Friction Stir Welding


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Friction Stir Welding

  1. 1. Friction Stir Welding in Automotive Applications – Present and Future by Gajendra Tawade
  2. 2. Friction stir welding - Introduction • Friction stir welding uses a spinning pin tool to heat the materials by friction and plastic deformation to between 70% - 90% of the solidus temperature. • The spinning pin tool extrudes material from one side to the other in distinct flow patterns where it is forged together and consolidated behind the pin
  3. 3. Friction stir welding (FSW) • Let us try to find answers to following questions – What materials can be welded with FSW? – What is maximum and minimum thickness that can be welded? – What does FSW machine look like? – What are the different components of FSW machine? – What is the material for pin-tool? – What is pin-tool life? – What are the welding parameters? (RPM, IPM, Force etc.) – What is the cycle time for FSW weld? (Better than spot weld???) – Can FSW replace spot welding, MIG or TIG welding? – Is robotic FSW possible to achieve current jobs/hour rate? – How is the control system for FSW technology?
  4. 4. Friction stir welding (FSW) • What are the parameters that can control the weld? • Is robotic FSW application is in production currently? • Why use FSW why not improve cycle time on spot welding? • What are real challenges in FSW at this time? • Is the process flexible? • What is the most potential application for FSW in future?
  5. 5. Base metal thickness for FSW • Aluminum:(2xxx to 7xxx series, A356,A357, Al-Li alloy) – 0.94/0.94 mm lap joint...(Oak Ridge National Lab) Single pass, 12.7 mm Butt Max, 7mm economical Double pass, 25.4. mm Butt max , 14mm economical
  6. 6. Base metal thickness for FSW • Steel: (SAE 1018, stainless steel 304L, 316L) – 6 / 6 mm 304L/316L alloy lap joint...(Naval surface warfare center, USA) – 6.3 / 6.3 mm Mild steel butt joint...(Edison Welding institute,USA) – 1.6 /1.6 mm DP600 steel and 1300 martensite steel lap…(Oak Ridge N.L.-Freedom Car Project) • Magnesium (AZ31) – 6 / 6 mm butt joint...(Oak ridge national lab, USA) • Al-Li alloy – External fuel tank of space shuttle (30% more strength than fusion welds)
  7. 7. FSW joint geometry Butt weld Lap weld
  8. 8. Joint designs for FSW Full penetration Partial penetration
  9. 9. FSW joint geometry • T-Section • Corner welds • For each joint geometry, specific tool designs are required which can be further developed and optimized.
  10. 10. FSW joint geometry • The FSW process can also be applied to – Circumfrential, annular, non-linear, and three dimensional welds. – Since gravity has no influence on the solid-phase welding process, it can be used in all positions, e.g.: • Horizontal • Vertical • Overhead • Orbital
  11. 11. FSW Machine • Machines are available in various sizes • Components of machine – Pin tool – Shoulder – Actuators and control system – Fixture (Holding the part) • Types of motion – Pin rotation – Longitudinal pin travel – Anvil or table travel (X and Y direction) – Vertical motion of the pin
  12. 12. FSW Machine - pin tool • Pin tool and shoulder design has significant effect on the weld • Pin tool - Acts as cutting tool • Shoulder - Squeeze the elasticized material • The better contour design on pin tool can increase the mixing of the material at the interface, thereby increasing the strength of the joint. Patent: July, 2003, FULLER CHRISTIAN, US2005121497
  13. 13. PCBN Pin Tool • First tools were very simple • Features on the pin tool were quickly worn • PCBN (Poly-crystalline boron nitride) is new famous material
  14. 14. FSW Machine - pin tool • The tool profile helps reducing the deformation of the work-piece • All the efforts were done to get better mixing of the weld pool and having a better joint strength with good pin and shoulder profile. Patent: July, 2003, FULLER CHRISTIAN, US2005121497
