See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/27424417 Study of Microstructure and Residual Stresses in Dissimilar Al/Steel Welds Produced by Cold Metal Transfer 2007-09-24 - 2007-09-26 Article in Materials Science Forum · March 2008 DOI: 10.4028/www.scientific.net/MSF.571-572.347 · Source: OAI CITATIONS READS 9 1,013 9 authors, including: Leonardo Agudo Jácome Haroldo Pinto Bundesanstalt für Materialforschung und -prüfung University of São Paulo 31 PUBLICATIONS 415 CITATIONS 122 PUBLICATIONS 585 CITATIONS SEE PROFILE SEE PROFILE Aleksander Kostka Sharna Weber Ruhr-Universität Bochum Century College 158 PUBLICATIONS 3,161 CITATIONS 97 PUBLICATIONS 863 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Phase Transformation Kinetics in Titanium Alloys View project Synchrotron X-ray Diffraction Analyses of Microstructure, Mechanical Behavior, Residual Stresses and Crystallographic Texture View project All content following this page was uploaded by Haroldo Pinto on 18 June 2015. The user has requested enhancement of the downloaded file. Materials Science Forum Vols. 571-572 (2008) pp 347-353 online at http://www.scientific.net © (2008) Trans Tech Publications, Switzerland Online available since 2008/Mar/07 Study of Microstructure and Residual Stresses in dissimilar Al/Steels welds produced by Cold Metal Transfer L. Agudo1,a, S. Weber1,b, H. Pinto 1,c, E. Arenholz2,d, J. Wagner2,e, H. Hackl3,f, J. Bruckner3,g, A. Pyzalla1,h 1 Max-Planck Institut für Eisenforschung GmbH, Max-Planck Str. 1, 40237 Düsseldorf, Germany2 voestalpine Stahl GmbH, voestalpine – Strasse 3, 4020 Linz, Austria 3 Fronius GmbH, Buxbaumstraße 2, 4600 Wels, Austria agudo@mpie.de, bweber@mpie.de, cpinto@mpie.de, dEnno.Arenholz@voestalpine.com, e Juergen.Wagner@voestalpine.com, fHackl.Heinz@fronius.com, gBruckner.Juergen@fronius.com, h pyzalla@mpie.de a Keywords: CMT, Welding, Aluminum, Residual Stresses, Microstructure Abstract. Recently a new welding technique, the so-called ‘Cold Metal Transfer’ (CMT) technique was introduced, which due to integrated wire feeding leads to lower heat input and higher productivity compared to other gas metal arc (GMA) techniques. Here microstructure formation and residual stress state in dissimilar steel to aluminum CMT welds are investigated. The intermetallic phase seam between the filler and the steel is only a few micrometers thick. Residual stress analyses reveal the formation of the typical residual stress state of a weld without phase transformation. Both in longitudinal and in transversal direction compressive residual stresses exist in the steel plate parent material, tensile residual stresses are present in the heat affected zone of the steel and the aluminum alloy. The area containing tensile residual stresses is larger in the aluminum alloy due to its higher heat conductivity than in the steel. Due to the symmetry in the patented voestalpine welding geometry and the welding from bottom and face side of the weld, the residual stress distributions at the top and at the bottom side of the weld are very similar. Introduction Tailored blanks combining steel and aluminum alloys could be very advantageous for light-weight engineering in automotive industry [1-3]. The production of these tailored blanks necessitates the development of reliable and cost efficient joining methods, which yield joints with good mechanical properties and in particular good forming characteristics as well as sufficient corrosion resistance. Fusion welding of steels to aluminum alloys, however, still remains a challenge because of the large differences in their electrochemical potential, their melting point, their thermal expansion coefficient and their thermal conductivity, and in particular due to the formation of brittle intermetallic aluminum-rich FexAly intermetallic phases [4] (Fig. 1) in the weld seam. Among the fusion joining techniques for joining steel to aluminum sheets is laser beam joining by heat conduction through the steel into the aluminum alloy which then melts and bonds to the steel surface. Main drawbacks of the laser welding process are the necessity of a flux medium, the limitation of the steel sheet thickness to about 1.5mm to ensure heat transfer, high requirements on the flatness and restrictions in allowable variation in size of the sheets to be joined and the high investment costs [5-7]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 192.12.81.1-07/03/08,14:05:04) 348 Stress Evaluation Using Neutrons and Synchrotron Radiation Fig. 1. Fe-Al Phase Diagram [4] Another fusion joining technique for steel to aluminum welds is the so-called ‘Cold Metal Transfer’ (CMT) technique, which was introduced by Fronius International GmbH, Wels, Austria. From the point of view of welding, ‘cold’ is a relative concept. In the CMT technique the work pieces to be joined, remain considerably ‘colder’ than in conventional ‘Gas Metal Arc’ (GMA) processes. The main characteristic that distinguishes the CMT welding process from a conventional metal arc welding technique is the incorporation of wire motion into process control (Fig. 2a), which substantially reduces heat input [8]. In addition the patented voestalpine welding geometry [9,10] is symmetric (Fig. 2b), which improves mechanical properties as well as corrosion protection. Due to the large difference in the melting points in case of fusion joining steel to aluminum alloys, the aluminum alloy usually melts while the steel remains in solid state. Therefore, joining of Al-alloys to steel is usually achieved via aluminum alloy filler. The aluminum alloy is welded to the aluminum alloy filler material while brazing occurs between the filler and the steel. The filler material is chosen under the pre-conditions that it has sufficient strength and ductility, and that the formation of intermetallic phases can be controlled with respect to both structure of the intermetallic phases and thickness of the intermetallic phase seam. Based on earlier work [11-13] AlSi3Mn1 was chosen as filler here and Al99.5 serves as a reference filler material. Experimental Details Welding. Using the CMT method, 1mm thick DX54D+Z200 steel sheets were joined to 1.5mm thick AW5182-H111 sheets at Fronius International GmbH using 1.2mm diameter Al99.5 and AlSi3Mn1 filler wires, all parent materials being provided by voestalpine Stahl GmbH. The optimized voestalpine welding geometry [9-11] (Fig. 2) and optimized parameters for the CMT welding process were chosen (Table 1). The samples were clamped during the welding process in order to avoid distortion. Metallography and Microscopy. For optical (OM), scanning electron (SEM) and transmission electron (TEM) microscopy, specimens were cut, ground and polished. Specimen microstructure was characterized in the SEM using electron back scattering diffraction (EBSD). Materials Science Forum Vols. 571-572 349 Fig. 2. (a) Sequence of CMT process stages and b) Geometry of the steel-aluminum CMT-weld (detail from (a), right) Table 1. Welding parameters Weld CMT vS [cm/min] vD [m/min] I [A] U [V] f [Hz] 4.5 70 12 70 60 vS: Welding speed, vD: Wire feeding speed, I: Current, U: Voltage, f: Frequency. Table 2. Properties of the aluminum alloy, steel parent material and the AlSi3Mn1 filler [11] (α α: coefficient of thermal expansion, κ: thermal conductivity). Rp0.2 [MPa] Rm [MPa] α [1/K] κ [W/mK] DX54D+Z 182±1 313±2 ~12x10-6* ~75* AW5182-H111 142±2 276±1 AlSi3Mn1 50 120 ~24x10-6** ~240** *for pure Iron; ** for pure Aluminum Residual Stress Analysis. Residual stress analysis was carried out using the instrument G3 at HASYLAB, DESY, Hamburg, Germany at 6.4keV energy. Residual stresses in longitudinal (along the weld) and in transversal direction were determined by the sin2ψ method (Al311 and α-Fe200 reflections) at the top and bottom of a AW5182-H111 aluminum to DX54D+Z200 steel butt-joint with AlSi3Mn1 filler, in the center of the weld seam and along both the steel and aluminum sheets up to 80mm distance from the weld centerline (Fig. 3). Face and bottom sides of the butt joint were electro-polished in order to access residual stresses in a depth of 100µm beneath the original surface. After electro-polishing residual stresses were determined in the weld seam, and in the aluminum and steel sheets up to a distance of 35mm from the weld seam. The diffraction elastic constants were calculated using Kröner’s model [14]. 350 Stress Evaluation Using Neutrons and Synchrotron Radiation Fig. 3. Scheme of stress measurement locations. Iconography corresponds to Fig. 5 and Fig. 6. Results and Discussion Microstructure. The welds did not contain any welding defects. Few pores (Fig. 2b) are visible, which are commonly formed in fusion-welded aluminum alloys due to their small solidification interval, which hampers the release of dissolved gas. Micrographs of the welded samples reveal different intermetallic phase morphologies (Fig. 4) at the interface between the molten filler and the steel when using Al99.5 and AlSi3Mn1 filler materials. The chosen welding parameters ensured that the thickness of the intermetallic phase seam (IMP) in both welds with Al99.5 filler (IMP thickness about 3.5µm) and AlSi3Mn1 filler (IMP thickness about 2.5µm) was well below the 10µm limit, which often has been claimed to be critical with respect to weld toughness [15,16]. The phases identified in the IMP of the weld with the Al99.5 filler are θ-phase Al3Fe (adjacent the filler) and η-phase Al5Fe2 (adjacent to the steel). The intermetallic phase seam in case of the AlSi3Mn1 filler contains also the θ-phase Al3Fe and the ηphase Al5Fe2 and additionally an Al(Fe,Mn)Si – phase. Fig. 4. SE-SEM micrograph showing the intermetallic phase seam (IMP) in welds with (a) Al99.5 filler and (b) AlSi3Mn1 filler. Residual Stresses. Because of its higher technical relevance due