Influence of surface enhanced treatment on microstructure and fatigue performance of 6005A aluminum alloy welded joint

Abstract

The application of 6005A aluminum alloy in transportation is increasing, which puts forward higher requirements for the fatigue performance of welding process. In this study, friction stir welding (FSW) enhanced treatment was applied on the traditional metal inert gas (MIG) weld. The results show that the fatigue life of FSW plate is three times higher than that of original MIG welded plate at 140 MPa stress level. Under 10⁷ cycles, the fatigue life limit was increased by 16 MPa compared with as-received MIG plate. The main reason of FSW leading to the increasing fatigue life are the reduction of porosity to reduce crack initiation and grain refinement to delay crack propagation.

Introduction

6005A aluminum alloys are increasingly used for structural applications, particularly in transportation industry such as high-speed railway, subway, ship and aviation due to their small density, corrosion resistance and high machinability, which are much better than steel products[1][2][3]. Recently, the 6005A aluminum alloys are extensively used for some important structural applications, which require higher fatigue property and reliability for welding process [4].

Welding jointed components have wide applications in subways, MIG (metal inert gas welding) becomes the main welding process for different aluminum alloy welded joints because of its several advantages, such as ideal fast welding effect, low cost and easy operability [5][6]. However, this traditional technique also faces some material defects such as porosity, hot cracking, strength reduction, deformation and residual stress are often brought about by MIG process [7]. These defects, especially porosity, will seriously soften the joints, leading to fatigue fracture of the fragile welded joint, and even significant safety accidents [8]. Some researchers have revealed that the cracks are initiated by porosity, which also lead to the fatigue life decreasing relative to the base metal [9] [10]. Recently, friction stir processing (FSP) has become a new technology to improve the fatigue performance of MIG welds [11].

FSP has been proven to be an effective technique which could bring an improvement of fatigue strength in the surface due to the reduction of welding defects and refined grain [12]. Mishra.R.S [13] reported the current state of understanding and development of the FSW and FSP, which reported that serve plastic deformation and temperature rise result in significant microstructural evolution within the welding zone with fine recrystallized grains arranging from 0.1 to 18 μm. Cavaliere[14] also revealed a 20% increase for the fatigue properties of a FSP Zr-modified 2014 aluminium alloy compared to the base metal after 10⁶ cycles, resulting from the fine and stable recrystallized microstructure during the FSP. In Husain Mehdi’s [15] work, they reported the influences of FSP on the tungsten inert gas (TIG) welding with dissimilar aluminum alloy, which indicated that after FSP the maximum compressive residual stress decrease from 73 MPa to 37 MPa.

Although FSP has many advantages, it only improves the performance of the material surface. Firstly, FSP is difficult to operate because of the high requirements of welding surface smoothness and flatness, and it also need a rotating tool with a specially designed pin and shoulder [16]. Secondly, though there are almost no pores and large grains on the surface after FSP, the internal pores, dendrites and coarse phases are not eliminated. Thirdly, FSP usually only works on one surface so that the other side cannot be strengthened.

In contrast, friction stir welding requires no special preparation and only two clean metal plates can be easily joined together in the form of butt or lap joints, without major consideration of the surface conditions of the plate [13]. Although MIG is the most widely used, most mature and lowest cost welding method, its welding coefficient and fatigue performance are not as good as BM. The important components in high-speed train need high mechanical support, so FSW technique is applied to locally reinforce these MIG samples. In order to improve the fatigue strength and prolong the service life of MIG welded thick plate, the influence of friction stir welding treatment on the properties of MIG welded thick plate was studied, especially for the tensile properties and high cycle fatigue properties. The fatigue failure mechanism of MIG welded aluminum alloy plate was analyzed by means of fatigue test, mechanical property test and microstructure observation.