  15. 15. FSW Machine - pin tool • The ratio of Volume of the Probe swept during rotation to the volume of the probe itself should be high • It is also called as “Static to dynamic volume ratio” • Volume of probe should be as small as possible, • Surface area should be larger Publication: Stephan Kallee, EWI, 2nd Eurostir workshop, Nov 2002
  16. 16. MX Triflute • A coarse helical ridge around the triflute lands, is used to reduce the tool volume, and help break up and disperse surface oxides. • Smaller tool volume help better mixing of metals • Helical flutes increase the surface area of the Publication: Stephan Kallee, EWI, 2nd Eurostir workshop, Nov 2002
  17. 17. MX Triflute • When welding thin sheets, heat generated between shoulder and the sheet is the main source of heat • Profiled or threaded surface of pin facilitates a downward auguring effect • Odd number of equally spaced flutes maintain maximum cross-section opposite to any re-entrant feature • Change in section between the shoulder and the probe is well radiused in order to reduce the stress concentration Publication: Stephan Kallee, EWI, 2nd Eurostir workshop, Nov 2002
  18. 18. Flared triflute • For conventional tool, static to dynamic volume ratio is 1.1:1 and for Tri-flute it is 2.6:1 • In the flared probe, the core remains as a taper frustum but the flute lands are flared out at an inverted angle so as to increase the tip diameter. • A tip profile has been included in the shape of a three pronged whisk. • Improved ratio of swept volume and the static volume further improves the flow path around and underneath the probe. Publication: Stephan Kallee, EWI, 2nd Eurostir workshop, Nov 2002
  19. 19. Skew-Stir • Improved dynamic to static volume ratio • Good re-entrant feature • Can be used where complex shape pins can not be used • Better dispersion of the surface oxides films in the • Greater volume of molten metal Publication: Stephan Kallee, EWI, 2nd Eurostir workshop, Nov 2002
  20. 20. WhorlTM Tool • The seashell-shaped probe is machined with a progressively changing pitch and angled at a critical rake. This shape needs less effort to go through the plasticised work-piece than a conventional cylindrical tool. • The benefits are as follows: – Very encouraging tensile, fatigue, de-formability and ballistic impact properties – No joint preparation is necessary, even when butt welding 75mm thick plates – The welding speeds are higher than those of mechanized MIG welding when welding plates of more than 10mm thickness – Low distortion and high reproducibility Publication: Stephan Kallee, EWI, 2nd Eurostir workshop, Nov 2002
  21. 21. Tool Material • MN100 – High PCBN content – Difficult to machine – Expensive • MN50 – Lower PCBN content – Easier to machine (e.g. EDG possible) – Less expensive • PCBN properties – Hard, wear resistant, survives temperature to 1500 K – Chemically inert – Poor tensile strength, low toughness – Difficult to manufacture – Limited to relatively small pieces – High thermal conductivity
  22. 22. Tool-Details • 19 mm WC shank • 19-25 mm diameter CBN disk • 6º face angle • Pin lengths up to 6 mm • Radius about 1 mm • Pin – Smooth for short pins – Flats on end for long structure • Produced in high temperature, ultra high pressure press (1450 C, 870,000 psi)
  23. 23. Tool-Details • Integrated thermocouple to measure tool temperature • Integrated force sensor can be embedded for measuring – Axial tool load – Lateral tool load
  24. 24. Managing Thermal Load • Stainless steel: 15 W/m-K • PCBN: 250-300 W/m-K • WC: 70 W/m-K • More heat flows into tool holder than weld! • Insulator to keep heat in weld zone • Need to protect machine bearings
  25. 25. Tool holder • Technara tool holder • Chilled water-glycol • Air/gas cooling on locking collar • Fits in milling machine (#50 Taper) • Instrumented for tool temperature
