool into the contacting sheets is gradually increased and the rotating tool is then withdrawn. The metallographic features of transverse sections following tool withdrawal at different penetration depths and/or dwell time settings are investigated following etching in either 5 vol.-% nital solution and/or acetic picral solution comprising 10 mL acetic acid, 4?2 g picric acid, 10 mL H2O and 70 mL of 95 vol.-% ethanol. It is worth pointing out that material within the stir zone adheres to the rotating tool when it is withdrawn at the end of each spot welding operation. Material adjacent to the keyhole periphery is plastically deformed and displaced upwards during tool retraction so that tensile straining can facilitate crack propagation when regions of weakness such as melted eutectic films are present in the stir zone, TMAZ or HAZ regions. With this in mind, care must be taken when interpreting the results produced when the weld zones produced at different plunge depth settings are examined.
Results Temperature output Figures 1 and 2 show the temperatures measured at the locations 0?2 mm from the tip of the rotating pin and 1?3 mm from the outer periphery of the tool shoulder during dissimilar AM60/AZ91 and AZ91/AM60 friction stir spot welding, when AZ91 is the lower sheet (Fig. 1)
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2 Measured temperatures during AZ91/AM60 spot welding using tool rotational speed of 2500 rev min21, plunge rate of 1?0 mm s21 and dwell time of 4 s: AZ91 was upper sheet in dissimilar sandwich
and the upper sheet (Fig. 2) in the dissimilar sandwich. When AM60 is the upper sheet in the dissimilar AM60/ AZ91 sandwich the temperature in the stir zone reaches 504uC at the beginning of the dwell period in dissimilar spot welding and then decreases to 475uC when the dwell period is extended to 4 s. In contrast, when AZ91 is the upper sheet in the dissimilar AZ91/AM60 sandwich, the temperature in the stir zone increases from y440uC at the start of the dwell period to 476uC at the end of the 4 s long dwell period in spot welding, see Fig. 2. It has been confirmed that tool shoulder contact occurs when material expelled during tool penetration becomes trapped between the bottom of the tool shoulder and the upper surface of sheet that is being spot welded.20 Consequently, tool shoulder contact occurs much earlier in the spot welding cycle than expected, before the rotating pin is fully penetrated. The axial force measured during spot welding increases dramatically when contact occurs between the bottom of the tool shoulder and the upper surface of the sheet material. The different sections examined during the tool penetration stage in spot welding are specified on the axial force and tool displacement versus time relationship shown in Fig. 3. The microstructural features when the rotating pin has just begun to penetrate into the lower AZ91 sheet material (at stages D-2 and D-3) are shown in Fig. 4. The lower sheet material (AZ91) is displaced upwards during tool penetration and a dynamically quiescent region21 is formed beneath the tip of the rotating pin. Thermomechanical deformation during the tool penetration process produces a microstructure in the TMAZ region immediately adjacent to the stir zone extremity comprising elongated a-Mg grains and aligned Mg17Al12 particles, see Fig. 4. Figure 5 shows the microstructural features in a traverse from the as received thixomoulded AZ91 base material across the HAZ and TMAZ regions and into the stir zone at stage D-7 in Fig. 3. The thixomoulded AZ91 base material has a microstructure comprising aMg nodules embedded in a matrix of a-Mg plus (aMgzMg17Al12) eutectic. Mg17Al12 begins to dissolve in the HAZ region, a partially recrystallised microstructure is formed in the TMAZ region and in the TMAZ close to the stir zone extremity, a-Mg grains are elongated and Mg17Al12 is aligned in the direction parallel to the stir zone boundary.
