Effects on the Surface Texture, Superplastic Forming, and Fatigue Performance of Titanium 6AL-4V Friction Stir Welds

Abstract

The speed and feed effects of the friction stir welding (FSW) process on the surface texture along the top of a butt welded nugget were studied. The tests were conducted using fine grain (0.8-2 lm) titanium alloy 6Al-4V with a nominal thickness of 2.5 mm. It was shown that the pin tool marks along the top surface of the weld can be highly detrimental to both the superplastic forming (SPF) characteristics and the fatigue performance of welded panels. Removing the marks by machining the top surface after FSW was found to eliminate the predominant tearing of the weld during SPF and most of the fatigue life of across the weld was also restored. Through additional development of the FSW process parameters, the butt welded nugget was made to have equivalent SPF characteristics as the parent sheet material. By using a water-cooled pin tool and other cooling techniques, it is believed that the weld zone can be kept below the beta transus temperature during FSW, which enables the formation of a grain structure that is uniquely conducive to superplastic behavior, when compared to conventional fusion welding processes.

Fatigue performance of Friction Stir Welded Ti–6Al–4V subjected to various post weld heat treatment temperatures

“…Previous research showed that the marks left by the FSW tool result in sharp notch features [14], thus a detrimental impact to the fatigue performance would be expected due to the high notch sensitivity factor; more so in titanium welds than aluminum. Machining or burnishing the as welded surface provided enhanced fatigue performance and the combination of the two led to the best performance, albeit still lower (11%) than the respective base material with respect to fatigue strength at a given life [28]. Edwards and Ramulu [29] then evaluated the performance of 3 and 6 mm gage FSW's that were subjected to a stress relief cycle and surface machining then tested at two load ratios (R = 0.1 and R = 0.6).…”

“…Microstructures and mechanical properties have also been well documented, but fatigue performance has only begun to be studied in Ti FSW [28][29][30]. In Aluminum, many studies have been conducted on the fatigue behavior of FSW joints and it has been shown that the performance is dependent on a number of variables including alloy, thickness, orientation, process parameters and post weld processes such as surface milling, peening, and heat treatment [1,[33][34][35][36][37].…”

“…Sanders et al [28] evaluated the effect of surface condition, weld orientation, stress relief and burnishing on fatigue life in thin gage (2.5-3 mm) Ti FSW's. It was found that in the as-welded surface condition the fatigue performance was significantly lower than the parent material, regardless of weld orientation or post weld stress relief.…”

Friction Stir-Welded Titanium Alloy Ti-6Al-4V: Microstructure, Mechanical and Fracture Properties

“…[11][12][13][14] Enabling this machine and tooling technology has so far proven to be difficult, but is currently possible at a handful of laboratories around the world. [15][16][17][18][19][20] The titanium welds and superplastic-formed joint mechanical properties reported previously [21][22][23][24][25][26][27][28][29][30][31][32] were essential for variable FSW process conditions. Since then, the process has been further developed and refined, and optimal process conditions for a given tool geometry have been identified.…”

Achieving superior low temperature and high strain rate superplasticity in submerged friction stir welded Ti-6AI-4V alloy

“…In the past several years, some preliminary results about the combination process of SPF and FSW of Ti alloys have been reported by Sanders, Edwards and their coworkers [4, [15][16][17]. In these studies, it was found that similar to fusion welds, for the FSW Ti alloy joints, the NZ was still the key zone to the SPF of the whole welds, since there was usually a very narrow thermo-mechanically affected zone (some even <100 μm in width) in the FSW Ti alloy joints and its microstructure was closer to that of the BM [8].…”