  26. 26. Tool Life - Failure Mechanisms • Wear when temperature is too high or too low • Pin fracture when retracting tool • Shoulder fracture during welding • Fracture elimination • Tool redesign • Better process control • Design of custom CBN grades
  27. 27. FSW Machine - pin tool • The proper mixing of the semi-molten material can be achieved by applying • Variable rotational speed of the pin tool • Using different diameter of rotational pins. • Preheated work-piece (Induction, lasers, resistive, gas or flame heaters) Patent: JUNE 2003, VYAS AMITABH, BOEING CO, CN1593834
  28. 28. FSW Machine - pin tool • The tool having a pin with two independently rotating portions – (25a) = Top steel plate , speed according to material properties – (25b) = Bottom steel plate • Two parts are integrated together. Same RPM, different angular speed (Fig. 5) • Fig. 6 pin tool with 3 parts, independent or integrated • Dissimilar metals can be welded Patent: JUNE 2003, VYAS AMITABH, BOEING CO, CN1593834
  29. 29. Trivex tool • 'TrivexTM' tool (Fig. 1) minimizes the traversing force • Three convex sides that prevent entrapment of material • This machine permitted the measurement of traversing and down forces, which were reduced by 18- 25% and 12% respectively with the new tools. • In particular, the TrivexTM tool without threads shows remarkable promise, because it is inherently much easier to manufacture
  30. 30. Rollers for controlling the weld depth • It is necessary to control the depth of the pin in the weld • A set of rollers associated with the welding head eventually reaches the surface of the work-piece when the welding pin-tool reaches the proper depth, and prevents further penetration of the welding post. • The rollers can be with the crowned surfaces • The force can be applied by a hydraulic cylinder
  31. 31. FSW Machine - pin tool • Pin tool with two shoulders • Control over the deformation on the plasticized metal • Fins allow better heat dissipation • Part # 37 allows stirring at the bottom Patent: Feb 2002, Waldron Douglas, Boeing Co, US2005103824
  32. 32. FSW Machine - pin tool • Reduced force is required for a grooved pin to advance through the metal than plane pin • Increased throwing power is necessary for the better weld strength Patent: June 2003, HEMPSTEAD GEORGE, Boeing co, EP1510279
  33. 33. FSW Machine - pin tool • Pin tool with internal flow channel – The threads on the pin are designed to flow/push molten metal towards the bottom of the weld. – The threads help better circulation of molten metal inside the weld cavity – One or more flow channels extending from the side wall of the pin to the internal flow cavity induce a continuous path of plasticized metal through the pin – Dimensions of pin 0.3 inch in diameter and 0.25 inch length. Patent: June 2000, MAHONEY MURRAY, US6206268
  34. 34. FSW Machine - pin tool • The pin tool can be designed to have – A greater surface roughness at the distal end – Shot pinning can be carried out after machining and before heat treatment – Distal end of the pin can roughened to generate more heat • Patent: August 2001, DUNCAN FRANK GORDON, US2002190100
  35. 35. FSW Machine - pin tool • Benefits of adjustable pin extension – Accurately control the pin extension while welding. – Weld variable thickness plates. – Weld plates having different thickness without having to change tools. – Eliminate the holes in the work left by traditional tools. – To have a variable diameter shank on the friction stir-welding tool. Patent: June 1996, WYKES DONALD, BOEING North American INC US5697544
  36. 36. FSW Machine - pin tool • Adjustable length friction welding pin Patent: June 1996, WYKES DONALD, BOEING North American INC US5697544
  37. 37. FSW Machine - pin tool • Consumable pin tool design – Contamination of pin material in the weld is one of the challenges – Consumable pin is continuously fed into the joint – Pin material same as base metal – Joint thickness can be increased locally Patent: Feb 2004, SUBRAMANIAN, GEN ELECTRIC, JP2005074520
