Comparison of Mechanical Properties of Joints in AA7050-T7451 Using Different FSW Tools

Transcription

1 Comparison of Mechanical Properties of Joints in AA7050-T7451 Using Different FSW Tools Daniel André Sequeira de Sousa Abstract Friction Stir Welding, (FSW), has become a good alternative for mechanical joining and fusion welding processes. The positive metallurgical features of this relatively new process allowed new possibilities of joining aluminium alloys like the 2000 or the 7000 series. This research work is based on the analysis of the mechanical properties of friction stir welded joints of the aluminium alloy AA7050-T7451 with 8 mm of thickness. A previous parameter study was done in order to determine the best set of parameters with the best results of static and hardness properties. These were used to perform the welds for distortion and fatigue assessments as well as the tool wear assessment. The mechanical properties were compared by two different approaches, single side FSW (FSW) and double side FSW (DS-FSW). Unexpectedly, the distortions of double side FSW were higher than those of single side FSW. The welding procedure chosen for the double side process and the loss of properties after the first pass are the main causes for bigger distortions in the double side FSW. There was no big difference in the fatigue performance of both approaches. Tool wear is not the main cause for tool failure. The forces involved during the process and fatigue could be the reasons for tool failure. Keywords: Tool Wear Single side FSW (FSW), Double side FSW (DS-FSW), AA7050-T7451, Distortion, Fatigue, 1. Introduction The Friction Stir Welding (FSW) process is a relatively new joining technique invented and patented by Thomas Wayne at TWI, Ltd in 1991 [1]. With this new welding technique materials that were once considered as non-weldable are now weldable using FSW due to the lower temperatures involved during the process. For this reason, this new process is very well received in the industry, especially in the aeronautical sector which still uses rivets and fasteners in many structural components [2]. The distortion problem creates dimensional inaccuracies that lead to an increase in fabrication costs and to the poor quality of welded components [3]. S. R. Bhide et al. [4], compared the distortions in HSLA 65 plates (711.2 x x 6.35 mm) using three different welding processes, submerged arc welding (SAW), gas metal arc welding (GMAW) and friction stir welding (FSW). There are three different modes of distortions namely bowing, angular and buckling distortions. They have concluded that FSW resulted in a buckling distortion and SAW and GMAW have angular and bowing distortions 1

2 [4]. The higher compressive stresses in the outer edges of the plates welded by FSW resulted in the buckling shape distortion [4]. The relevance of fatigue testing is related with the increasing use of high strength materials and the need for higher performance of these materials [5]. Zhou et al. [6], made a comparative study of fatigue properties of friction stir and MIG pulse welded joints in 5083 Al-Mg alloy and results show that the friction stir welds had better appearance and better fatigue performance than the MIG pulse welds. The fatigue life of FS welds was times longer than the MIG pulse welds under a stress ratio of 0.1 [6]. The FSW process is relatively new and therefore there is very little investigation in tool wear in thick aluminium plates [7]. A preliminary study of tool wear was carried out with the intention of filling this knowledge gap. In conclusion, Friction Stir Welding offers great possibilities for welding thick materials opening the technology to other applications [8]. Many research works have been using thin plates limited to thicknesses of 5 to 6 mm and there is little information on thicker plates with several mechanical tests [8], [9]. This research work will expand the knowledge of thicker plates and its mechanical behavior in terms of distortion, fatigue and tool wear. 2. Experimental Procedure The scope of this work is to understand and evaluate the application of the Friction Stir Welding process in the AA7050-T7451 aluminium alloy with 8 mm thickness with the intention of producing structural elements for the next generation aircrafts. Due to the challenge of welding thick materials, two different approaches (single and double side welds) were selected in order to compare and establish which one is the most appropriate for an industrial application. Single side FSW is done in one single pass where the probe welds through the entire thickness of the workpiece and double side FSW is done in two different passes in each side of the plate (see Figure 1). Some specifications for the present work were defined by Embraer in order to maintain the competitiveness with other welding processes in terms of quality and productivity. These are defined as follows: Sound welds Welding speed 5 mm/s Weld efficiency 80 % (mechanical properties of the welds above 80% of the base material properties) Tool Life Performance 25 m A previous study was carried out in order to define the optimum parameters for single and double side FSW. This set of parameters were chosen after metallography, hardness and tensile evaluation that had to answer to the specifications already here mentioned. The parameters were then used to 2

