Rev. Téc. Ing. Univ. Zulia. Vol. 39, Nº 1, 168 - 176, 2016
Study of Friction Stir Spot Welding Process and its Parameters for Increasing Strength of Dissimilar Joints
1 2 S. Siddharth , Dr. T. Senthilkumar
1 Department of Mechanical Engineering U C E, BIT Campus Anna University Tiruchirappalli, Tamil Nadu, India, [email protected] 2 Department of Mechanical Engineering U C E, BIT Campus Anna University Tiruchirappalli, Tamil Nadu, India, [email protected]
Abstract--- Joining of dissimilar materials poses a challenge due to the difference in physical properties (melting point, thermal coefficient of expansion and density), mechanical properties (strength, hardness and toughness) and metallurgical properties (crystal structure, microstructure and chemical composition). Solid state joining comes as a novel method for joining these.
In this paper, friction stir spot welding, a new variant of friction stir welding process is discussed. As it is a solid state joining process, its importance in joining dissimilar materials is studied. The important friction stir spot welding parameters and their effect on the strength of the joined materials are analyzed. The defects that pose a challenge reducing the mechanical properties of the joints a r e critically examined and suitable methods for improvement of these properties are evaluated.
1. INTRODUCTION Friction stir spot welding (FSSW) process was developed by Mazda Motor Corporation and Kawasaki Heavy Industry and patented in 2003, (Sakano R et al., 2001), (Iwashita T et al., 2003) which is a variation of linear friction stir welding method. To meet the challenges in the today’s automobile requirements, such as durability, reliability and sustainability, FSSW process proves to be a competent solid state process to weld the low carbon automotive steel (A. K. Lakshminarayana et al., 2015). It is a substitute to conventional resistance spot welding and riveting in joining light weight structural metals such as aluminium and magnesium alloys (Weny Li et al.), Chen et al. (2010) , (Bakavos et al., 2009) have shown the feasibility of producing dissimilar aluminium to steel friction stir spot welds in thin automotive sheet (1 mm thick).
The effects of the friction spot welding process parameters on the mechanical strength of AA6181-T4 and Ti6Al4V dissimilar joints were investigated using statistical analysis (A. H. Plaine et al., 2015). The most important advantage is the ability to join advanced high-strength steels (AHSS) (Shi S.G et al., 2003), which are very common materials in the automotive and aerospace industries, with over 90% in power saving and 40% in equipment saving compared with resistance spot welding (RSW) (Sakano et al., 2001)
2. PRINCIPLE OF OPERATION FSSW can be considered a transient process due to its short cycle time, unlike friction stir welding (FSW) (Badarinarayan H et al., 2009). In FSSW process, a non-consumable tool consisting of a shoulder and pin is used to produce the weld. The pin length is selected such that it sufficiently penetrates into the lower sheet of the lap configuration of the work pieces.
168 Rev. Téc. Ing. Univ. Zulia. Vol. 39, Nº 1, 168 - 176, 2016
Figure 1. Plunging Figure 2.Stirring
Figure 3. Drawing Out
PLUNGING
During this process, a rotating tool with a probe is plunged into the material from the top surface for a certain period of time to generate frictional heat. At the same time, a backing plate contacts the lower sheet from the bottom side to support the downward force.
STIRRING
In this stage for a particular duration of time frictional heat is generated between the wear resistant welding components and work pieces. This heat, along with that generated by the mechanical mixing process softens the materials without melting. Heated and softened material adjacent to the tool causes plastic flow. In addition, the tool shoulder gives a strong compressive force to the material.
DRAWING OUT
After the dwell period, the tool is withdrawn from the plunged zone and drawn away from the material, a solid-phase weld is produced between the upper and the lower sheets. The material hardens on cooling thereby welding the two pieces together.