  38. 38. FSW Machine - pin tool • Auto adjustable pin tool – For welding materials of varying thickness – pin can be incrementally withdrawn from the work-pieces thus eliminating any crater or keyhole in the weld Patent: Sept 1997, DING R JEFFREY, NASA, US5893507
  39. 39. FSW Machine - pin tool • Various pin tool profiles
  40. 40. FSW Machine - pin tool • At higher spindle speeds during welding non-extrudable “Al” – higher heat causes the aluminum build up on the welding tool shoulder – material from the sides of the weld surface tear away – upper surface of the weld becomes rougher – Excessive buildup make further welding impossible – Additional machining may be required – points of initiation of fatigue cracks can be generated • Cooling of the pin is necessary • This method allows 20% to 100% more welding speed • 6.4 mm 6061 Al needs 1600 rpm 15 ipm for smooth weld • 3.17 mm 2024 (non extrudable) needs 500 rpm, 3.5 ipm for smooth weld. This can be improved to 800 rpm and 8 ipm with cooling Patent: May 1996, Boeing Co., Colligan, JP10052770
  41. 41. FSW Machine - pin tool • The shoulder 38 has an angle of about 10 DEG • A coolant rate 0.01 Gpm for direct cooling • A coolant rate of 0.1 Gpm for internal cooling • Cool air or gas is sprayed in the fins Patent: May 1996, Boeing Co., Colligan, JP10052770
  42. 42. FSW Machine - pin tool • Thermal expansion of pin reduces the distance between the pins • Respective tools 2 and 3 are • Cooled by air jets from respective air nozzles 4 and 5. • Thus, the thermal expansion of the tools 2 and 3 are prevented, and • The gap between the upper and lower tool 3 can properly be held. • In this way, a friction stir weld with proper “T” is formed • The welding strength of the product is secured Patent: August 2003, Toyota., Kozuka, JP2005074451
  43. 43. FSW Machine - pin tool • Control over pin temperature is necessary for – Achieving stable welding quality – Avoid overheating/ hot shearing of pin tool – Control the wear of pin tool – Maintain smoothness and avoid roughness • “We found through our trials that cooling tip is critical, If you keep the temperature down you get a nice smooth burr-free weld.”……Mr. Scafe from Tower Automotive • Tower is working on a tool that’s driven by a servo motor and that can be hollowed out so that coolant can flow right to the tip.
  44. 44. FSW Machine - pin tool • Other welding parameters can be varied to get a stable temperature curve (Curve-E) • Stable temperature curve will provide consistent weld quality Patent: Oct 2001, ANDERSSON CLAES-GORAN, US2005006438
  45. 45. Material for Pin tool • Material for Pin tool – Polycrystalline cubic boron nitride (PCBN) – CBN is the 2nd hardest known material (1st Diamond) • Other materials – Tool steel (H13) – molybdenum alloy, such as TZM, – nickel alloys, such as Rene 41 (UNS N07041), – iron-nickel alloys, Nimonic 118 • PCBN tool exhibits little or no wear when correct parameters are utilized during FSP 304L stainless steel.
  46. 46. Friction stir welding - Process control • FSW is robust enough for mass manufacturing • Friction stir welding defects: – Defects due to higher RPM – Defects due to higher IPM – Defects due to flow pattern Research paper: Sept 2004, Arbegast, South Dekota school of mines
  47. 47. Measurement of load distribution on the tool • The experiment measured x-forces on a dynamometer while using varying tool geometries. • Welds were made at several tool lengths with a constant pin diameter. • Welds were then processed at several pin diameters with a constant pin length. • In all cases, shoulder diameter and concavity were held constant.
  48. 48. Load Vs Pin length • Force generally increased with pin length • Unexpected variation occurred at lengths of 0.25, 0.265 and 0.28.
  49. 49. Load Vs Pin Diameter • It was anticipated that the force would increase with pin diameter; however the results did not support this hypothesis. • smallest and largest diameter pin was less than 7%.
