FSW TIG Welding of Cu 99 Copper

Transcription

1 FSW TIG Welding of Cu 99 Copper R. Cojocaru, C. Ciucă, L. Boţilă, V. Verbiţchi National Research & Development Institute for Welding and Material Testing ISIM Timişoara, Romania Keywords Friction stir welding, TIG assisted FSW, cooper 1. Introduction In order to improve working conditions for welding tools, reduce wear and increase durability of the friction stir welding tool, to increase welding speed values, some solutions have proposed to supplement the amount of heat developed during FSW welding process aimed at increasing the plasticizing degree of the processed material. As additional sources of heat can be used heaters based on resistive effect, inductors, high-frequency inductors, laser beam or electric arc [3], [5], [7]. The source of heat is used to heat the area in the close vicinity of the welding tool. This leads to a much better plasticizing of the material, because not only the heat generated by the FSW tool is used, but also the heat generated by an additional source of heat. After the preheating is achieved, joining is made by the FSW process [4]. 2. Conditions of the experiments 2.1. Material to be welded. Joining copper by welding is relatively difficult due to the very high thermal conductivity that determines the heat transfer from the joint zone to cooler areas of the welded components. This obstacle can be mitigated using the FSW welding process. The chemical composition of the Cu 99 sheet used in the experimental program is provided in the Table 1 and the mechanical properties in the Table 2. Table 1. Chemical composition of Cu 99. Chemical composition (%) Alloying element Cu Zn Al Si Mg Percentage (%) 98,80 0,1458 0,0326 0,0235 0,008 Table 2. Mechanical characteristics of Cu 99. Figure 1. Outfit for hybrid FSW-TIG welding. Name of the characteristic Tensile strength, Rm Yield strength, Rp0.2 Vickers hardness, HV1 Elongation at break, A5 260 MPa 206 MPa 85 HV1 40 % Value The experimental program on hybrid FSW-TIG welding was developed for butt welding Cu99 copper sheets, thickness s = 5mm and size 250mm x110mm. The main weakness of classic FSW welding of this material, is that welding speed of rather low values (max. 120 mm / min) could be used. At higher speed, vibration occurred in the welding system, due to instability of the welding process, which resulted in rapid destruction of the welding tool pin (especially in the beginning sequence of the actual welding process, when traveling of the tool starts by preset welding speed). Figure 2. Schematic of the FSW-TIG process. The method of TIG-assisted friction stir welding is a new FSW technique proposed by the author team. TIG-assisted FSW is a development of the FSW process, causing the emergence of a hybrid welding process, in solid form, which integrates the preheating of plates by the TIG welding process (Fig. 1 and Fig. 2) Technique of the experiments. To develop the new FSW-TIG welding technique, completion of the specialized FSW welding machine of the outfit of ISIM was necessary, with new components / modules, specific to the TIG welding process. The machine was also completed with an online monitoring system of the welding process, using infrared thermography (Fig. 3) [5]. 4 year XXIV, no. 1/2015

2 A welding device has been made with the following main functional characteristics (Fig. 4): It ensures safe positioning and fixing of the welding tool into the main shaft of the FSW welding machine; The design allows heat developed during the welding process (max. 1000ºC) to dissipate into the atmosphere, thereby protecting the main spindle bearings of the FSW machine. The shape and dimensions of the welding tool has been selected on the basis of the following considerations: Achieving optimal temperature for a good plasticizing of the material in the weld; Determining the size and geometry of the pin of the welding tool, to ensure sufficient helical movement of the material flow for forming the nugget in the thickness direction; Thickness of material to be welded Welding tools Due to ongoing expansion of the possibilities of application of the FSW welding process, welding tool geometries were developed continuously, used for joining of new materials, but also for achieving new types of joints [1]. Figure 5. FSW welding tool with threaded cylinder pin made of X38CrMoV5 steel, heat treated to HRC. Figure 3. Hybrid FSW-TIG welding system: a) FSW welding machine; b) Monitoring system of the welding process; c) Command interface of TIG welding process; d) TIG welding head; e) TIG welding equipment; f) Shielding gas supply installation. Figure 4. Welding device: a) grip area; b) body; c) welding tool. Figure 6. Welding tools made of material P20S: a) welding tool with smooth conical pin; b) welding tool with four plain bevels. By the hybrid FSW-TIG welding of copper Cu99, one-piece tools with threaded cylindrical pin and smooth shoulder were used (Fig. 5), made of material X38CrMoV5 (AISI H11, DIN ), heat treated to HRC, as well as tools with conical pin (Fig. 6a), respectively tools with four-bevel pin and smooth shoulder (Fig. 6b), made of sintered tungsten carbide P20S. 3. Program of experiments At the design stage of the program of experiments on classical FSW, respectively FSW-TIG welding processes of Cu99 material, by the initial setting of the welding parameters mainly the following factors of influence have considered: Physical and mechanical properties and characteristics of the material to be welded Technical characteristics of the welding system used in the experimental program. year XXIV, no. 1/2015 5