3. TOOL NOMENCLEATURE IN FSSW The non-consumable rotating tool is designed depending upon the type of materials to be joined. The shoulder length, type of pin, its profile significantly contributes to the stirring action during the plunging time and there by affects the quality of the weld. Tool life affects the number of tool changes made during production and also affects the cost of the process. There are several tool materials which have been used for FSW and FSSW of AHSS, including silicon nitride (SiN), polycrystalline cubic boron nitride (PCBN), tungsten rhenium (W-Re), tungsten carbide (WC), and other tungsten-based alloys (M. Santella et al., 2010). One identified drawback is that the tooling leaves a keyhole (the size of its dynamic volume) in the weld during retraction that requires removal via either post-processing, or costly, highly specialized tool design. As such, pin-less tool designs have been identified in the literature as a low-cost alternative (Bakavos D et al., 2010). For small thickness lap joints pin-less flat tools are identified, and for increasing the stirring action short flute and long flute, pin-less tools are used. For better plunge and stir of the materials in thicker materials,
169 Rev. Téc. Ing. Univ. Zulia. Vol. 39, Nº 1, 168 - 176, 2016
pinned tools with specific geometries like cylindrical and triangular are used. Welds made with triangular pin tools have displayed larger bond width and better lap-shear strength compared to welds made using cylindrical pin tools (Badarinarayan H et al., 2009).
Figure 4. Conical and concave pin Figure 5. Coarse threaded and triangular pin
Figure. 6. Pin-less flat Figure 7. Pin-less flat short flute
Figure 8. Pin-less flat long flute
4. FRICTION STIR SPOT WELDING PARAMETERS
Tool Penetration Depth in mm Tool rotation rate in rpm Tool shoulder plunge depth in mm Dwell time – Duration of plunge in seconds Tool plunge speed in mm/min Axial force of the rotating tool in N
EFFECT OF TOOL PENETRATION DEPTH
The tool which rotates at a constant speed is made to plunge and penetrate the materials to be joined. The work piece is held in a lap configuration in friction stir spot welding. The depth of penetration of the tool in the top plate is responsible for determining the overall quality of the joints. The depth of plunge in the bottom plate, the thickness of top and bottom plate are also the factors to be considered.
Baek et al., joined low carbon steel plates by friction stir spot welding (FSSW) with lap configuration and observed that the tool penetration depth exerted a strong effect on failure mode of joining samples and a
170 Rev. Téc. Ing. Univ. Zulia. Vol. 39, Nº 1, 168 - 176, 2016 weak effect on the joint shear strength. Pan et al studied the effect of tool penetration depth at a constant tool rotational speed and reported different failure modes such as interfacial separation at shallow insertion depths, nugget pullout at highest strength, and perimeter failure at deepest insertion (T.Y. Pan et al., 2004).
Gerlich et al., studied the effect of tool penetration during friction stir spot welding of aluminium and magnesium alloys. It was found that the highest temperatures attained during friction stir spot welding of Al 6111 and AZ91 were close to the solidus temperatures of each base material and correspond with 0.94 Ts (Al 6111) and 0.99 Ts (AZ91) where Ts is the solidus temperature of the material in degrees Kelvin. The melt wear was found to occur under the rotating tool shoulder provided that sufficient penetration of the upper sheet occurs during the spot welding operation (A. Gerlich et al., 2015).
EFFECT OF AXIAL FORCE
Axial force is the force with which the non-consumable tool that rotates at a constant speed comes in contact with the work piece. During the friction stir spot welding process, as the materials soften due to frictional heat, the axial force of the plunging tool plays a pivotal role in holding the two work pieces together. Axial welding forces affecting weld quality were measured during the execution of the experiments by means of a piezoelectric load cell (Aidan Reillya et al., 2015).