Cracking in dissimilar Mg alloy friction stir spot welds
3 Axial force and displacement outputs during dissimilar AM60/AZ91 friction stir spot welding: depths of rotating pin during dissimilar welding are indicated by points D1 to D-7; D-7 is point when rotating pin is fully penetrated and dwell period initiates
Figure 6 shows some large discontinuities formed close to the extremity of the stir zone following tool withdrawal at the penetration depth corresponding with stages D-3 and D-4 in Fig. 3. However, discontinuity formation is not apparent when the rotating pin penetrates further into the dissimilar AM60/AZ91 sandwich at the stage D-7 in Fig. 3, see Fig. 7a and b. The TMAZ material immediately adjacent to the stir zone extremity is displaced upwards and moves towards the periphery of the tool shoulder, see Fig. 7a. Figure 8a shows Mg17Al12 particles aligned parallel to the extremity of the stir zone which is formed beneath the tip of the rotating pin in a dissimilar AM60/AZ91 spot weld during plunging period at the stage D-2 in Fig. 3. Figure 8b shows evidence of liquation cracking in the TMAZ region at the extremity of the stir zone, which is formed beneath the tip of the rotating pin in a dissimilar AM60/AZ91 spot weld made using a dwell time of 2 s. a-Mg grains are elongated and Mg17Al12 particles are aligned parallel to the stir zone extremity and melting and liquation cracking occur when a dwell time of 2 s is applied, see Fig. 8. Figure 9 also shows evidence of liquation cracking in AZ91 material displaced upwards during tool penetration (in dissimilar friction stir spot welds made using dwells times of 1 and 2 s). When AZ91 is the upper sheet in the dissimilar sandwich, there is no evidence of cracking in the stir zone or in the TMAZ or HAZ regions of the lower AM60 sheet (in spot welds made using a dwell time of 2 s). However, some evidence of liquation cracking is apparent beneath the tool shoulder in the location close to its extremity, see Fig. 10.
Discussion Temperature output When AM60 is the upper sheet in the dissimilar sandwich, the stir zone temperature early in the dwell period in AM60/AZ91 spot welding is close to that found during AM60/AM60 spot welding.19 Su et al.21 confirmed that a layer of dynamically recrystallised AM60 material is formed immediately adjacent to the periphery of the rotating pin when it penetrates through the upper sheet and into the lower sheet. As a result, heat generation resulting from viscous dissipation occurs
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a and b showing upward displacement of lower sheet material at stage D-2 in Fig. 3 (see arrows): chemical composition at location A is 87?9Mg–11?1Al–1?1Zn (wt-%); c elongation of a-Mg grains and alignment of Mg17Al12 phase (see arrows) in TMAZ region immediately adjacent to stir zone extremity at stage D-2 in Fig. 3; d formation of dynamically quiescent region immediately beneath tip of rotating pin at stage D-3 in Fig. 3 (see arrows) 4 Microstructural features at stages D-2 and D-3 in Fig. 3: rotating pin has just started to penetrate into lower AZ91 sheet
in an annulus of dynamically recrystallised AM60 material surrounding the periphery of the rotating pin. This readily explains why the temperature in the stir zone during dissimilar spot welding when AM60 is upper sheet in the dissimilar sandwich is comparable to that when two AM60 sheets are welded. The stir zone temperature decreases from 504 to 475uC when the dwell time increases to 4 s during
dissimilar AM60/AZ91 friction stir spot welding, see Fig. 1. The stir zone temperature decreases since lower sheet material (AZ91) is incorporated into the stir zone during the dwell period in spot welding.22 The width of the stir zone increases during the dwell period when material from the locations beneath the tool shoulder and from the bottom of the rotating pin is incorporated at the top of the thread and is moved downwards before
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5 Microstructural features in traverse from as received AZ91 base material (shown on right hand side of figure), through HAZ, TMAZ and into stir zone region (shown on left hand side of figure) at stage D-7 in Fig. 3
being discharged from the bottom of the thread on the rotating pin. During dissimilar spot welding, a ribbon of dissimilar contiguous lamellae is discharged at the bottom of the rotating pin and a helical vertical rotational flow of material is created within the stir zone formed adjacent to the periphery of the rotating pin. This readily explains the formation of a microstructure comprising intermingled AM60 and AZ91 lamellae in the intermixed region formed adjacent to the periphery of the rotating pin, see Fig. 11. When AZ91 is the upper sheet in the dissimilar AZ91/ AM60 sandwich the temperature early in the dwell period (440uC) is close to the a-MgzMg17Al12 eutectic temperature in the Mg–Al binary equilibrium phase diagram (437uC) and increases to 476uC at the end of the found second long dwell period. When AZ91 is the upper sheet in the dissimilar sandwich, the penetrating tool is surrounded by an annulus of dynamically recrystallised AZ91 material so that the temperature at the start of the dwell period is similar to that found during AZ91/AZ91 friction stir spot welding.18 The temperature in the stir zone increases to 476uC when AM60 material is incorporated into the stir zone when the dwell period is extended to 4 s, see Fig. 2. Figures 1 and 2 indicate that similar stir zone temperatures are measured at the end of the 4 s long
dwell period no matter which sheet material (AM60 or AZ91) is the upper sheet in the dissimilar sandwich. It has recently been suggested that the chemical composition of the stir zone formed adjacent to the periphery of the rotating pin during both similar and dissimilar friction stir spot welding is principally determined by mechanical mixing and not by chemical mixing.22,23 As a result, the viscosity of material contained within the stir zone will depend on the relative proportions of AM60 and AZ91, which are incorporated at the top of the pin thread during the dwell period in friction stir spot welding. When equal amounts of AM60 and AZ91 are incorporated at the top of the thread on the rotating pin and the rule of mixtures is assumed, the estimated stir zone temperature is 472uC. This estimated value is similar to that measured during dissimilar friction stir spot welding of AM60 and AZ91 using a dwell time of 4 s, see Figs. 1 and 2.