3 Due to the large forces that develop during the process, in the case of the classical FSW welding process, it was necessary to limit the welding speed to below 120 mm / min [2]. Very good results have been achieved, using the following values of the welding parameters: Welding speed mm/min Welding tool speed rev/min Welding tool - shoulder diameter Ø = 20 mm - cylinder-conical smooth pin - pin length L = 4.8 mm - material - tungsten By the hybrid FSW-TIG welding of copper Cu99, experiments have been performed in the following conditions: Welding under similar conditions to those of classical FSW welding (similar process parameters, configurations and dimensions of the welding tools); Use of both different process parameters and welding tools, compared to classical FSW welding. The following welding parameters have been used: FSW: welding speed: mm/min tool speed: rev/min, clockwise welding tool: - shoulder diameter Ø = mm - pin diameter Ø max = 5.5 mm - pin length L = 4.8 mm - conical pin and conical four-bevel pin, made of sintered tungsten carbide. TIG: welding speed: v = mm/min welding current: I = A voltage: U = 20V shielding gas: Ar (flow rate 6 l/min) welded sample length: 150 mm 2,82 μm. These values are comparable to those obtained by the usual machining operations of copper. In the Fig. 8.a, the variation of the temperature during the classical FSW welding process is presented, and in the Fig. 8.b, the temperature evolution in the case of the hybrid FSW-TIG welding process is shown. In both cases presented in Fig. 8, a welding speed of 100 mm / min has been used, under similar process conditions, in terms of parameters and welding tools. The distance between the rotation axis of the FSW welding tool and the tip of the tungsten electrode of the TIG welding head was approximately mm. 4. Results. Discussions In the Figure 7, the appearance of the welded joints is illustrated by the application of the classical FSW process (welding speed v 1 = 100 mm/min, tool speed n 1 = 1000 rev/ min), respectively the FSW-TIG process (welding speed v 2 = 200 mm/min, tool speed n 2 = 1200 rev/min). Figure 7. Appearance of the welded joint of alloy Cu99: a) FSW; b) FSW-TIG. The quality of the welded joint in the case FSW-TIG was much improved, compared with classical FSW welding, the measured roughness values being in the range Ra = 2,54- Figure 8. a) Temperature evolution during the FSW welding process for Cu99: a) FSW; b) FSW-TIG. The average value measured during the stabilized FSW-TIG welding process was about 630ºC. It was found that the temperature by the TIG-assisted FSW welding is about 100ºC higher than by the classical FSW welding, that produced a better plasticizing of materials, which led to: Faster stabilization of the welding process, by reaching the required optimum temperature in a very short time; Comparative analysis of the temperature charts revealed that, besides the process becomes more stable in the case of FSW-TIG, the vibrations occurring within the working system are much lower. This demonstrates that the forces that accompany the welding process are greatly diminished in the case FSW-TIG. In the case of a welding speed of 200 mm/min, analyzing the temperature evolution during the process, it has founded that the recorded temperature was 550ºC (after stabilization). The temperature is lower than in the previous experiment, due to the much higher welding speed that was used, v 2 = 200 mm/min, v 2 = 2 v 1 (the amount of heat developed during the process is inversely proportional to the welding speed). 6 year XXIV, no. 1/2015

4 To analyze the welded samples in metallographic terms, the macrostructures were treated with 10% NaOH solution, respectively the samples for microstructures were etched with a solution of FeCl 3 hydrate and hydrogen fluoride HF. The macroscopic examination revealed no defects or imperfections (Fig. 9). It is noted that hardness values in the range of HV1 have been obtained, and the average hardness is approximately 85 HV1, comparable to that of the base material. Figure 9. Macroscopic analysis of the FSW-TIG weld of Cu99. The Figure 10 shows the microscopic examination of welds obtained by the hybrid FSW-TIG process. The base material has a structure composed of polyhedral grains with annealing twins. Figure 10. Microscopic analysis of the FSW-TIG weld of Cu99. In the nugget area (N), the joint microstructure presents the appearance of grains with more reduced plastic deformations, as a consequence of the effect of the thermal field which removed to some extent the effect of the strain field (Fig. 10 a). In the heat-affected zone (HAZ), the structure presents the specific characteristics of a high temperature, which also led, besides the beginning of re-crystallization, to a grain size growth, compared to the base material (Fig. 10 b). The heat and thermo-mechanically affected zone (TMAZ) shows a flow band structure, in which the grains were strongly deformed in the flow direction of the material (Fig. 10 c). Values recorded at the static tensile test: Rm MB (base metal) 260MPa, Rm IS (welded joint) 256MPa. By the static bending test, the welded joints showed a maximum degree of deformability. Regarding the hardness evolution (Fig. 11), there have not been significant differences in the characteristic areas of the FSW-TIG joint, compared to the base material (v = 200 mm / min). Figure 11. Hardness evolution by the FSW-TIG weld of Cu 99, v = 200 mm/min. In previous studies, the finite element analysis (FEA) has been approached, as a useful method in predicting the materials behaviour, subjected to the hybrid welding process. Thermal, mechanical and metallurgical modifications can be predictable using this kind of analysis [8]. The aim of the performed research was to compare the behaviour of copper, from the thermal point of view, when FSW, respectively TIG-assisted FSW procedures have been applied. The numerical data obtained by FEA have been validated by experimental measurements achieved by the thermography method [8]. Due to the additional thermal source, both the finite element analysis and the experimental method revealed that the peak temperature reached in the case of the hybrid welding is around 100ºC higher than the maximum temperature achieved in the first welding variant, even if the welding speed was higher in the TIGassisted FSW variant. The additional heat generated by the thermal source which develops a preheating effect can explain that. It is well known that the weld quality depends on the heat amount generated by the welding source(s). In order to avoid the flaws occurrence, heat flow should be high enough to maintain a maximum temperature of the process around % of the melting temperature of the base metal [8]. In this case the tensile strength of the welded joints had a value which was 92% of the tensile strength of the base material. Analysis of the results obtained from the application of the FSW-TIG process by the butt welding of Cu99 copper sheets, of thickness s = 5 mm, has shown that this technique can ensure joints with very good characteristics and properties to be achieved. They also highlighted the beneficial economic effects of the process (in terms of productivity, lastingness of the welding tools and complexity of the FSW welding machines). 5. Conclusions Addressing the issue of FSW-TIG welding was based on the need to develop methods for friction stir welding (FSW) that allow welding time as short as possible and excellent quality of the welded joint, especially for parts made of hard weldable materials by the classical FSW process. The method of TIG-assisted friction stir welding (FSW) is a new FSW technique proposed by the team of authors. The TIG-assisted FSW is a development of the FSW process, year XXIV, no. 1/2015 7

