Microstructure characteristics and performance of dissimilar welds between magnesium alloy and aluminum formed by friction stirring

Microstructure characteristics and performance of dissimilar welds between magnesium alloy and aluminum formed by friction stirring

Scripta Materialia 53 (2005) 585–589 www.actamat-journals.com Microstructure characteristics and performance of dissimilar welds between magnesium al...

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Scripta Materialia 53 (2005) 585–589 www.actamat-journals.com

Microstructure characteristics and performance of dissimilar welds between magnesium alloy and aluminum formed by friction stirring Jiuchun Yan, Zhiwu Xu *, Zhiyuan Li, Lei Li, Shiqin Yang State Key Laboratory of Advanced Welding Production Technology, School of Materials Science and Engineering, Harbin Institute of Technology, No. 92 West Da-Zhi Street, Harbin 150001, PR China Received 29 January 2005; received in revised form 28 March 2005; accepted 18 April 2005 Available online 13 May 2005

Abstract The friction stir welding of AZ31Mg/1060Al has been investigated. The welds were formed when the stirring pin was off the centerline to AZ31 or to 1060, and are characterized by intercalation lamellae. Al3Mg2 and Al12Mg17 cause the weld to crack during friction stir welding on the centerline of the weld.  2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Friction stir welding; Microstructure; Intermetallic compounds; Magnesium alloy and aluminum

1. Introduction The development of the automotive industry needs component weights to be reduced in order to improve the performance of automotive vehicles, so it is urgent that light aluminum alloys and magnesium alloys are used for these components. Thus, the problem of welding for aluminum/magnesium alloys must be faced; it is known that a variety of attempts to weld aluminum/ magnesium alloys have failed using arc welding, electron beams and laser beams, because much more intermetallic compounds form in the weld during fusion welding. Friction stir welding is a solid-state welding technique invented by The Welding Institute in 1991 [1]. Defectfree welds with good mechanical properties have now been made in a wide variety of aluminum alloys [2–6] and magnesium alloys [7–11]. For dissimilar materials, Murr et al. have compared the microstructure and *

Corresponding author. Tel.: +86 451 864 18695; fax: +86 451 864 16186. E-mail address: [email protected] (Z. Xu).

micro-hardness distribution of friction stir welded joints such as Al/Ag, Al/Cu, 6061/2024, 6061Al/20%Al2O3 [12–14]. Hirano et al. [15] have found that two kinds of phase occur in the intermediate layer during friction stir welding of AZ31/A1050: one phase was Al12Mg17 intermetallic compound, the other was not defined. The present work investigates the friction stir welding of 1060 and AZ31; the microstructure, chemical composition, performance and fracture profile of the friction stir weld were focused on. The feasibility of friction stir welding of Al and Mg is examined in order to lay a foundation for the practical use of Mg/Al joints.

2. Experimental procedure Samples (4 · 150 · 40 mm) of extruded 1060Al and AZ31Mg were used during friction stir welding. The chemical compositions of 1060Al and AZ31Mg alloy are shown in Tables 1 and 2, respectively. The welding equipment used was a FSW-3LM-003 welding machine, produced by the China Friction Stir Welding Center.

1359-6462/$ - see front matter  2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.04.022

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Table 1 Chemical composition of 1060Al (wt.%) Cu

Si

Fe

Mn

Mg

Zn

Ti

Al

0.05

0.25

0.35

0.03

0.03

0.05

0.03

Remainder

Table 2 Chemical composition of AZ31Mg (wt.%) Al

Mn

Zn

Cu

Ni

Si

Fe

Impurity

Mg

3.0

0.3

0.9

0.002

0.002

0.20

0.003

0.30

Remainder

The rotation tool is made from high-speed steel, and the pin (/6 · 3.8 mm) has standard right-hand threads and the shoulder (/20 mm) is perpendicular to the axis of the tool. The couple samples were friction stir welded with the tool angle of 3, rotation speeds of 200, 315, 500, 600, 800, 1000 rpm and welding speeds of 19, 24, 30, 48, and 75 mm/min. There are two sides to the centerline of the weld in one couple samples. In the welding process, one side is referred to as the ‘‘advancing side’’ when the rotational motion of the pin and its welding motion are in the same direction. The ‘‘retreating side’’ is the side in which the rotational motion is in the opposite direction to that of the welding motion. The sample AZ31 was put on the retreating side at all times in this study. The relatively mixing content of AZ31 and 1060 along the faying surface of the weld was decided by the position of the pin between the Al alloy and magnesium alloy. Experi-

ments were performed when the stirring pin was just perpendicular to the centerline of the weld (called Mode I), off the centerline 4 mm to the magnesium alloy (called Mode II) and off the centerline 4mm to the aluminum (called Mode III). The cross sections of welded joints were prepared for metallographic analysis by a standard polishing technique. The microstructures of bonded joints were examined by electron probe X-ray microanalyser (JEOL-733), and X-ray diffractometer (XRD, XÕPertPw3040). The tensile strength of welded joints was evaluated by means of an electron tension-testing machine (Instron-5569).

3. Results and discussion Fig. 1 shows the profiles of the welds for the three kinds of friction stirring welding modes. When using Mode I, the cracks in the weld occur immediately after the stirring pin passed; the welding speed was 30 mm/ min and the rotation speeds were 200, 315, 500, 800, and 1000 rpm, as shown in Fig. 1a. Using Mode II, the welding speed of 30 mm/min was kept constant, and with the rotation speed at 200 rpm, the welding process is likely to be a milling process, and no integrated weld was shaped. As the rotation speeds were increased to 500, 600 and 800 rpm, the weld can be shaped at the beginning. But the excessive heat caused the weld metal to melt and extrude as the welding continued, rather than stirring the weld metal from the

Fig. 1. Profiles of the welds by using friction stir welding: (a) Mode I; (b) Mode II; (c) Mode III.