3 perform the welds for the distortion, fatigue and tool wear assessments which are the main focus of this work. Table 1 illustrates the set of parameters defined for both approaches. a) b) c) Rotational Direction Rotational Direction Rotational Direction Tilt Angle Travel Direction Travel Direction Travel Direction Figure 1 Single and double side FSW representation (a) Single side FSW, b) first pass of double side FSW and c) second pass of double side FSW) Table 1 - Summary of tool dimensions and mechanical test results for single and double side FSW and base material AA7050-T mm thick [10] Design. Base Material Single Side FSW Double Side FSW Tool Diameter (S-Shoulder; P- Probe) S: 20 mm S: 15 mm Rot. speed, rpm Weld speed, mm/s Vertical force, kn P: 8 mm P: 6 mm Min. Hardness, HV ± ± 2.5 (80.2%) ± 2.5 (86.2%) Yield Strength, MPa ± ± 2.5 (80.4%) ± 0.7 (93.5%) UTS, MPa ± ± 2.4 (93.7%) ± 1.3 ( 100%) Elongation, % 16.4 ± ± 0.4 (43.3%) 10.7 ± 0.2 (65.2%) Heat Input, kj/mm / 0.23 The welds were carried out in a FSW gantry system with a stiff clamping system. All welds were performed at room temperature and butt joint configuration was chosen for this project. The tilt angle was fixed at 1 for all weld experiments. For the distortion assessment, an optical measurement system called PONTOS was used. Two static photos were taken; the first was taken before welding and served as a reference stage and another photo was taken immediately after welding. For the fatigue test, an X-ray analysis was performed in the plates as-welded to ensure that there were no types of defects in the welded area. Then, the plates were machined in order to reduce the thickness of the plates to 6.5 mm and also to produce the fatigue specimens. The axial fatigue tests were conducted at room temperature using servo-hydraulic machines with a horizontal configuration with a maximum load capacity of 125 kn. The stress ratio and frequency used for fatigue testing was, respectively, 0.1 and 10 Hz. A two parameter Weibull distribution was used to determine the reliability curves and the fatigue life of the specimens. The fracture surface analysis was done using a scanning electron microscope (SEM) Quanta TM 650 FEG. The machine used for tool wear analysis was a Keyence TM VK Laser scanning microscope. The scanning of the probe and shoulder was done every 2.4 m of weld length, i.e. after two set of plates, until tool failure. The tool wear assessment was only performed 3

4 in the tool for single side FSW. Samples for metallography analysis of the welded specimens was carried out in a Struers Tegramin-30 polishing machine and post observation in an optical microscope Leica TM DM IRM. 3. Results and Discussion In order to complement this work and to decide which approach is the most appropriate, complementary mechanical tests such as distortion and fatigue assessments were performed. The following chapters compares both approaches in order to sum up all the results and also to better understand the final conclusions Distortion Assessment The relative displacements of double side FSW were higher than those achieved for single side FSW. It was expected that the distortions of double side FSW would be smaller than single side FSW since the heat input is less in each pass and is delivered in a more phased manner. Table 2 shows a summary of the relative displacements of both approaches. Table 2 Maximum and minimum displacement values for single and double side FSW of AA7050-T7451 Single Side FSW Double Side FSW First Pass Second Pass Relative Displacements Max. Min. dx [mm] dy [mm] dz [mm] dx [mm] dy [mm] dz [mm] dx [mm] dy [mm] dz [mm] However, the free surface left during the double side process and the welding direction done in the same way in both passes were the causes for the bigger distortions in the double side FSW. Contrary to fusion processes, there was not a compensation after the second pass, instead, the deformation after the second pass was even higher. This may be related to the experimental procedure used during the double side process. The welding direction was done in the same way for both passes, causing the end of the plates to be submitted to higher heat inputs during each pass. In order to control the distortion of the plate, an alternate direction can be used during each pass as a first solution. The loss of properties (softening) due to heat input after the first pass is also a factor for the susceptibility to higher deformations. Richter-Trummer et al. [11], concluded that the higher the clamping forces, the more restrained the material is and, therefore, the lower the distortions [11]. Following this idea, another solution could be the use of an efficient clamping system that would allow a good fixation of the workpiece. Naturally, there would be an increase in the residual stresses in the workpiece that 4