Microstructural changes during tool penetration Figures 4–7 show the microstructural changes produced when the rotating pin penetrates into the dissimilar AM60/AZ91 sandwich. The temperature increases rapidly during the tool penetration stage in friction stir spot welding and lower sheet material (AZ91) is displaced upwards, see Figs. 1, 2, 4a and b. A combination of rapid
6 a discontinuity formed at stage D-3 in Fig. 3 and b SEM graph of failure region at stage D-4 in Fig. 3: chemical composition at location A in Fig. 6b indicated is 76?8Mg–23?2Al (wt-%)
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heating, plastic deformation and material displacement during tool penetration produces a TMAZ microstructure in the location immediately adjacent to the stir zone extremity, comprising elongated a-Mg grains and aligned Mg17Al12, see Fig. 4c. In this connection, the alignment of material in the TMAZ region immediately adjacent to the stir zone extremity is also observed during the tool penetration stage in Al alloy spot welding.22 As mentioned earlier, tool retraction during friction stir spot welding plastically deforms and displaces material immediately adjacent to the keyhole periphery and tensile straining can promote crack propagation when a melted eutectic film formation produces a line of weakness. With this in mind, the large discontinuities formed close to the extremity of the stir zone (see Fig. 6) can be explained as follows. It has been shown that the stir zone produced during the tool penetration stage in friction stir spot welding comprises two quite distinct material flows.21 Close to the periphery of the rotating pin, material is moved downwards; further from the pin periphery within the stir zone, material is moved upwards. A spiral flow of material is involved in each case. However, the upward displacement of lower sheet material (AZ91) during tool penetration (see Fig. 4a and b) occurs in the TMAZ region adjacent to the stir zone extremity. Figure 12 shows a horizontal cross-section through the keyhole region at the location corresponding with the upper surface of the AZ91 sheet material when the 2 s dwell time was applied. It is apparent that TMAZ material displaced upwards during tool penetration has a circumferential flow component since a-Mg grains are elongated and Mg17Al12 particles are aligned in the direction of tool rotation. The temperature increases rapidly and exceeds 437uC, the a-Mg plus a-MgzMg17Al12 eutectic temperature in the Al–Mg binary equilibrium phase diagram during dissimilar AM60/AZ91 spot welding, see Fig. 1. Melted eutectic films are formed in lower sheet material (AZ91) displaced upwards and crack propagation occurs when the rotating tool is withdrawn; see Fig. 6b. However, there was no evidence of discontinuity formation when the rotating tool penetrates further into the dissimilar AM60/AZ91 sandwich and was then withdrawn, see Fig. 13. It is suggested that compressive loading by the tool shoulder squeezes the contacting AM60 and AZ91 sheets together and forces the crack faces together when the depth of tool penetration is increased.