EFFECT OF DWELL TIME
The time duration for which the tool remains in contact with the work piece during the plunging, stirring and drawing out stages is the dwell time. Dwell time is found to have a great influence on tensile shear fracture load (TSFL) followed by tool rotation speed and plunge depth (A. K. Lakshminarayana et al., 2015). Amancio-Filho et al. investigated the influence of FSSW process parameters on the strength of overlap welds on AA2024-T3 alloy produced using a 32 FFD. They showed that dwell time (DT) has the main effect on the weld strength, followed by rotational speed (RS) and DT interaction. In joining 0.93 mm thick 6111-T4 aluminium and 1 mm thick DC04 (un-galvanized) and DX54Z (galvanized) low carbon steel sheet the cycle time was less than one second, and high lap-shear strengths were achieved by the joints failing in nugget pull- out mode (Uematsu Y et al., 2008). For FSSW of dissimilar aluminium to steel, Chen et al. found that it was difficult to achieve high strength joints within one second, particularly with zinc coated steel grades (Chen et al., 2010).
5. HEAT AFFECTED ZONES The frictional heat developed induces a change in the grain structure which significantly affects the strength of the joints. Four regions (Mohamed Merzoug et al., 2011) that can be specifically defined when viewed through the cross section of a FSSW joint are 1. An area of no contact. 2. A region of only mechanical bonding “kissing bond”. 3. An interface of partially metallurgically bonded region. 4. A zone with full metallurgical bonded zone.
Figure 9. Cross Section of a typical FSSW joint (Al6061)
171 Rev. Téc. Ing. Univ. Zulia. Vol. 39, Nº 1, 168 - 176, 2016
Figure 9. A. Heat Affected Zone
Figure 9. B. Thermo mechanically affected zone
Figure 9. C . Stir Zone
STIR ZONE Also called the nugget, dynamically recrystallized zone is a region of heavily deformed material that roughly corresponds to the location of the pin during welding. The grains within the stir zone are roughly equiaxed and often are in an order of magnitude smaller than the grains in the parent material.
THERMO MECHANICALLY AFFECTED ZONE (TMAZ) It occurs on either side of the stir zone. In this region the strain and temperature are lower and the effect of welding on the microstructure is correspondingly smaller. Unlike the stir zone the microstructure is
172 Rev. Téc. Ing. Univ. Zulia. Vol. 39, Nº 1, 168 - 176, 2016 recognizably that of the parent material, albeit significantly deformed and rotated.
HEAT AFFECTED ZONE The Heat Affected Zone (HAZ) is the region which lies closer to the weld-center and has experienced a thermal cycle during welding which has modified the microstructure and/or the mechanical property, there is no plastic deformation in this region.
PARENT MATERIAL (PM) It is the material that is remote from the welded region that has not been deformed; however it may have experienced thermal cycling from the weld. This is not affected by the heat in terms of the microstructure or the mechanical properties.
From literatures the importance of the heat affected zones can be clearly understood. Uematsu et al., joined T4 treated 6061 using a double acting tool consisting of outer flat shoulder and inner retractable probe, which could re-fill probe hole. The microstructures of the weld zone were classified into MZ (mixed zone) and SZ, where fine equiaxed grains were observed due to dynamic recrystallization during FSSW process. They further found that the tensile strength of the joint was improved by a re-filling process because the effective cross sectional area of the nugget was increased
Merzoug et al., conducted experiments on AA6060-T5 using a tool steel of the type X210 CR 12 and the rotational speed of the tool ranged from 1000 to 2000 rpm. The tensile tests made it possible to establish that the sample produced at 1000 rpm and 16 mm/min has a good quality of welding, which has 5 kN to 16 mm/min and 1000 rpm compared to 1.98 kN for 25 mm/min and 2000 rpm. The micro hardness approached the maximum value as they moved away from the nugget zone.
Zhang et al., spot welded AA 5052-H112 of 1 mm thickness. They concluded that softening occurs in the welds. A minimum hardness of 19.2 HV, which equals to 45.7% of that of the PM, was measured in the HAZ. In addition, hardness in the TMAZ and SZ improved due to the recrystallization which makes the hardness distribution exhibit a W-shaped appearance. The joint strength decreases with increasing tool rotational speed, while it is almost independent of the given tool dwell times.