5 could be detrimental in terms of performance. For this reason, a heat treatment would be necessary for stress relaxation of the workpiece. Either way, despite there is no design criteria for the distortion measurements, the results for both approaches can be considered as acceptable taking into consideration the dimensions of the plate (1200 mm x 400 mm). As mentioned before, there is no information about the residual stresses of the plates, therefore it is not possible to conclude the effect of the process in terms of distortion. It would be interesting to evaluate the residual stresses from the manufacture process and compare with the residual stresses from the clamping system and the process itself in order to conclude which influences the most in terms of distortion. WD WD WD Figure 2 - Distortion assessment for single and double side FSW (WD Welding Direction) 3.2. Fatigue Assessment The S-N diagram of double side FSW is very similar to single side FSW (see Figure 3) which is an indication that the different approaches have little influence in the fatigue performance. 5

6 Max. Stress, σ max [MPa] S-N Diagram for Single and Double Side FSW of AA7050-T7451 FSW y = 546,91x -0,056 R² = 0,8999 DS-FSW y = 650,41x -0,071 R² = 0, Fatigue Life, N, cycles Double Side FSW Run-Out (4 M) Single Side FSW Figure 3 - S-N diagram of double and single side FSW of AA7050-T mm thick According to the literature the fatigue strength of the welded specimens is close to the base material s [12]. The fatigue strength of both single and double side FSW specimens is around MPa. The same fracture mechanism occurred in both approaches; the specimens that broke in the base material evidenced an agglomerate of particles on the surface of the specimen that caused stress concentrations leading eventually to the complete failure after a certain amount of cycles (see Figure 4). The specimens that broke in the stir zone demonstrated a mixture of intergranular and transgranular fracture which is a characteristic feature of a ductile material (see Figure 4). In addition, for both specimens that broke in the stir zone and base material, a combination of striations and dimpled surfaces were observed indicating that the material, AA7050-T7451, has a natural tendency to resist to fatigue and also the process has little influence in fatigue failure because the same failure mechanism is happening in the stir zone and base material. This was observed for both approaches. Overall, the different approaches have little influence in fatigue performance in spite of the better static properties of double side FSW. The presence of inclusions and the quality of the surface are determining factors to fatigue behavior. Given the fact that the presence of inclusions in the base material caused the premature failure of the specimens, it is of most importance to verify and control the quality of the base material after the manufacture process. On top of that situation, other specimens broke in the base material, so there is a great possibility that the inclusions may be affecting the fatigue performance of the other samples. 6