Cracking in AM60/AZ91 spot welds
a lower sheet material (AZ91) displaced upwards in direction towards periphery of tool shoulder (see arrow); b elongation of a-Mg grains and aligned Mg17Al12 phase at grain boundary regions in TMAZ adjacent to extremity of stir zone formed during tool penetration; c elongation of a-Mg grains and aligned Mg17Al12 phase at grain boundary regions in TMAZ adjacent to extremity of stir zone formed during tool penetration (see arrows): chemical composition at location A in Fig. 7c is 72?3Mg– 23?5Al–2?3Mn–1?9Zn (wt-%) 7 Microstructural features in TMAZ region at stage D-7 in Fig. 3
The friction stir spot welding process can be visualised as two distinct phases, namely: (i) the tool penetration stage during which frictional heating, thermomechanical deformation and viscous dissipation promote rapid heating adjacent to the periphery of the rotating pin. Mg17Al12 particles begin to dissolve in the HAZ region, a partially recrystallised microstructure is formed in the TMAZ region and the microstructure immediately adjacent to the stir zone extremity comprises elongated a-Mg grains and aligned Mg17Al12 particles, see Figs. 4, 5 and 7 (ii) the dwell period in spot welding during which material incorporation increases the width of the stir zone and a stir zone microstructure comprising
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8 a alignment of a-Mg grains and Mg17Al12 particles at extremity of stir zone formed beneath tip of rotating pin (see arrows) during tool penetration stage in spot welding at stage corresponding with D-2 in Fig. 3 and b cracking at extremity of stir zone formed beneath tip of rotating pin in AM60/AZ91 spot weld made using dwell time of 2 s
intermingled AM60 and AZ91 lamellae is formed, see Fig. 11. As mentioned earlier, the temperature in the stir zone and in the TMAZ and HAZ regions is determined by the selection of the upper sheet material AM60 or AZ91 in the dissimilar sandwich, see Figs. 1 and 2. Melted eutectic films form in the TMAZ region adjacent to the stir zone extremity when the temperature in the stir zone during the dwell period in dissimilar spot welding exceeds the a-Mg plus Mg17Al12 eutectic temperature (437uC). The liquation cracking tendency in the TMAZ region is also increased since the high heating rate during the tool penetration stage in dissimilar friction stir spot welding limits the dissolution of Mg17Al12 particles. Figure 8 shows evidence of liquation cracking in the location beneath the tip of the rotating pin. A dynamically quiescent region forms beneath the tip of the rotating pin immediately following contact between the rotating tool and the surface of the upper sheet. Material contained within the dynamically quiescent region is retained in this location during the whole of spot welding operation and plays no role when the stir zone formation occurs beneath the tip of the rotating pin.21 a-Mg grains are elongated and Mg17Al12 are aligned parallel to the extremity of the stir zone and eutectic melting occurs when a dwell time of 2 s is applied, see Fig. 8a and b. In this connection, cracking has recently been found in the same location beneath the tip of the rotating pin in Al 7075 friction stir spot welds made using a rapid plunge rate (10 mm s21) and an extremely short dwell time (0?05 s).22 Figures 3, 4 and 7 show the upward displacement of AZ91 material during the tool penetration stage in spot welding and its movement towards the periphery of the tool shoulder. Figure 9a–c shows evidence of liquation cracking in AZ91 material, which is displaced upwards (in spot welds made using dwell times of 1 and 2 s).
A flow reversal occurs when the dwell period initiates since material beneath the tool shoulder is moved downwards towards the top of the thread before and is incorporated into the expanding stir zone.23 The flow reversal at the start of the dwell period is readily illustrated when an 80 mm thick layer of copper is placed between the upper AM60 and lower AZ91 sheets before dissimilar friction stir spot welding. The copper interlayer material is displaced upwards in the direction towards the periphery of the tool shoulder during the tool penetration stage in spot welding, see Fig. 14a. However, when the dwell period initiates, the copper interlayer material is drawn in towards the top of the thread on the rotating pin, see Fig. 14b. The flow reversal at the beginning of the dwell period has important consequences since the downward movement of material from beneath the tool shoulder dramatically alters the material flow pattern produced during dissimilar AM60/AZ91 friction stir spot welding. During the dwell period, lower sheet material (AZ91) in the TMAZ region adjacent to the stir zone extremity is moved upwards and follows the boundary of the stir zone in the direction towards the top of the thread on the rotating pin, see Fig. 14a. This readily explains the location of the liquation cracking shown in Fig. 9. The above results suggest that liquation cracking is a feature of the dissimilar AM60/AZ91 friction stir spot welding process using the welding parameter settings applied during the present investigation. However, it should be noted that it might be possible to avoid cracking using a different combination of welding parameter settings. For example, cracking might be avoided through the use of longer dwell time settings, which would facilitate dissolution of melted eutectic material in the high temperature stir zone. Also, it is possible that the cracking tendency during dissimilar friction stir spot welding might be altered through the
