Research on the FSW of Thick Aluminium

Transcription

1 Research on the FSW of Thick Aluminium Peng Chai, Guohong Luan, Xiaojuan Guo, Shan Wang China FSW Center Beijing FSW Technology Co., Ltd Tel: , Fax: Web: Abstract: The Friction Stir Welding (FSW) technology has the widely prosperous applications in joining light metals such as Aluminium alloy because of no fusion in its process and its insensitivity to the oxide on the weld surface. This paper represented the FSW tools, FSW equipment as well as necessary auxiliary devices for heavy section of Aluminium alloy (thickness 40mm from one side while 70mm from two sides) and analyzed relevant technical difficulties in the process. The research results showed that the joint strength can be equal to base metal and the elongation percentage of the joint can be up to 90% of base metal by FSW, meanwhile, other auxiliary devices such as cooling device needed to be introduced for the excessive heat input to make the even distribution of the heat in the weld. Key words: Friction Stir Welding, 5XXX Aluminium alloy, heavy section, heat sink 0 Introduction Friction stir welding is a revolutionary solid-joining process patented by The Welding Institute (TWI) [1,2]. During welding, a non-consumable friction stir welding tool is plugged into the joint with certain rotating speed, and the joint material will be solid welded together by the friction heat and deformation energy [3] along with axial force. Therefore, any material in any thickness can be joined by FSW on condition of the sufficient strength and rigidity of FSW tool which is capable at high welding temperature, as well as providing sufficient force in welding area and overcoming relevant welding torque. The fusing temperature of Aluminium alloy is about 600, and the plastic joining temperature may be at 450~500, 80% of its fusing temperature [4-5], so the workpiece of Aluminium alloy can thus be joined by FSW while the tool is made of the materials that is capable at the temperature up to or more than 450. As the welding thickness increases, the rigidity and strength of the FSW equipment as well as the force and welding torque should be correspondingly improved. So, the more strict requirements should be satisfied for heavy section of Aluminium alloy. 1

2 In Aerospace, Shipbuilding and Electronics Industries, a large quantity of Aluminium alloy blanks are needed in their components manufacturing. Because of the limited processing method especially for the no so high rolling level, plenty of heavy section structures of Aluminium alloy can be obtained by joining the narrow materials piece to piece and machining correspondingly for the lack of the satisfied materials, the possible welding methods are: electron beam, pole-changing plasma and MAG. The joint welded by MAG with thickness 70mm of Aluminium alloy is shown in Fig. 1, which was finished by dozens of pile-up welding processes from two sides by adopting X groove, and the weld quality detection needed to be done after each welding process, what s more, the defective weld with the blow holes or slags needed to be burned-on and be detected again until eliminating all the defects completely. Thus, one weld with length 4-6m can be always finished in about 2 weeks time. Fig. 1 The Joint Welded by MAG with Thickness 70mm of Aluminium Alloy Aiming to similar structures, China FSW Center developed and manufactured the first FSW Equipment for heavy section in Asia last year (Fig, 2), which can join the plate with dimensions of 6000mm 6000mm 40mm. The blanks with thickness 70mm of Aluminium alloy can be joined from two sides, once only from one side (Fig. 3), the weld with same length can be finished in two days time, it is about 1/5 of that by fusion welding methods, and thus it saves 80% in the consumptions of materials and personnel as well as the quality detection cost. 2

3 Fig. 2 FSW Equipment for Heavy Section in CFSWC FSW Weld Fig. 3 The Roughcast Blanks with Thickness 70mm (4m 4m) Welded by FSW This paper will analyze the structure and performance of FSW weld in heavy section of Aluminium alloy by a series of experiments, and show relevant researches on the special FSW technology with the increasing thickness in workpieces. 1 Experiment Method In the experiment, the workpiece of 5A06 Aluminium alloy (its chemical compositions are shown in Table 1) with dimensions of 300mm 150mm 40mm was adopted separately in three conditions: no cooling device in welding process for sample 1; cooling device for workpiece and tool installed for sample 2; optimized parameters for sample 3 with cooling device as that for sample 2. The cooling in welding process is carried out by introducing one cooling jet behind the FSW tool while moisture mixture as the cooling medium. 3

