Microstructure characteristics of thick aluminum alloy plate joints welded by fiber laser Zhihui Zhang a,b,c , Shiyun Dong b , Yujiang Wang b , Binshi Xu a,b , Jinxiang Fang a,b , Peng He a, ⁎ a State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China b National Key Laboratory for Remanufacturing, Academy of Armored Forces Engineering, Beijing 100072, China c Department of Mechanical Engineering, Academy of Armored Force Technology, Changchun, Jilin 130117, China abstract article info Article history: Received 8 January 2015 Received in revised form 11 June 2015 Accepted 13 June 2015 Available online 25 June 2015 Keywords: Super narrow gap Fiber laser Zr and Er micro-alloying Aluminum alloy It's difficult to weld high strength thick plate since the groove is huge when using traditional arc welding, and the weld tends to be softened and large deformation could occur after multi-layer welding. All of these can affect the industrial application of high strength thick plate wielding. In this case, developing advanced welding technology and welding material is necessary to optimize the microstructure and performance of the welds. Fiber laser has many advantages such as good monochrome and high quality laser beam. In order to decrease the heat damage to the base metal from the welding heat source, low heat input is employed for welding thick plate. Fiber laser is applied in the welding of 20 mm thick Al–Zn–Mg–Cu alloy with super narrow gap filler wire. The microstructure comparison of Al–Mg–Mn alloy and Al–Mg–Mn–Zr–Er alloy welded joints reveals that a huge amount of fine equiaxed grains is formed in the weld zone of Zr and Er micro-alloying Al–Mg–Mn alloy welding wire and a great number of precipitation strengthening phases are precipitated in the weld zone after the heat treatment of welded joints in the entirety. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The welding of thick metallic plates usually involves wide-angle groove, multilayer, multipass, large amount of metal filling and tremen- dous residual stress. The narrow gap welding method for welding thick metal plates can-not only reduce the transverse section of the groove drastically and cut down the amount of weld metal deposition signifi- cantly, but also realize efficient welding with small weld heat input [1–3]. The commonly used narrow gap welding technologies include nar- row gap submerged arc welding (SAW), narrow gap argon tungsten-arc welding, narrow gap electroslag welding and narrow gap laser welding. Nonetheless, super narrow gap fiber laser welding with filler wire is rare- ly reported. It applies linear heat input smaller than commonly adopted narrow gap welding, which can be minimized to 0.5 kj/mm. Fiber laser boasts the advantages of high quality light beam, perfect monochromatic- ity, short wave length and favorable technological adaptability [4,5]. Combining the edges of fiber laser and super narrow gap, it can reduce the heat losses upon the welding structure remarkably by welding heat source. As a result, an appropriate selection of welding materials can enhance the overall mechanical properties of welded joints. Aluminum alloy is the priority material for aircraft and spacecraft, which is extensively applied in the aerospace industry [6]. For example, 180 t thick aluminum plates are used in each airbus. However, in practical application, the aluminum alloy thicker than or equal to 20 mm accounts for 50%. The purpose of the present work aims at adopting small power fiber laser for effective welding of 20 mm thick Al–Zn–Mg–Cu alloy, with single “I” pass groove whose horizontal width is 2.5 mm. Micro-alloying functions as a pivotal method to boost the properties of aluminum alloy. Studies indicate that the additions of trace rare earth elements into the aluminum alloy can remarkably refine the grains [7], and could purify the composition of alloy so as to improve the mechan- ical properties at last. The addition of Sc into the aluminum alloy can os- tentatiously boost the strength of alloy while the low cost element Er shares similar functions with Sc in the aluminum alloy [8]. In order to compare the influence of different welding materials upon the micro- structure of joints welded by super narrow gap fiber laser, the present study selects 5183 alloy and 5E06 as the welding wire. 2. Experimental procedures YLS-4000 YB-doped fiber laser with a wave-length of 1070 nm and maximum rate output power of 4000 W produced by IPG Photonics is applied in the experiment. During the welding, the laser head vertically deviates from the direction of the work piece by 10° to prevent reflective laser from damaging the focus lens. The motor execution sys- tem adopts two-dimensional welding machines. The experimental ma- terials are 7A52 high strength aluminum alloy with a dimension of 160 mm × 80 mm × 20 mm. Filler wires with different chemical Materials and Design 84 (2015) 173–177 ⁎ Corresponding author. E-mail address: 576492531@qq.com (P. He). http://dx.doi.org/10.1016/j.matdes.2015.06.087 0264-1275/© 2015 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/jmad