Improvement in fatigue performance of friction stir welded A6061-T6 aluminum alloy by laser peening without coating

Abstract

The authors have applied laser peening without coating (LPwC) to fatigue specimens cut out from friction stir welded (FSWed) A6061-T6 aluminum alloy plates with a thickness of 3 mm. Both crown and root sides of the specimens were peened by laser pulses with an energy of 60 mJ and a peak power density of 2 GW/cm² from a frequency-doubled Nd:YAG laser. The effects on the fatigue properties were studied through plane bending fatigue tests with a stress ratio of R = −1. The results showed that the fatigue strength of unwelded specimens (base material; BM) was 110 MPa at 10⁷ cycles and LPwC enhanced the strength by 60 MPa in spite of increase in surface roughness due to the direct irradiation of the laser pulses to the bare surface of the specimens. Meanwhile the fatigue strength of FSWed specimens was 90 MPa and LPwC enhanced it by 30 MPa to 120 MPa. This increment is a half compared to that in the BM, however the fatigue strength of the FSWed specimens after LPwC was higher than that of the BM. The surface roughness, hardness and residual stress were assessed and characterized as well.

Introduction

Friction stir welding (FSW) is a solid state joining process and primarily used on aluminum alloys that are difficult to fusion weld [1], [2], [3]. FSW induces less distortion and residual stresses because of the smaller heat input required for joining compared to the fusion welding. These benefits could eliminate heat treatments after joining in some cases, which is expected to save the cost and time for production. Hence the applications of FSW are currently spreading over components in various industries such as transportation, shipbuilding and aeronautics [4].

Fatigue performance is one of the most important factors to be cleared before utilizing materials and components in actual applications. Therefore, investigations of fatigue properties have been made for various materials joined by FSW with different parameters [1]. These investigations revealed that the fatigue performance of the friction stir welded (FSWed) joints is superior to that of the conventional fusion welded joints, however inferior to that of the base material (BM) in most cases. The reduction in the fatigue strength is considered due to material softening and/or tensile residual stresses arising from FSW [1]. To restore the hardness and moderate the tensile residual stresses, surface treatments such as shot peening (SP), laser peening (LP) and low plasticity burnishing (LPB) have been examined [5], [6], [7], [8].

Laser peening is a rapidly deploying surface technology which introduces work-hardening and deep compressive residual stresses on metallic components [9], [10], [11]. Many studies on the effects of laser peening on base materials have appeared and the results showed that laser peening prolonged the fatigue lives and enhanced fatigue strengths remarkably. Hatamleh et al. [5], [6] have studied the effects on FSWed aeronautic aluminum alloy joints and concluded that laser peening could improve the fatigue performance of FSWed A7075 and A2195 aluminum alloy joints. They utilized a specially designed high power laser system firing laser pulses with 10–100 J and a sophisticated beam delivery system. Furthermore, the process involves a so-called “sacrificial overlay” or “protective coating” to prevent the surface from being damaged by the irradiation of high power laser pulses. The overlay is formed with adhesive metal foil or black paint prior to laser irradiation [12] and the remaining overlay is removed after the treatment. Accordingly, it is not practicable in some cases to apply the system and the procedure to actual components as it is.

The authors have developed a novel process requiring no “sacrificial overlay” [13]. We abbreviate this simple process as LPwC (Laser Peening without Coating) hereafter in the present paper. LPwC utilizes a compact laser system with pulse energy of about 100 mJ, much less than that of the previous one. The fired laser pulses can be delivered through an optical fiber cable [14], which dramatically expands the adaptability to actual objects with complicated geometry. The detail of the LPwC process was described elsewhere [13], [15], [16] and also the effects on residual stress, stress corrosion cracking (SCC) susceptibility and fatigue properties [17]. The effects on fusion welded joints were also studied for an SM490 carbon steel and an HT780 high strength steel [18], [19]. The results showed that LPwC dramatically enhanced the fatigue strength of the joints. However, there are no preceding studies on the fatigue performance of FSWed material after LPwC.

In the present study, the authors applied LPwC to the BM and FSWed material of an A6061-T6 aluminum alloy [7]. Plane bending fatigue tests were performed for both materials and the results were compared with those of unpeened materials. The surface roughness, hardness and residual stress were also measured and compared to discuss the effects of LPwC on the fatigue properties of the FSWed joints.