Effect of Tunneling Defect in Friction Stir Welding of Al-Mg Alloys S. Balos, P. Janjatovic, D. Labus Zlatanovic, M. Dramicanin, D. Rajnovic, L. Sidjanin Faculty of Technical Sciences Novi Sad, Serbia

Friction stir welding – FSW? FSW is a solid state metal joining process that uses a specialized tool to join two work pieces. Main advantages: The ability to weld age hardened alloys Low distortion Excellent mechanical properties No porosity with optimized parameters Energy efficient Dissimilar materials can be successfully welded Ability to join materials without protective atmosphere gases and consumable materials Deliberate tunnel formation, called friction stir channeling (FSC) is a method proposed of complex closed channels from the lateral position. Friction stir processing (FSP) is a way of modifying the surface and subsurface layer through grain refinement and/or the introduction of ceramic particles to form metal-matrix composite (MMC) layer. Based on microstructural characterization, there are three distinct zones, stirred (nugget) zone, thermo-mechanically affected zone, and heat-affected zone Intense plastic deformation and frictional heating during FSW/FSP result in generation of a recrystallized fine-grained microstructure within stirred zone. Unique to the FSW process is the creation of a transition zone—thermo-mechanically affected zone between the parent material and the nugget zone. The TMAZ experiences both temperature and deformation during FSW Beyond the TMAZ there is a heat-affected zone. This zone experiences a thermal cycle

Friction stir welding - FSW The mechanical properties strongly depend on the microstructure of FSW welds, depending on: tool geometry welding parameters Two most critical regions characteristic for FSW process are: Nugget TMAZ (thermo - mechanically affected zone) The nugget and the surrounding region can represent a potentially sensitive zone for defects like tunneling defect. The mechanical properties strongly depend on the microstructure of FSW welds, which depend on both tool geometry and welding parameters. Two most critical regions for FSW process are the nugget and the thermomechanical zone Especially the region between the nugget and the TMAZ That means the nugget and the surrounding region can represent a potentially sensitive zone for defects like tunneling defect, that occurre if the material flow around the tool pin is not optimal The result of such anomaly is the irregular weld filling, which can be overcome by tool design and optimization of tool rotation and welding speed. However, there has been a considerable interest in a deliberate tunnel formation, called friction stir channeling (FSC). This is a solid – state technology proposed as a method of heat exchanger fabrication, that is, the fabrication of complex closed channels from the lateral position, perpendicular to the heat exchanger device.

Aim: To study the effect of various types of tunnel like formation in friction stir welding (FSW) with: different rotation and welding speeds to compare the obtained mechanical properties with: -oxy-fuel welded (OFW) -gas-metal arc welded (GMAW-MIG) -gas-tungsten arc welded (GTAW-TIG), OFW, MIG and TIG with consumable materials using CRMs: AA5356 (4.5-5.5 % Mg) and AA4043 (4.5-6 % Si)

Experiment: Base material: aluminium – magnesium alloy AA5052 plates, 3 and 8 mm thick (H32 and H38, respectively) dimensions: 300x65 mm   Cu Mn Mg Si Fe Zn Ti Al AA5052-H32 0.04 0.11 2.46 0.19 0.73 0.061 0.010 balance AA5052-H38 0.03 0.29 2.33 0.011 0.022 In this study, we used aluminium – magnesium alloy AA-5052-H32 plates (in ½ hard condition). Their chemical composition and mechanical properties are given in Tables 1 and 2 Sample dimensions were 300x65 mm, to form a welded joint of 300x130 mm.   Rp0.2% [MPa] Rm [MPa] A [%] HV5 AA5052-H32 157 214 19 67 AA5052-H38 204 245 12 90

2. Tool geometry 3 mm plates were welded with polygonal tool 8 mm plates were welded with threaded tool The reservoir volume was larger than the pin volume (ratio 1.1:1 and 5:1), to help the tunneling defect to occur The tool position was vertical, without tilt The plunge depth of tool shoulder was 0.2 mm In fig is shown Tool geometry . Tool shoulder is of the conventional concave type, however, the reservoir volume was larger than that of the pin. This ratio was 5:1, which enables the tunnel defect formation because it is larger than needed to store the piled-up material from the tool pin indentation. The tool position was vertical, without tilt The plunge depth of tool shoulder was 0.2 mm in all FSW welded specimens, to enable a constant pressure during all tests The control specimen was welded with oxy-fuel technique, by using 99 % Al consumable rod.