to the higher strength of the AlSi3Mn1 filler compared to the Al99.5 filler residual stress analyses focused on the weld with the AlSi3Mn1 filler material. The residual stress distributions at the surface of the face side of the weld and 100µm beneath the surface of the weld face side are very similar (Fig. 5a), the same is true for the residual stresses at the surface of the bottom side of the welds (Fig. 5b, Fig. 6) and 100µm beneath it. The residual stress distribution in longitudinal direction both at the face (Fig. 5a) and the bottom side of the weld (Fig. 5b) are characterized by compressive residual stresses within the steel plate parent material, which turn into tensile residual stresses in the heat affected zone with a steep gradient. The filler in the weld seam and the heat affected zone in the aluminum alloy are under tensile residual stresses, and the aluminum parent material contains balancing compressive residual Materials Science Forum Vols. 571-572 351 stresses in longitudinal direction. The maximum value of the residual stresses in the aluminum alloy, on the top side of the weld near the weld seam, is as high as 100 MPa and, thus, about 2/3 of its yield strength. The residual stress distribution in transversal direction at the bottom side of the weld (Fig. 6) is very similar to the residual stress distribution in longitudinal direction. The residual stress values encountered are slightly lower in transversal direction compared to the longitudinal direction. The residual stress distributions determined are typical for welds without phase transformation. Martensitic phase transformation (proven by temperature measurements and also by metallography) does not occur in the steel because the temperature reached in the joining process basically is only slightly higher than the melting temperature of the aluminum alloy. Fig. 5. Residual stress state along the longitudinal direction (↓) of a joint with AlSi3Mn1 aluminum filler on the surface and ~100µm beneath on (a) top and (b) bottom side. Fig. 6. Residual stress state along the transversal direction (→) of a joint with AlSi3Mn1 aluminum filler on surface and ~100µm beneath on bottom side. The cold parent materials hinder shrinkage of the weld seam and the heat affected zone, thus introducing compressive residual stresses into the base material and tensile residual stresses into the weld seam and the heat affected zone [16]. 352 Stress Evaluation Using Neutrons and Synchrotron Radiation The thermal conductivity of the aluminum alloy is larger than the thermal conductivity of the steel (Table 2). Therefore, the heat affected zone and the region containing tensile residual stresses is larger in the aluminum alloy than in the steel. The larger extension of the stress field conductivity and the higher thermal expansion coefficient of the aluminum alloy (Table 2) contribute to its slightly higher tensile residual stresses compared to the steel plate and to the compressive residual stresses in the steel plate parent material. The strong similarity between the residual stress at the face and the bottom side of the weld can be attributed both to the welding process, which includes welding at both the top and the bottom side of the weld and to the symmetry of the voestalpine welding geometry. Summary and Conclusions The microstructure formation and the residual stress state in joints of dissimilar steel to aluminum alloys were investigated. The welds were manufactured by the CMT method (a modified gas metal arc welding technique) in the voestalpine welding geometry using an AlSi3Mn1 filler material. Microstructure analyses show welding between aluminum alloy and filler and brazing between filler and steel. The intermetallic phase seam at the interface between filler and steel is only few micrometers thick. Residual stress analyses reveal a typical welding residual stress state of a weld without phase transformation. Both in longitudinal and in transversal direction compressive residual stresses are present in the steel plate parent material, while tensile residual stresses were determined in the heat affected zone of the steel and the aluminum alloy. The area containing tensile residual stresses due to its higher heat conductivity is larger in the aluminum alloy than in the steel. The residual stress distributions at the top and at the bottom side of the weld are very similar due to controlled welding parameters and the symmetric welding geometry. Acknowledgements The authors thank Dr. A. Rothkirch and Dr. T. Wroblewski, HASYLAB at DESY and in particular Dipl.-Ing. (FH) Benjamin Breitbach, MPIE, for experimental support. References [1] G. Kobe, Aluminium/steel welding, Automotive Industries 7 (1994) 44. [2] S. Ramasamy, Welding Journal 2 (2003) 35 -39. [3] J. Bruckner, E. Arenholz, G. Schmid, Gas-Metal-Arc-joining of steel to aluminium, 5. 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