  50. 50. Load distribution along the pin length • The following figure shows the load distribution along the pin length
  51. 51. Temperature Distribution at the tool • Infrared imaging camera is used to measure the “surface” temperature • Three thermocouples were ued to measure “inside” temperature • Tool temperatures were transmitted using data acquisition system and RF telemetry system
  52. 52. Temperature distribution at the tool • Temperature at the pin center was higher than than the surfac
  53. 53. Coordinate system for the FSW • sdgfg
  54. 54. 316 L Stainless Steel
  55. 55. 316 L Stainless steel
  56. 56. Cu-Ni-Cr Alloy
  57. 57. Thermal Model Results
  58. 58. Friction stir welding - Process control • Successful tests for the 6 / 6mm, 304 L stainless steel • PCBN tool with 15mm diameter shoulder • PCBN tool 2mm length , Liquid cooled • Tool supplied by...Tecnara tooling systems Research publication: Brigham Young University, 2003
  59. 59. Friction stir welding - Process control • Relationship between rotational speed and joint strength Patent: Dec 2002, UNIV XIBEI POLYTECH (CN), CN1435290
  60. 60. Friction stir welding - Process control • Controlling certain attributes of plasticized region are the key to control quality of resulting welds • The plasticized region may be monitored by – Force on the tool – Torque applied to the pin tool – Work-piece temperature – Physical dimension of the plasticized region – Changing surface characteristics such as color and reflectivity. • During joining, – The FSW tool is moved through the joint at a constant speed – Or the work-pieces are moved relative to the FSW tool at a constant speed Patent: April 2003, STOTLER TIMOTHY, US2005040209
  61. 61. Friction stir welding - Process control • The load required to force the tool through the work-pieces may vary due to – Temperature variations in the work-pieces – Thickness variations – Intended, or unintended, heat sinks • Forcing the tool through such regions of variable resistance at a constant speed often results in – Reduced weld quality due to inadequate mixing of the plasticized materials and – Reduced aesthetic quality due to overheating of the materials. • The correct amount of plasticized region throughout the length of the weld is necessary for the consistent weld quality Patent: April 2003, STOTLER TIMOTHY, US2005040209
  62. 62. Friction stir welding - Process control • Pin tool traveled quickly, resulting weld is narrow weld, incomplete fusion of side walls and surface fusion high stress on tool and premature failure of the tool • Pin tool traveled too slowly generating wide weld bead, bigger heat affected zone, surface blistering, excessive surface indentation, reduced mechanical properties, loss of corrosion resistance • With the load controlled pin tool, optimum plasticized region can be formed
  63. 63. Friction stir welding - Process control • The reference signal representative of at least one of the plasticized region dimensions may be generated in a number of ways. • The forces on the pin tool can be measured with the following experimental set-up. It uses a load cell mounted on the receiver of the drive screw • Present method can generate a reference signal which will represent the amount of plasticized material generated.
  64. 64. Friction stir welding - Process control • Reference signal is then compared with one of the predetermined signal and error signal is then generated • Initials trials can be taken to get the optimum weld pool/plasticized material dimension. The load representative of best weld geometry is recorded. Margin of error signal is decided • The error signal then acts to control the motor 46 to produce required travel axis load, plus or minus an acceptable margin of error throughout the length of the weld • Thus the motor 46 would adjust the travel speed to obtain the predetermined load. Patent: April 2003, STOTLER TIMOTHY, US2005040209
  65. 65. Friction stir welding - Process control • Figure shows actual results of experiments for welding 3.17 mm thick Al • Travel speed changes as the tool warms up • Travel load is kept constant at around 875 lbs, speed kept on changing depending on material warm up and heat sink Patent: April 2003, STOTLER TIMOTHY, US2005040209
  66. 66. Force measurement • When investigating the milling or grinding process, the workpiece to be examined is mounted on the cover plate of the dynamometer. • The dynamometer measures the reaction forces of the rotating tool via the workpiece. • Torque is the most important evaluation criterion for the drilling or milling process. • The tool sits directly on the rotating dynamometer. • Kistler's rotating multicomponent dynamometers measure cutting forces reliably and accurately at speeds of up to 25,000 rpm.
