Effect Of Annealing Treatment On Joint Strength Of AluminumSteel Friction Stir Lap Weld

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Accepted Manuscript Effect of annealing treatment on joint strength of aluminum/steel friction stir lap weld M Movahedi, A.H Kokabi, S.M Seyed Reihani, W.J Cheng, C.J Wang PII: DOI: Reference: S0261-3069(12)00567-5 http://dx.doi.org/10.1016/j.matdes.2012.08.028 JMAD 4743 To appear in: Materials and Design Received Date: Accepted Date: 25 May 2012 12 August 2012 Please cite this article as: Movahedi, M., Kokabi, A.H., Seyed Reihani, S.M., Cheng, W.J., Wang, C.J., Effect of annealing treatment on joint strength of aluminum/steel friction stir lap weld, Materials and Design (2012), doi: http://dx.doi.org/10.1016/j.matdes.2012.08.028 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Effect of annealing treatment on joint strength of aluminum/steel friction stir lap weld a a a M Movahedi ,1, A.H Kokabi , S.M Seyed Reihani , W.J Chengb, C.J Wangb a Department of Materials Science and Engineering, Sharif University of Technology, P.O Box 11365-9466, Azadi Ave., Tehran, Iran b Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan, R.O.C Abstract In this work, the effects of the annealing temperature and duration on the joint strength of the steel/aluminum lap joints made by friction stir welding were investigated The strength of the welded joints was evaluated by shear-tensile test Formation of intermetallic layer at the joint interface and the fracture locations were studied using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) The results indicated that longer time of annealing treatment resulted in higher joint strength at 300 and 350 °C and that the rate of the increase in joint strength with time was about 72% higher at the latter temperature than at the former one In comparison with the atomic diffusion through the joint interface, the formation and development of a thin intermetallic layer in the joint after annealing at 300 and 350 °C (with the thickness of less than 0.49 m) may have a higher impact on the improvement of the joint strength However, the joint strength decreased with the increase in annealing duration at 400 oC In the samples annealed at 400 oC for more than 90 min, the joint strengths were degraded drastically From the joint strength point of view, intermetallic layer thickness of ~2.6 m was a critical thickness Keywords: aluminium; steel; annealing treatment; intermetallic; friction stir welding; joint strength Corresponding author's e-mail: movahedi@mehr.sharif.edu; Tel and Fax: (+98) 2166165243 1 Introduction Joining of steels to aluminum alloys can be used for producing steel/aluminum bimetallic parts in a wide range of industrial areas such as transportation, shipbuilding and aerospace [13] However, it is difficult to obtain a sound steel to aluminum joint by using the conventional fusion welding processes due to the large difference between the melting points of steel and aluminum alloys and also the formation of thick brittle Al/Fe intermetallic compounds at the joint interface [1, 2, 4] Compared to the fusion processes, low-heat generation during solid state welding makes it a highly potential approach for aluminum to steel joining Friction stir welding (FSW) is a solid state process during which a rotating tool containing a pin and a shoulder is plunged into the joint and so, heat is generated by the friction between the tool and the work pieces as well as the plastic deformation [5-8] In the last decade, some researchers have studied the friction stir butt and lap welding of steels to various aluminum alloys [1, 2, 9, 10] An annealing treatment is carried out after welding of dissimilar metals to improve the metallurgical bond at the bond interface, to relieve the residual stresses and to modify the weld zone microstructure It should be noted that the annealing temperature and duration must be controlled accurately in the case of steel/aluminum alloys joint as there is a serious concern about the formation of brittle intermetallics of aluminum and iron at the bond interface The growth of intermetallic layer is governed by chemical reaction at the interface and interdiffusion of reacting species through the formed layer Since diffusion phenomenon depends on annealing conditions, i.e temperature and duration, the Al/Fe intermetallic layer composition and thickness can be controlled by using proper annealing conditions [11, 12] Generally, the intermetallic layer thickness plays an important role in obtaining strong joints Some researchers believe that an intermetallic layer with a thickness of less than 10 µm impose no detrimental effect on the joint strength and even may improve the joint quality However, when the layer thickness is more than 10 µm, the joint strength declines significantly [11, 13-18] It is noteworthy that some investigations revealed that the formation of intermetallic phases, regardless of their thicknesses, reduces the bond quality of the joint [19, 20] As can be seen, there is no consistency between the results presented in the literature about the effects of the Al/Fe intermetallic thickness on the joint strength There are few papers discussing the effects of the annealing treatment on the joint strength of aluminum/steel friction stir welded joints [21] Springer et al [21] studied the effects of annealing in the range of 200–600 oC for 9–64 on the formation of intermetallic reaction layers and their influence on the joint strength in friction stir ‘butt’ welding between a low carbon steel and pure Al (99.5 wt.%) and Al–5 wt.% Si In general, their results showed that as the temperature and duration of annealing was increased, the joint strengths decreased In the reaction layers with a thickness of ∼7 m with pure Al and 1.6 m with Al–5 wt.% Si, the fracture location changed from the Al-weld region to the joint interface It means that the strength of the joint interface decreased significantly with the increase in the thickness of the intermetallic layer over and 1.6 m for pure Al and Al–5 wt.% Si, respectively Moreover, they noted that the variations of the tool rotation and the travel speeds and so, the resulting changes in the plastic deformation and the amount of the stored energy in the Al/Fe interdiffusion systems can affect the atomic diffusion and the intermetallic phases nucleation through the interface during the annealing treatment To the best knowledge of the authors, no attempt has been made so far to study the effect of the post-weld annealing treatment on the joint strength of the aluminum/steel friction stir ‘lap’ welds Therefore, in the present study, the effects of the annealing temperature and duration on the joint strength of the St-12/Al-5083 lap joints made by friction stir welding were investigated The focus was on the formation of the intermetallic layer at the joint interface and its effect on the joint strength in a constant combination of the tool travel and rotation speeds Thus, investigation on the effect of the friction stir welding parameters such as tool travel and rotation speeds on the formation of the intermetallic layers and joint strengths was not at the scope of the present paper and may be followed in the future works Materials and experimental procedures Al-5083 (Al-4.45 wt.% Mg) and St-12 (Fe- 0.6 and >0.25, respectively (See the electronic color version of the article.) 14 Table Intermetallic layer thickness Anneal temp (oC) 350 Anneal duration (min) 30 0.35±0.20 Thickness (m) 350 90 0.41±0.20 350 180 0.49±0.19 400 30 0.77±0.34 15 400 45 1.43±0.33 400 60 2.60±0.43 400 90 5.11±0.62 400 180 7.80±1.31 Fig Fig Fig Fig Fig Fig Fig Fig Fig Research Highlights  Effects of annealing conditions on the joint strength were investigated  Longer time of annealing resulted in higher joint strength at 300 and 350°C  Rate of increase in joint strength with time was higher at 350oC than at 300oC  Fracture loads decreased with increase in annealing duration at 400oC  Intermetallic layer thickness of 2.6 m was a critical thickness
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