7 a) b) Figure 4 SEM observations of fatigue fracture surfaces (a) agglomerate of precipitates present in the specimens that broke in the base material and b) intergranular and transgranular fracture in the specimens that broke in stir zone) 3.3. Tool Wear Assessment One of the conclusions of this study is that the wear is not the main cause for tool failure. It was possible to weld around 3.8 m until the complete failure of the tool which is quite far from the 25 m specification. Profiles before and after welding were taken in the probe and shoulder in order to quantify the amount of wear in the tool. The relative height between peak and valley was measured in different locations in order to assess the amount of wear observed in the respective locations. The height differences were compared between the reference stage and after 2.4 m of weld. Figure 5 to Figure 8 represent the profiles of the probe and shoulder before and after welding (reference stage and after 2.4 m of weld). 3,0 Probe Profile Tool 1 TW m Reference Stage 3,0 Probe Profile Tool 1 TW m 2.4 m Height [mm] 2,5 2,0 1,5 1, Height [mm] 2,5 2,0 1,5 1,0 0,5 0,5 0, Length [mm] 0, Length [mm] Figure 5 Probe profile before and after welding (left reference stage; right 2.4 m stage) Relative Height [mm] 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0, Relative Height between Peak and Valley Probe - Reference Stage Probe - After 2.4 m Figure 6 Relative height between peak and valley of the probe in different locations 7

8 Profile A 1,50 Shoulder Profile Tool 1 TW m Reference Stage Profile A 1,50 Shoulder Profile Tool 1 TW m 2.4 m Height [mm] 1,25 1,00 0,75 0, Height [mm] 1,25 1,00 0,75 0,50 0,25 0,25 0, Length [mm] 0, Length [mm] Figure 7 Shoulder profile before and after welding (Profile A) (left reference stage; right 2.4 m stage) Profile A 0,8 Relative Height between Peak and Valley Relative Height [mm] 0,6 0,4 0, Shoulder - Reference Stage Shoulder - After 2.4 m 0, Figure 8 Relative height between peak and valley of the shoulder in different locations As it can be seen, the amount of wear in the probe and shoulder after 2.4 m is almost inexistent leading to the conclusion that the wear is not the main cause for tool failure. The material of the probe is MP159 which is a nickel-cobalt based alloy and is a high strength material and the shoulder is Hotvar, a molybdenum-vanadium based alloy. The hardness of both is, respectively, above 200 and 500 HV [13], [14] and are much higher than the hardness of the AA7050-T7451 ( 158 HV) so it is expected that the wear of the tool to be almost inexistent. A metallography analysis was carried out and there were no defects in the microstructure confirming that the wear has no influence during the process. There could be other reasons for tool failure such as fatigue or even defects in the base material of the probe, either way these are mere assumptions. The forces involved during the process could also be a viable explanation for the failure of the tool. These forces induces a bending action in the probe causing it to fail in the plane of the shoulder. The probe tends to break in the plane of the shoulder where the stress concentrations are more predominant due to the threads of the probe. Since the tool does not break instantly, there could be eventually a combination of failure mechanisms such as fatigue and the forces involved during the process. The fact that the tool is completely destroyed makes the study of the fracture surface under SEM observation impossible. This would contribute to take more information about the mechanism of fracture and reach more solid conclusions about the reason for tool failure. Other attempts were made in order to reach better tool life performances but all of them evidenced volumetric defects in the microstructure of the weld making these analysis unacceptable for comparison and for future tests. However, these new attempts showed great improvements; around 8