  67. 67. Development of process control algorithm -Future work • The “weld power” is the power supplied by the weld tool by spindle torque (T) rotation speed (ω) and spindle efficiency (η) – Weld power (P) = (T) (ω) (η) • Weld specific energy = Power input (P)/unit length • A new parameter called pseudo heat index can be defined which will represent heat generated, which will be directly proportional to RPM and depth of plunge, inversely to IPM • (χ) Pseudo heat index is a arbitrarily defined dimensionless function which can be helped to explain the hot or cold friction stir welds – (χ) = RPM/IPM * Depth of plunge • (χ) has good correlation with heat input Research paper: Sept 2004, Arbegast, South Dekota school of mines
  68. 68. Development of process control algorithm-Future work • (χ) is being developed as process control algorithm Research paper: Sept 2004, Arbegast, South Dekota school of mines
  69. 69. FSW - Challenges - Automation • Emergency stop policy • If emergency stop is pushed when pin tool is welding the robot – Pin will stop and after some time, stick with the solidified metal – Pin might break – Continuing welding after stop will be difficult due to solidified burrs • Recommended emergency policy – Pin rotation and robot will not stop if pin is welding – Emergency stop signal will be hold for predetermined time – Robot and tool will come to stop after completion of weld – Gate will be locked while robot is welding emergency stop is pushed Patent: March 2002, MURAKAMI, MAZDA MOTOR (JP), EP1415754
  70. 70. Multi-pass friction stir welding • For FSW weld there are two sides – Advancing side – Retracting side • Material on advanced side mixed more vigorously than material on the retracting side • The multi-pass weld joint is formed of at least 2 FSW joints in parallel configuration. • The first- and second-pass joints define transversely opposite advancing and retreating sides • The second-pass joint is disposed to at least partially overlap the retreating side of the first-pass joint • Thus, material at the retreating side of the first-pass joint that may be insufficiently mixed during formation of the first-pass joint is re-mixed Patent: July 2003, KAY ROBERT M (US), US2005139640
  71. 71. Multi-pass friction stir welding Patent: July 2003, KAY ROBERT M (US), US2005139640
  72. 72. Control system • Pin length controller - Controls pin length relative to the shoulder • Pin force sensor - Senses force being exerted on pin during welding • Position sensor - Sensor for knowing the current shoulder position • Work piece stand off signal - Determines distance between current location and standoff signal at the end of the weld • In this invention, a control system is provided for precisely controlling the depth or location of the distal end of the stirring pin of a rotating pin tool of a friction stir welding machine or apparatus so as to control the penetration of the pin into the work-piece. Patent: June 2000, DING JEFFREY, NASA, US6497355
  73. 73. Friction stir welding • Friction stir welding around a pipe – The pin tool around the OD – A moving mandrel rotates inside the pipe – But welds can be achieved – Process extremely suitable for under water welding and repair Patent: May 2003, Russell Steel, BABB JONATHAN, US2005082342
  74. 74. Friction stir spot welding - The process • This process is specially developed for welding thin steel sheets • No - continuous weld seam • Process can be classified – Plunge – Refill …..1. Shoulder First 2. Fixed position • RPM: 1500, Weld time 1.6s to 3.2s, Thickness 1.6 to 1.6 mm • Tool material: polycrystalline cubic boron nitride (PCBN), Initiation Full Plunge Full Retract Plunge Initiation Full Plunge Full Retract Research paper: Jan 2005, Arbegast, South Dakota School of mines, SAE-2005-1252
  75. 75. Friction stir spot welding Research paper: Jan 2005, Arbegast, South Dakota School of mines SAE-2005-1252
  76. 76. Friction stir spot welding • Two approaches for friction stir spot welding • Fixed pin approach – Hole in middle of the joint – Fast processing time • The second approach : (Not much work done yet) – Shoulder to refill the pin hole. – Long processing time to fill hole Research paper: Jan 2005, Arbegast, South Dakota School of mines SAE-2005-1252
  77. 77. Friction stir spot welding - The process • The welding cycle Force/Speed
  78. 78. FSSW classification • FSSW process – Refill method Shoulder – Plunge method • Refill is New and innovative Pin – Pin & shoulder rotate separate Clamp – Separate load application – Clamp holds the part – Uses “ADAPT” MTS spindle head Workpiece Initiation Full Plunge Full Retract Research paper: Jan 2005, Arbegast, South Dakota School of mines SAE-2005-1252