6. MICROSTRUCTURAL EVALUATION
Figure 10. Stir Zone
173 Rev. Téc. Ing. Univ. Zulia. Vol. 39, Nº 1, 168 - 176, 2016
Figure 11. Thermo Mechanically Affected Zone
Figure 12. Heat Affected Zone
Figure 13. Parent Material
Microstructures of FSSW 7075-T6 aluminum alloy joints are shown. From the microstructural observation under 50 x magnifications, the variation in grain sizes in the width and thickness can be seen.
7. CHALLENGES IN FSSW
Formation of void in the middle of the weld zone after welding. Very small contact zone. Ineffective removal of oxides and other contamination from the weld interface (as there is no relative motion between them). Lower weld strength. Poor mixing of materials (sheets) used for lap joint welding (as there is no relative motion between them).
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Large heat effected zone, etc.
8. PARAMETER OPTIMIZATION METHODS In order to overcome various difficulties during the FSSW process, reducing the intensity of defects and increasing the quality of the joints optimization methods are adopted for finding the optimum parameters and their range for obtaining perfect joints. Design of experiments – For making low carbon automotive steel joints using FSSW design of experiments was used to conduct the experiments for exploring the interdependence of the process parameters. A second order quadratic model for predicting the lap shear tensile strength of friction stir spot welded low carbon automotive steel joints was developed from the experimental data (A.K Lakshminarayana et al., 2015).
Grey relational analysis - FSSW parameters for joining Al6061 was optimized through grey relational analysis showed that applying ultrasonic vibration and selections of 1200 RPM tool rotary speed, 6 mm plunge depth and 6 s dwell time causes the highest value of grey relational grade and guarantee maximum lap shear force as well as maximum hardness (Wenya Li et al., 2014).
Response surface methodology - The effect of the process parameters on the lap shear strength of AA6181-T4/Ti6Al4V single joints was investigated and based on the experimental results following response surface methodology, a mathematical model to predict lap shear strength was developed using a second order polynomial function (.A.H. Plaine et al., 2015).
9. JOINT ANALYSIS Fracture morphology- Z. M. Su conducted fracture morphology analysis and demonstrated that the shear fracture of the FSSW-FSW joints took place along the boundary between the upper and lower sheets through the weld nugget, and the faying surface between the two sheets was completely sheared off.
Fatigue analysis – A.H. Plaine investigated the fatigue behaviors of swept friction stir spot welds in lap- shear specimens of al clad 2024-T3 aluminum sheets are carefully examined them by optical and scanning electron micrographs.
10. CONCLUSION Thus in this paper a novel method for solid state material joining has been discussed. The process and stages involved in FSSW has been analyzed. The effect of the non-consumable tool on weld quality and its various types have been studied. The various process parameters of FSSW have been listed and analyzed for its effect on the joint strength.
The effect of stirring has been discussed with reference to the various heat affected zones and the changes in grain structure are studied. Process parameter optimization methods have been studied and the joint analysis techniques are evaluated.