9 13 m and 11.5 m were achieved. A different configuration probe and a slight parameter change were the modifications done in these new attempts. The new configuration would contribute to a stronger probe being capable to withstand higher forces during the process. The new process parameter should be such that would contribute to lower forces and better microstructure free of defects. This must be an iterative process because both the mechanical and tool life performance must be taken into consideration. The use of different materials in the probe could be an interesting variable to be studied since wear is not the main cause for tool failure. Off course, the materials must be resistant enough in order to wear not become a factor in the quality of the weld. 4. Summary and Conclusions The 8 mm aluminium alloy AA7050-T7451 was successfully welded with remarkably good mechanical results. The challenging requirements mentioned in chapter 2 were achieved with exception of the tool wear performance. Two different approaches were taken, single and double side FSW, both turned out to have overall similar results in terms of mechanical properties. It is important to mention that the main focus of this work was distortion, fatigue and tool wear assessment and according to the experimental work carried out on AA7050-T mm thick aluminium alloy it can be concluded: 1. The distortions of double side FSW were higher than in single side FSW. The maximum relative displacement measured in double side FSW was 1.5 mm for the first pass and 3.3 mm for the second pass and the maximum relative displacement for single side FSW was 1 mm on the outer edges of the plates. It was expected that the distortions of double side FSW would be smaller than single side FSW due to the less energy delivered during each pass. However, the distortions of either single or double side FSW can be considered as small. 2. The material AA7050-T7451 showed a natural tendency to resist to fatigue. Both fatigue S-N diagrams for single and double side FSW were very similar (see Figure 3) which indicates that the different approaches have no influence in fatigue performance. The inclusions and the surface finish marks are responsible for stress concentrations leading to crack propagation and then eventually to complete failure. 3. The tool wear measured in the probe and shoulder was considerably small and therefore the wear is not the main cause for tool failure. Additionally, there was no influence on the microstructure of the weld. The tool life performance was around 3.8 m which is much lower than the 25 m requirement. Although the trials with the new probe configuration evidenced volumetric defects in the microstructure, the new probe showed a great improvement in tool life performance, around 13 m and 11.5 m. 9

10 Overall, both approaches evidenced similar results. Taking into consideration that nowadays production costs and productivity are crucial factors in the industry, single side FSW seems to be the most appropriate approach to be used in the future given the need for less preparation and less time to weld the same amount of length. This research work will contribute for a better understanding about the mechanical behavior of thicker materials and will serve as a base for future works. 5. References [1] TWI, Friction Stir Welding, TWI. [Online]. Available: [Accessed: 01-Mar- 2015]. [2] B. J. Dracup and W. J. Arbegast, Friction Stir Welding as a Rivet Replacement Technology, in Automated Fastening Conference & Exposition, [3] D. Deng, H. Murakawa, and W. Liang, Numerical simulation of welding distortion in large structures, Comput. Methods Appl. Mech. Eng., vol. 196, no , pp , Sep [4] S. R. Bhide, P. Michaleris, M. Posada, and J. DeLoach, Comparison of Buckling Distortion Propensity for SAW, GMAW and FSW, Weld. J., no. American Welding Society and Welding Research Council, [5] F. C. Campbell, Elements of Metallurgy and Engineering Alloys, 1st ed., USA: ASM International, 2008, pp [6] C. Zhou, X. Yang, and G. Luan, Comparative study on fatigue properties of friction stir and MIG-pulse welded joints in 5083 Al-Mg alloy, Trans. Nonferrous Met. Soc. China, vol. 15, [7] P. Fleming, T. Bloodworth, T. Prater, and et. al, An investigation in Tool Wear Detection in Friction Stir Welding, Vanderbilt School of Engineering. [8] J. G. Perret, J. Martin, P. L. Threadgill, and M. M. Z. Ahmed, Recent developments in friction stir welding of thick section aluminium alloys, in Paper presented at 6th World Congress, Aluminium Two Thousand, Florence, Italy, [9] D. A. Kumar, P. Biswas, S. Tikader, M. M. Mahapatra, and N. R. Mandal, A study on friction stir welding of 12mm thick aluminum alloy plates, J. Mar. Sci. Appl., vol. 12, no. 4, pp , Jan [10] V. Correia, Friction stir welding processes in AA7050 thick plates, MSc Thesis, University of Porto (FEUP), [11] V. Richter-Trummer, E. Suzano, M. Beltrão, a. Roos, J. F. dos Santos, and P. M. S. T. de Castro, Influence of the FSW clamping force on the final distortion and residual stress field, Mater. Sci. Eng. A, vol. 538, pp , [12] Department of Defence of United States of America, Military Handbook - Metallic Materials and Elements for Aerospace Vehicle Structures, [13] Aircraft Materials - Alloy MP159 (AMS 5841). [Online]. Available: [14] Uddeholm Hotvar,