  79. 79. Stage-1 FSSW Process
  80. 80. Stage-2
  81. 81. Stage-3 Hold/Dwell Time
  82. 82. Stage-4
  83. 83. Classification of “Refill” • Refill can be classified as – 1. Shoulder first – 2. Fixed position Initiation Full Plunge Full Retract Research paper: Jan 2005, Arbegast, South Dakota School of mines SAE-2005-1252
  84. 84. Shoulder first • Shoulder first observations: – Need a precision ratio of shoulder and pin diameter Initiation Full Plunge Full Retract – Weld diameter directly proportional to shoulder diameter – The larger diameter shoulder displaces a significant volume of material and requires the smaller diameter pin to retract to a greater distance to maintain constant volume exchange. – This large pin retraction distance drew the plasticized material into cooler regions of the shoulder where it subsequently adhered to the cold inner walls. – This caused the pin to periodically stick and become lodged within the shoulder between spot weld cycles. Research paper: Jan 2005, Arbegast, South Dakota School of mines SAE-2005-1252
  85. 85. Shoulder first with threaded pin • Threaded pin can refill effectively in shoulder first approach 1/2 Plunge Full Plunge 1/2 Retract Full Retract (No Dwell) 1/2 Plunge Full Plunge 1/2 Retract Full Retract (No Dwell) Research paper: Jan 2005, Arbegast, South Dakota School of mines SAE-2005-1252
  86. 86. Advantages of “threaded pin” • At full plunge, the threaded pin features result in more effective filling of the top surface cavity • At the ½ retract position the shoulder material is being forced toward the center section by the scroll features where it is augured downward by the pin threaded features. • This results in significantly more plasticization, refill of the hole and formation of effective shear area. • Little void or cavity in the weld can be avoided by adding – Dwell time and – Re-forge step Research paper: Jan 2005, Arbegast, South Dakota School of mines SAE-2005-1252
  87. 87. Friction stir spot welding • Control of the process can be categorized into 2 modes – Displacement control – Load control • Displacement control - Tool is driven into the sample by a controlled plunging rate to reach a pre- determined maximum depth • Nominal force - Relatively low when pin plunges into the sample then it increases to higher value, when shoulder of the tool touches the sample Research publication: Tsung, Ford, Dearborn, Oak Ridge National Lab
  88. 88. Friction stir spot welding • Load controlled - The rotating pin is driven into the sample by controlling the rate at which the force on the pin tool increases. The tool keep on plunging into the sample until a preset time is reached. Research publication: Tsung, Ford, Dearborn, Oak Ridge National Lab
  89. 89. Friction stir spot welding • Experiments with displacement control – Tool - H13 tool steel – Work-piece 0.94 mm to 0.94 mm (1.88 mm total) Al alloy (6111-T4) - butt joint – Tool RPM - 2000 RPM – Displacement 1.6 mm to 1.9mm Research publication: Tsung, Ford, Dearborn, Oak Ridge National Lab
  90. 90. Friction stir spot welding • Total thickness : 1.88 mm Research publication: Tsung, Ford, Dearborn, Oak Ridge National Lab
  91. 91. Friction stir spot welding • The current discussion is for displacement controlled method • Load controlled process has the same results • The standard / acceptable failure mode / criterion is not yet defined • It is reasonable to assume that best failure mode should be the one for which shear strength is maximum Research publication: Tsung, Ford, Dearborn, Oak Ridge National Lab
  92. 92. Friction stir spot welding • Anvil - Can be decorative • Diameter 12.7 mm circular Research publication: Tsung, Ford, Dearborn, Oak Ridge National Lab
  93. 93. FSSW Intellectual property • US6601751, August 2003 (Mazda) • EP1149656, October 2001 (Mazda) • JP2002336977, Nov 2002 (Mazda & Kawasaki) • JP2001314983, Nov 2002 (Mazda)
  94. 94. FSW system at Mazda RX-8 plant • Mazda’s first application to 2003 RX-8, a mass production car. The entire aluminum rear door was friction stir spot welded • Rear door Hem flange and hood applications • 90% energy savings • 40% capital reduction (Compared to RSW)
  95. 95. Friction stir spot welding of AHSS • FSSW of Al is common, FSSW of “AHSS” is not – AHSS welds at higher temperatures – It requires much higher mechanical loading • Oak Ridge National lab (Freedom car project) has successfully friction spot welded – 1.6/1.6 mm DP600 and – 1.6/1.6 mm 1300 MPa martensite – Weld time 2 to 3 seconds (laboratory scale) – polycrystalline cubic boron nitride tool is good – Narrow weld ligament width restricts tensile strength. ligament width