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front to the back of the pin to fill the hole formed when the pin advanced. Thus, a groove began to appear from the middle of the weld. When the rotation speed was 1000 rpm, too much heat was generated by friction,

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and the weld cracked when the weld temperature reached the eutectic temperature. The integrated weld could only be formed when the rotation speed was 315 rpm (see Fig. 1b). So it is necessary that the weld

Fig. 2. Back-scattered electron images of a cross section of weld nugget friction stir welded with a rotation speed of 315 rpm and a welding speed of 30 mm/min by using Mode II: (a) the back-scattering spectroscopy image (·600); (b) the scanning image of Al (·600).

Fig. 3. XRD patterns from the friction stirring welds by using the three modes: (a) Mode I; (b) Mode II; (c) Mode III.

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temperature should be controlled below the eutectic temperature to avoid an intermetallic compound occurring while the weld metal can plastically deform. Using Mode III, the integrated weld could be formed when experiments were performed with the welding speed of 20 mm/min and the rotation speed of 315 rpm (see Fig. 1c). Experimental results show that it is hardly possible to form the weld when the weld speed was below 30 mm/min. Fig. 2 shows the back-scattered electron images of the cross section of friction stirring weld of AZ31/1060 with the rotation speed of 315 rpm and the welding speed of 30 mm/min, using Mode II. In the bimetallic weld zone, the light regions and the dark regions are composed of Al and Mg alloy, respectively. The magnesium alloy and aluminum are in the form of strip sheets. Obviously, the volume of Mg alloy is more than that of Al. If we use Mode III, the result will be reversed. It can be seen from Fig. 2a that the magnesium alloy and aluminum at the interface penetrate each other in the form of a vortex, rather than being uniformly blended. Moreover, the phenomenon of complex vortex flow is characterized by intercalation lamellae. These should be attributed to the material undergoing a helical motion within the rotational zone that rotates, advances, and descends in the wash of the threads on the pin and rises on the outer part of the rotational zone. However, the vortex flow will be destroyed if high-rotation speed is used. This illustrates not only the visualization of fascinating solid-state flow phenomena, but also complex chaoticdynamic patterns characteristic of fluid mixing. Since the differences in yield strength between these dissimilar

grains may promote the superplastic-like flow between these different lamella regions, the dynamic recrystallization process provides a mechanism for superplastic flow between lamella regimes to make it possible that the metal can be joined in the solid state. Fig. 3 shows XRD patterns from the friction stir welds using the three welding modes. The results of XRD analysis in Fig. 3a shows that besides Al(Mg) and Mg(Al) solid solutions, approximately 5% of b-Al3Mg2 and 1% of c-Al12Mg17 simultaneously appear in the friction stir weld by using Mode I. These phases are all brittle compounds with eutectic compositions, which are the main reason for the formation of the weld crack. It can be seen from Fig. 3b and c that no new phase besides Al and Mg solid solution with different content

Fig. 4. Results of the tensile strength of friction stir welded joints of 1060/AZ31 made using the three kinds of welding modes.

Fig. 5. Fracture morphologies of friction stirring weld of AZ31/1060 in Mode II: (a) macroscopic fracture morphology (·200); (b) microscopic fracture morphology (·1000).

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(70%Mg, 29%Al in Mode II and 69%Al, 29%Mg in Mode III) forms in the weld by using Mode II and Mode III. Furthermore, this proves that the weld temperature during friction stirring is below the eutectic temperature of Al and Mg. Thus, the brittle phases of b, b 0 , c and c 0 do not form and the weld crack has been avoided during friction stirring by using Mode II and Mode III. Fig. 4 shows the results of the tensile strength of friction stir welded joints of 1060/AZ31. It can be seen that the highest strength of the welded joints in Mode II can be up to 82.4 MPa, which is approximately 67% of the tensile strength of 1060Al and 30% of the tensile strength of AZ31Mg alloy. The tensile strength of the joints made using Mode III is lower than that of the joints made by Mode II because the strength of 1060 is much lower than that of AZ31. The tensile strength of the joints by Mode I is much lower than the others. The tensile strength of gas tungsten arc welded joints of 1060/AZ31 without filler metal is nearly zero, that is, no strength. Fig. 5 shows the fracture morphologies of friction stirring welds of AZ31/1060 in Mode II. The fracture is located near the centerline of the weld within nugget. It can be seen that the fracture characteristic is like a ‘‘stream’’, and that every branch of the ‘‘stream’’ has a different height and parallel step of cleavage plane. These are typical characteristics of cleavage fracture.

4. Conclusions Friction stir welding of AZ31/1060 has been investigated, and the microstructure, chemical composition and fracture profile of the friction stir weld were examined. The welds were formed when the stirring pin was off the centerline to magnesium alloy or to the Al. But when the axis of the pin was just perpendicular to the welding centerline, the brittle intermetallics formed

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cause the weld to crack. The friction stir weld shows complex vortex flow characterized by intercalation lamellae. The results of XRD indicate that the brittle phases of Al3Mg2 and Al12Mg17 are the cause of the weld cracking. The highest strength of the welded joints can be up to 82.4 MPa, which is approximately 67% of the tensile strength of 1060Al and 30% of the tensile strength of AZ31Mg alloy. Fracture morphologies are characterized by the typical characteristics of cleavage fracture.

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