5 causing the emergence of a hybrid welding process, in solid form, which integrates the pre-heating of the plates by the TIG welding process. It was founded that preheating by additional external heat source contributes to the reduction of forces, determines the reduction of the stress status on the pin and theoretically can be an option to increase either welding speed or tool life. Analysis of the results of the performed experiments demonstrates that by applying the FSW-TIG process for welding Cu99 copper, the following advantages can be certainly obtained: - Increasing welding speed by 100%; - Obtaining welded joints with comparable characteristics with the classical FSW welding; - Obtaining a more stable welding regime (without vibration) which provides a better protection of both the machine and welding tools. References [1]. G. Kohn et al.: Laser assisted friction stir welding. Patent No.: US , WO 99/39861; [2]. G. Kohn, Y. Greenberg, I. Makover and A. Munitz: Laser- Assisted Friction Stir Welding. wj/2002/02/feature; [3]. Laser Assisted Friction Stir Welding ; [4]. B.M. Tweedy, W. Arbegast and C. Allen: Friction stir welding of ferrous alloys using induction preheating. Friction Stir Welding and Processing. The Mineral, Metals & Materials Society, 2005; [5]. A. Murariu, V. Bîrdeanu, R. Cojocaru, V. Safta, D. Dehelean, L. Boţilă and C. Ciucă: Application of Thermography in Materials Science and Engineering, Chapter in the book Infrared Thermography, pp , available at intechopen.com /books/infrared-thermography, ISBN: , Publisher InTech, March, 2012; [6]. C.B. Fuller, Friction Stir Tooling: Tool Materials and Designs, Friction Stir Welding and Processing, pp.7-37, ASM International, 2007; [7]. D. Dehelean et al., Innovative and ecological technologies to process advanced materials by friction stir welding, Project CEEX 66/ ; [8]. E. Scutelnicu, D. Bîrsan and R. Cojocaru, Research on Friction Stir Welding and Tungsten-Inert-Gas-assisted Friction Stir Welding of Copper, published in Proceedings of 4th International Conference on Manufacturing Engineering, Quality and Production Systems MEQAPS 11. Barcelona, Spain, 15th -17th of September 2011, pp , ISSN: , ISBN: (ISI indexed). Calendar of international and national events Calendarul manifestărilor ştiinţifice şi tehnice internaţionale şi naţionale 2015 May Eurojoin 9 Bergen, Norway May th International Conference on Emerging Technologies Brussels, in NDT Belgium Jun. 3-5 The 3 rd IIW South-East European Welding Congress - Welding and Joining Technologies for a Sustainable Development and Environment Jun. 3-5 International Fair 2015 Innovation in Welding and Non-destructive Examination Jun. 28 Jul. 03 Aug Sept. 7-9 Sept th IIW Annual Assembly and International Conference on High-Strength Materials - Challenges and Applications Timisoara, Romania Timisoara, Romania Helsinki, Finland The 15 th Nordic Laser Materials Processing Conference Lappeenranta, Finland 3 rd Conference on Smart Monitoring, Assessment and Rehabilitation of Structures International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE 2015) Antalya, Turkey Berlin, Sept. 16 Welding Trainer Conference 2015 Nürnberg, Sept th IIW Welding Research and Collaboration Colloquium Munich, year XXIV, no. 1/2015