4 In this experiment, it would prove that the heat produced in FSW process are excessive rather than not sufficient, so some measures should be taken for the elimination of heat to avoid the overheated structure and the lowered joint performance. Mechanical property test, metallurgical structure analysis and tensile fracture analysis as shown in Fig.5 were carried out separately after finishing welding. In Fig.5, four tensile samples were obtained from the welding starting position (,, and ) whose thickness was 10mm (8mm after treatment), width was the same as the weld thickness (40mm), and length was the same as the samples after welding (300mm). To separate the samples, put the sample number as in Table 2 separately 1-,2-,3- before each sample, sample with dimensions of 60mm 40mm 15mm (thickness 15mm, and more than 10mm after treatment) so as to observe the microstructure of the weld cross-section and compare the HBW of the weld with that of the base metal. Table 1 The Chemical Compositions of 5A06 Aluminium Alloy Brand Si Fe Cu Mn Mg Zn Ti others Al 5A the rest Table 2 Welding Experiment Conditions Welding Parameters Cooling Tool Parameters(mm) Sample Rotating Speed (rpm) Welding Speed (mm/min) In Process Shoulder Tool Pin Pin Length No Yes Yes Aerosol Nozzle FSW Tool FSW Weld Fig. 4 FSW Process with Cooling Device 4 Welding Workpiece

5 Weld Welding Direction Fig. 5 Tensile Sample and Metallographic Specimen Sampling Corroding treatment for the Metallographic Specimen in 25%NaOH after being burnished, and measured the HBW hardness of the weld cross-section on condition of 250kg for on-board load, 5mm for the diameter of load-on steel ball and 30s for the load time. 2 Experiment results and conclusions Fig.6 Weld Surface Formation of Sample 1 Fig.7 Weld Surface Formation of Sample 2 In the welding process for Aluminium alloy with thickness 40mm, it is very important to cool the weld and the tool, or some defects may form for the overheat. Comparing the weld surface formation in the condition of cooling device installed (Fig. 7) with that without such device (Fig. 6), it showed that some corrugations existed on the weld surface, obvious rarefaction and holes presented in the key hole, and ineffective joining with big hole occurred near the weld surface for the overheat; while with cooling device only some aluminium scraps and flashs formed on the weld surface and no obvious holes in key hole. 5

6 Therefore, the cooling during the welding process is one of the essential requirements for Aluminium alloy by FSW with thickness 40mm. and it is the same for Aluminium alloy with thickness 70mm. Table3.The joints mechanical property under different weld conditions. SN Tensile(MPa) Elongation(%) SN Tensile(MPa) Elongation(%) B B Table3 show the mechanical property of the joints under different weld condition. All the mechanical property tests were done along the rolling fiber. Some phenomenon can conclude from the mechanical property data in the table. The tensile and elongation of the samples (from 1-1 to 1-4) with overheat weld structure were far lower than those of the base metal. The tensile value is just 45% compared to the base metal s. The Elongation value is also just 16% compared to the base metal s. The tensile and elongation of the samples (from2-1 to 3-4) without overheat weld structure were obviously improved. The tensile value was as high as 95.6% compared to base metal s, in the same time, The Elongation value is also get to 93% compared to base metal s. Under the same cooling condition different mechanical property joint would be obtained by FSW with different parameters including spindle rotational speed and travel speed. With 360 rpm spindle rotational speed and 60mm/min travel speed we can get the best mechanical property joint