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a cracking in location beneath tool shoulder close to its periphery (see arrows); b SEM graph of fracture surface indicated by arrow in Fig. 10a above: chemical composition of location A in Fig. 10b is 80?1Mg–19?9Al (wt-%) 10 Dissimilar AZ91/AM60 spot weld, where AZ91 is upper sheet in dissimilar sandwich, during dissimilar spot welding tool rotational speed was 3000 rev min21, plunge rate was 2?5 mm s21 and dwell time was 2 s
9 a liquation cracking in dissimilar AM60/AZ91 spot weld made using dwell time of 1 s, b and c liquation cracking in dissimilar AM60/AZ91 spot weld made using dwell time of 2 s: chemical composition at location A in Fig. 9c is 74?3Mg–25?7Al (wt-%)
11 Intermingled lamellae of AM60 and AZ91 in stir zone produced during dissimilar AM60/AZ91 friction stir spot welding; tool rotational speed was 3000 rev min21, plunge rate was 2?5 mm s21 and dwell time was 2 s; chemical compositions at locations A and B are 91?1Mg–7?5Al–0?8Mn–0?6Zn (wt-%) and 93?5Mg–6?3Al–0?3Mn (wt-%)
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12 Horizontal cross-section through dissimilar AM60/ AZ91 spot weld showing microstructure in TMAZ region at extremity of stir zone in lower sheet (AZ91) material: tool rotational speed was 3000 rev min21, plunge rate was 2?5 mm s21 and dwell time was 2 s
use of different tool geometries and slower plunge rate settings. A TMAZ microstructure comprising elongated a-Mg grains and aligned Mg17Al12 particles forms as a natural consequence of the tool penetration stage in spot welding and since the temperature in the stir zone (504uC) exceeds the a-MgzMg17Al12 eutectic temperature (437uC) there will be a strong likelihood of melted eutectic film formation in material adjacent to the stir zone extremity.
Cracking in AZ91/AM60 spot welds There was no evidence of liquation cracking in the TMAZ region produced in the lower AM60 sheet or of LPI cracking in the stir zone formed during dissimilar AZ91/ AM60 friction stir spot welding, see Figure 10. It has already been confirmed that cracking during AM60/AM60 spot welding is only apparent very early in the dwell period in spot welding and crack free joints are produced when the dwell period in spot welding is .0?1 s.19 However, during dissimilar friction stir spot welding, cracking is apparent in upper sheet material (AZ91), see the arrows in Fig. 10a and is associated with melted eutectic film formation in the location beneath at the tool shoulder close to its extremity, see Fig. 10b. It is apparent from Fig. 2 that the temperature measured at the location 1?3 mm from the outer periphery of the tool shoulder closely corresponds with the a-MgzMg17Al12 eutectic temperature (437uC).
Conclusions Cracking during dissimilar friction stir spot welding of thixomoulded AM60 and AZ91 sheet materials was investigated. The following have been confirmed.
a displacement of copper interlayer material upwards during tool penetration stage; b incorporation of copper interlayer material into intermixed region formed adjacent to periphery of rotating pin during spot welding using dwell time of 2 s 14 Transverse cross-sections showing displacement of 80 mm thick copper interlayer during tool penetration stage in dissimilar AM60/AZ91 spot welding: tool rotational speed was 3000 rev min21 and plunge rate was 2?5 mm s21
1. During dissimilar friction stir spot welding of AM60 and AZ91 the temperature in the stir zone at the beginning of the dwell period is determined by the upper sheet in the dissimilar sandwich. When AM60 is
13 Weld profiles at stages D-2 and D-5 in Fig. 3: a before and b after formation of discontinuity shown in Fig. 6
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the upper sheet, the temperature in the stir zone at the beginning of the dwell period (504uC) is similar to that when AM60 sheets are spot welded. Also, the temperature in the stir zone at the beginning of the dwell period (437uC) AZ91 is the upper sheet is similar to that when AZ91 sheets are spot welded. Similar stir zone temperatures produced at the end of the 4 s long dwell period during dissimilar AM60/AZ91 and AZ91/AM60 friction stir spot welding as a result of incorporation of AM60 and AZ91 into the stir zone. 2. Liquation cracking occurs in the TMAZ region during friction stir spot welding when AZ91 material is the lower sheet in the dissimilar sandwich. A TMAZ microstructure comprising elongated a-Mg grains and aligned Mg17Al12 is created during the tool penetration stage in spot welding and melted eutectic film formation is facilitated since the temperature in the stir zone (504uC) is much higher than the a-MgzMg17Al12 eutectic temperature (437uC). 3. There is no evidence of liquation cracking in the stir zone or the TMAZ region of AM60 sheet when it is the lower sheet in the dissimilar sandwich. However, liquation cracking is observed in upper sheet material (AZ91) in the location beneath the tool shoulder close to its periphery.