  96. 96. Friction stir spot welding of AHSS • Joint efficiency = – Tensile shear strength / tensile strength of base metal • 55 to 70% for friction stir welding • Better approaches than fixed pin approach are necessary
  97. 97. Applications
  98. 98. Automotive applications - ford GT 2005 • Multi-piece “Aluminum” tunnel • Improved dimensional accuracy and 30% more strength than MIG Al Stamping FSW Al Extrusion
  99. 99. Automotive applications - Engine cradle • Prototype engine cradle developed by Tower Automotive
  100. 100. Friction stir welding - Cycle time • Tower participated in a trial with Ford Motor Co. – To produce 1,000 “Al” TBDs – MIG and laser welder never got to 1,000 samples in the allotted time. – Conclusion: FSW can achieve required Jobs/hour rate
  101. 101. Problems in wide spread application FSP • STANDARDIZATION (?) – Pin tools, process parameters and essential variables – Acceptance criteria – Process control algorithms – Joint design allowable and service life assessments – Standard design, analysis and testing protocols – Operator qualification – Fixture and tooling qualification – Corrosion prevention and control – High initial cost
  102. 102. Future work in progress • Following aspects are under development: – Rapid configurable & flexible tooling to reduce implementation costs – Low cost clamping systems – Low cost cast-able support fixtures – Control algorithms to sense change in the alloy and thermal heat sinking – Process forces are being determined to facilitate design – Circumferential pipe welding with internal “pig” (Speed 15 ipm) Control algorithms to sense change in alloy
  103. 103. MTS
  104. 104. ISTIR process development system by MTS • The ISTIR PDS (Process Development System) • Fully instrumented research system • Capable of simultaneous force-controlled operation of three independent axes (X, Z, and Pin). • It has MTS Adjustable Pin Tool (AdAPT™) weld head technology • Enabling it to perform self reacting welds and join materials of varying thickness. • The ISTIR PDS can support up to 5 DOF to produce welds with double curvature. • The ISTIR PDS has successfully joined materials less than 1mm and up to 40mm in thickness.
  105. 105. ISTIR process development system by MTS • The ISTIR PDS Jr. solution is a single-axis system designed to perform linear welds. • As a lower cost alternative to ISTIR PDS, • MTS AdAPT weld head technology. • The PDS Jr. hasbeen deployed to perform process research.
  106. 106. MTS Universal Weld
  107. 107. Flow pattern • Dynamically recrystallized zone (DXZ). • Zones I advancing side extrusion zones, • Zones II retreating side extrusion zones respectively • Zone III is the flow arm of material dragging across the nugget top by the pin tool shoulder. • Zone IV is the swirl zone of material processing near and beneath the pin tool tip. • A fifth Zone V (recirculation zone) may form under very hot processing conditions where the downward motion of material is greater than that which can be accommodated by the space behind the pin tool (excess flow) with the material changing direction and circulating back up towards the top surface.
  108. 108. Process parameters development for FSW • Typical FSW butt joints typically exhibit 65% - 100% joint efficiencies (Refer table) • At colder processing parameters (low rpm, high ipm), the static strength is influenced and lowered by the formation of the characteristic “wormhole” or “tunnel” defect.
  109. 109. Weld strength comparisons • Typical Lap Shear Strength (pounds per inch of weld) of Partial Penetration Lap Joints in Aluminum Alloys Compared to Industry Specified Minimum Strengths for Rivets and Resistance Spot Welds • In thin sheet 2xxx, 7xxx and 5xxx partial penetration lap joints, weld strength exceed those minimums specified in the industry standards for resistance spot welds and riveted structures • For 6xxx “Al” alloys static strength is decreased by sheet thinning defect
  110. 110. Effect of RPM • Once a joint design, pin tool and alloy have been selected, there are three essential variables (rpm, ipm, and plunge depth) which must be considered • During processing, sliding (X), separation (Y) and forge (Z) forces are introduced into the part by the rotating pin tool and flowing metal
  111. 111. Effect of RPM • X - Sliding force
  112. 112. Spindle torque and RPM