7 Fig8. The fracture pattern of tensile sample marked 1-1and 2-4 Compare the tensile fracture of the sample marked 1-1 to 2-4 as showed in Fig 8, it can be found that 2-4 sample with uniform fracture pattern while just 3/5 fracture area of the sample1-1 has the same fracture pattern with, the other area is vertical laid regularly. SEM was used in order to find out more information about the phenomenon. Fig 9 shows the micro fracture pattern of the sample 1-1. There is not any dimple in the vertical laid area as showed in Fig 9 a, while there are lots of dimples in the other area as showed in Fig9c. A transition area as showed in Fig 9 b existed between the area a and the area c. This indicated that the joint fracture pattern in the area a was fragile but ductile in the area c. Both fragile and ductile fracture pattern existed in the area b. The different fracture patterns to different area probably caused by the different heat input for the different distances from the heat source. The shoulder produces lots of heat on the work-piece which lead to overheat structure in the area a. c b a a b c Fig9. micro fracture pattern of the sample 1-1 After sanded, polished and corroded the metallographic specimen(2-5#) selected from the sample 2, the section shape of the specimen can be observed obviously. Because of the different structure compatibility,the corroded weld joint pattern is completely different from base pattern which can be obtained by the comparison among 10a-c. Graph 10a is the 7

8 magnification of the weld center zone, and graph 10c is the magnification of the base metal. Comparatively, the grains in weld zone are refining in high level, and strengthened components are distributed uniformly in the weld zone which is closely connected with the friction, stir and rolling. In the welding process, the grain structure of weld is continuously stirred, rolled to crush, refined, nucleated and recrystallized. The grain sizes of the transition zone (graph 10b) which experiences the same process are between the grain sizes of weld center and base metal, because the zone is a little distant from tool pin. At the same time, because the transfer process is driven by the materials in the periphery of the tool pin, so the materials in transition zone distribute in the shape of belt, not homogeneously. 4 a b c Adv. 6 Ret. a b c Fig10.The macrostructure and microstructure of typical sample 2-5 The hardness measurement (table 4) of the weld zone and base zone (1-6 dots in graph 10) indicate that hardness distribution of the base zone is relatively uniform, while the hardness distribution in weld zone presents variation. In the midline of the cross-section (1,2,3,5), dot 1 and dot 3 are on the base metal, dot 2 is close to the weld edge, dot 5 is in the center of the weld. Comparatively, the hardness of dot 5 is the lowest and dot 2 s is the highest that maybe results from the larger test dot(relative to micro-hardness) which press the strengthening phase on the position of dot 2, because the similar rule like this is not discovered in other experiments. In the direction of weld thickness(4,5,6), in which dot 4 and dot 6 are close to the upper surface and lower surface respectively, the hardness of these three dots are basically the same. All this show that if the welding parameters are selected and controlled appropriately, the properties variations in thickness direction may be the small in 40mm, even 70mm thick aluminum structure friction stir welding process. 8

9 Table 4. the HBW of typical sample Conclusions All the analyses illustrate: If the equipment and pin tool possess enough capacity, 40mm to 70mm thick aluminum structure can be welded. Cooling the weld from time to time is critical to heavy section aluminum friction stir welding, or it is hard to get acceptable quality weld. With the appropriate welding parameters, the strength and elongation of 5A06 aluminum friction stir weld can reach above the 90% of the base. In the tensile test of the overheated friction stir weld, the fracture pattern showed longitudinal laid characteristic and appeared typical brittle fracture. In heavy section(40mm) aluminum friction stir welding, the hardness distribution in thickness direction is homogeneous, without obvious variations. References 1. Thomas, W.M.: Friction Stir Butt Welding, International Patent App. No. PCT/GB92/02203 and GB Patent App. No , Dec. 1991, U.S. Patent No. 5,460, Tweedy, M., Arbegast, W., Allen, C.: Friction Stir Welding of Ferrous Alloys Using Induction Preheating, Friction Stir Welding and Processing III. TMS 2005 Annual Meeting, San Francisco, CA February Smith S. D.: A review of models for simulation of material flow during friction stir welding. TWI Report, Oct Seidel T.U.: The development of a friction stir welding process model using computational fluid dynamics. PhD, University of South Carolina, USA, Threadgill P.L. and Nunn M.E.: A review of friction stir welding Part 1 process overview, TWI report, /02/1150.3, Feb