REFERENCES
Aidan Reillya, Hugh Shercliff , Yingchun Chenb, Philip Prangnell – “Modelling and visualisation of material flow in friction stir spot welding” - Journal of Materials Processing Technology 225 (2015) 473–484. A. Gerlich, P. Su, T. H. North – “Tool penetration during friction stir spot w e l d i n g of Al a n d Mg a l l o y s ”- Journal of Mat e r i a l s Science, 40 (2005) 6473–648 A.K. Lakshminarayanan, V.E. Annamalai, K. Elangovan – “Identification of optimum friction stir spot welding process parameters controlling the properties of low carbon automotive steeljoints”- Journal of Materials Research & Technology, ELSEVIER, (2015) 262-272 A.H. Plaine, A.R. Gonzalez, U.F.H. Suhuddin, J.F. dos Santos, N.G.Alcantara – “The optimization of friction spot welding process parameters in AA6181-T4 and Ti6Al4V dissimilarjoints”- Materials & Design 83 (2015) 36–41 Badarinarayan H, Yang Q, Zhu S. – “Effect of tool geometry on static strength of friction stir spot-welded aluminum alloy”- International Journal Machine Tools 49 (2009) 142–8
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Baek SW, Choi DH, Lee CY, Ahn BW, Yeon YM, Song K, et al. – “Structure–properties relations in friction stir spot welded low carbon steel sheets for light weight automobile body” - Mater Trans 51(2) (2010) 399–403. Bakavos D, Chen Y, Babout L, Prangnell P – “Material interactions in a novel pin less tool approach for friction stir spot welding thin aluminum sheet”- Metal Material Transaction 42A (2010) 1266–82. Bakavos, D., Babout, L., Prangnell, P – “Optimisation of short cycle time friction spot welding of thin aluminium automotive sheet” Proc. 8th I n t . Friction Stir Welding Symposium, Timmendorfer Strand, Germany, S7A, 18–20 May (2010), pp. 546–554. Chen, Y.C., Haddadi, F., Prangnell, P.B., - “Feasibility study of short cycle time friction stir spot welding thin sheet Al to ungalvanised and galvanised steel”, Proc. 8th Int. Friction Stir Welding Symposium, S10B, Timmendorfer Strand, Germany 18–20 May (2010), pp. 893–903 G. D. Urso – “Thermo- mechanical characterization of friction stir spot welded AA6060 sheets: Experimental and FEM analysis”- Journal of Manufacturing Processes 17 (2015) 108–119 Iwashita T. – “Method and apparatus for joining”- US patent 6601751 B2. August 5; 2003. Mohamed Merzoug, Mohamed Mazari, Lahcene Berrahal, Abdellatif Imad “Parametric studies of the process of friction spot stir welding of aluminium 6060-T5 alloys” - Materials and Design 31 (2010) 3023– 3028 Y. Rostamiyan, A. Seidanloo, H. Sohrabplow carbonoor, R. Teimouri – “Experimental studies on ultrasonically assisted friction stir spot welding of AA6061” - A rchives of C ivil and M echanical Engineering 15 ( 2015 ) 335 - 346 Sakano R, Murakami K, Yamashita K, Hyoe T, Fujimoto M, Inuzuka M, Nagao U, Kashiki H. – “Development of spot FSW robot system for automobile body members” - Proceedings of the 3rd international symposium of friction stir welding 2001; Kobe, Japan; 2001 M. Santella, Y. Hovanski, A. Frederick, G. Grant, M. Dahl – “Friction stir spot welding of DP780 carbon steel”- Sci. Technol. Weld. Join. 15 (2010)271–278. Shi SG, Westgate SA. – “Resistance spot welding of high-strength steel sheet”,- The Welding Institute (TWI) corporate research program report, corporate research program 767 T.Y. Pan, A. Joaquin, D.E. Wilkosz, L. Reatherford, J.M. Nicholson, Z. Feng, M.L. Santella – “Spot friction welding for sheet aluminum joining”- 5th International Symposium on Friction Stir Welding, The Welding Institute, Metz, France, (2004), paper no. 11A-1 Uematsu Y., Tokaji K., Tozaki Y., Kurita T., Murata S. - “Effect of refilling probe hole on tensile failure and fatigue behavior of friction stir spot welded joints in Al–Mg–Si alloy” - International Journal of Fatigue 30 (2008) 1956–1966. Wenya Li, Jinfeng Li, Zhihan Zhang , Dalu Gao, Weibing Wang, Chunlin Dong – “Improving mechanical properties of pinless friction stir spot welded joints by eliminating hook defect” – Materials and Design, Vol 62 (2014) 247-254 Zhaohua Zhang, Xinqi Yang Jialong Zhang, Guang Zhou, Xiaodong Xu, Binlian Zou “Effect of welding parameters on microstructure and mechanical properties of friction stir spot welded 5052 aluminum alloy” Materials and Design 32 (2011) 4461–4470.
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