Acknowledgement The authors wish to acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada during this project.
References 1. B. Porro and P. Beatrice: Proc. 3rd Int. Magnesium Conf., (ed. G. W. Lorimer), 167–176; 1997, London, Institute of Materials. 2. G. S. Cole: Automot. Light Met., 2001, 1, 1. 3. in ‘Metals handbook’, 9th edn, Vol. 6, ‘Welding, brazing, and soldering’, 429–435; 1983, Materials Park, OH, ASM International. Author: please supply the name/s of author/s.
Cracking in dissimilar Mg alloy friction stir spot welds
4. M. Avedisian and H. Baker (eds.): ‘ASM specialty handbook: magnesium and magnesium alloys’, 106–118; 1999, Materials Park, OH, ASM International. 5. K. Nakata, S. Inoki, Y. Nagano, T. Hashemoto, S. Johogan and M. Ushio: Proc. 3rd Int. Symp. on ‘Friction stir welding’, Kobe, Japan, September 2001, TWI. 6. S. Lathabai, K. J. Barton, D. Harris, P. G. Lloyd, D. M. Viano and A. McLean: Magnesium Technology 2003, (ed. H. I. Kaplan), 157– 162; 2003, Warrendale, PA, TMS. 7. A. Stern, A. Munitz and G. Kohn: ‘Magnesium technology 2003’, (ed. H. I. Kaplan), 163–168; 2003, Warrendale, PA, TMS. 8. C. Schilling, A. von Strombeck, J. F. dos Santos and N. von Heesen: Proc. 2nd Int. Symp. on ‘Friction stir welding’, Gothenburg, Sweden, June 2000, TWI. 9. R. Sakano, K. Murakami, K. Yamashita, T. Hyoe, M. Fujimoto, M. Inuzuka, Y. Nagao and H. Kashiki: Proc. 7th Int. Symp. JWS, Kobe, Japan, November 2001, JWS, 645–650. 10. A. Gerlich, M. Yamamoto and T. H. North: J. Mater. Sci., 2008, 43, (1), 2–11. 11. M. Yamamoto, A. Gerlich and T. H. North: Proc. GKSS Int. Semin. on ‘Friction based spot welding processes’, Hamburg, Germany, March 2007, GKSS. 12. A. Gerlich, G. Avramovic-Cingara and T. H. North: Met. Trans. A, 2006, 37A, 2773–2786. 13. C. G. Rhodes, M. W. Mahoney and W. H. Bingel: Scr. Mater., 1997, 36, 69–75. 14. T. Shibayanagi and M. Maeda: Trans. JWRI, 2004, 33, 17–23. 15. R. S. Mishra and M. W. Mahoney: Mater. Sci. Forum, 2001, 357– 359, 507–514. 16. J.-Q. Su, T. W. Nelson, M. Mishra and M. Mahoney: Acta Mater., 2003, 51, 713–729. 17. J.-Q. Su, T. W. Nelson and C. J. Sterling: Scr. Mater., 2005, 52, 135–140. 18. M. Yamamoto, A. Gerlich, T. H. North and K. Shinozaki: Sci. Technol. Weld. Joining, 2007, 12, (3), 208–216. 19. M. Yamamoto, A. Gerlich, T. H. North and K. Shinozaki: J. Mater. Sci., 2007, 42, (18), 7657–7666. 20. A. Gerlich, P. Su, T. H. North and G. J. Bendzsak: Mater. Forum, 2005, 29, 290–294. 21. P. Su, A. Gerlich, T. H. North and G. J. Bendzsak: Sci. Technol. Weld. Joining, 2006, 11, (1), 61–71. 22. A. Gerlich, M. Yamamoto and T. H. North: Sci. Technol. Weld. Joining, 2007, 12, (6), 472–480. 23. P. Su, A. Gerlich, T. H. North and G. J. Bendzsak: Metall. Trans. A, 38A, (3), 584–595.
Science and Technology of Welding and Joining
2008
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7
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