3. Welding parameters Sample Rotation speed [rev/min] Welding speed [mm/min] Rotation to welding speed [rev/mm] Tool rotation direction Material flow direction AA5052-H32; 3 mm thickness; polygonal tool pin 1 1230 17 72.35 right - 2 12 102.50 3 23 53.48 4 925 68 13.60 5 91 10.16 AA5052-H38; 8 mm thickness; threaded tool pin 6 645 129.00 left Up 7 Down 8 53.75 9 49 13.16 We determined mechanical properties of welded workpieces on specimens cut perpendicular to the weld. Also tensile testing, root bending and hardness testing were performed, accompanied by metallographic examination. OFW specimen was welded with 4043 alloy rod (4.5-6 % Si)

AA5052-H32; 3 mm thickness; AA5052-H38; 8 mm thickness; polygonal tool pin AA5052-H38; 8 mm thickness; threaded tool pin Specimen 6:Multiple-closed tunnel; RS 645 rev/min, WS 5 mm/min, Material flow - up Specimen 1:Double triangular-closed tunnel; RS 1230 rev/min, WS 17 mm/min Specimen 2:Single triangular-closed tunnel; RS 1230, WS 12 mm/min Specimen 7:Complex shaped-closed tunnel; RS 645 rev/min, WS 5 mm/min, Material flow - down Specimen 3:Triple triangular-closed tunnel; RS 1230, WS 23 mm/min Specimen 8:Complex shaped-closed tunnel; RS 645 rev/min, WS 12 mm/min, Material flow - down Specimen 4:Double triangular-closed tunnel; RS 925 rev/min, WS 68 mm/min Specimen 9:Double triangular-closed tunnel with crack; RS 645 rev/min, WS 49 mm/min, Material flow - down Specimen 5:Crack type-open tunnel; RS 925 rev/min, WS 91 mm/min Weld metal HAZ BM Specimen 10:Oxy-fuel welded specimen: -intensive porosity near the root

Tensile properties Sample Rp0.2% [MPa] Rm [MPa] Joint effectiveness Welding parameters RpFSW/ RpBM100 [%] RmFSW/ Rm BM100 [%] 1 118 137 75 64 1230 rpm; 17 mm/min 2 117 138 1230 rpm; 12 mm/min 3 120 56 1230 rpm; 23 mm/min 4 105 110 67 51 925 rpm; 68 mm/min 5 96 101 62 46 925 rpm; 91 mm/min 6 91 109 45 44 645 rpm; 5 mm/min; up 7 112 127 55 52 645 rpm; 5 mm/min; down 8 140 65 57 645 rpm; 12 mm/min; down 9 80 89 40 36 645 rpm; 49 mm/min; down 10 86 106 48 OFW, AA4043 alloy rod 11* - ~78 MIG, AA5356 wire 12* ~75 TIG, AA5356 alloy rod In Table we can see Tensile properties of welded specimens. We can see that the highest joint efficiencies were obtained with the high-end rotational speed. This refers to efficiency calculated taking into account proof strength, ultimate tensile strength and elongation, in relation to base material corresponding values. Specimen 1 and 2 proof and ultimate tensile strength effectivenesses are very similar, in spite of their different tunnel shape and size. It can be seen that the overall size of the tunnel in specimen 2 is larger, however, its surroun0ding area, particularly towards the other side of the specimen contains inhomogenities and cracks. Therefore, specimen 2 effectiveness calculated by elongation is the highest, although roughly half the effectiveness calculated by proof and ultimate tensile strength. The effectiveness figures obtained for specimens 3 – 5 are significantly lower. The advantage of specimens with increased tool rotational speed and a lower welding speed (specimen 2 and partially specimen 1) is most probably due to a more effective friction heating. The major influence on weld filling comes from tool geometry, as well as rotation and welding speeds. Both higher rotation and welding speeds influence the increase in the frequency of impulses generated from the tool square pin at a given weld length. That means, a more effective weld filling is obtained, in spite of a relatively large reservoir in the tool shoulder. Oxy-fuel welded specimen exhibited a considerably lower proof and ultimate tensile strength effectiveness even compared to the lowest performing specimens 4 and 5. However, their elongation is on the level of specimens 1 and 2. The main reason is the absence of tunnel-like formations and cracks. **V. Singh and V. Paroothi, Study of Microstructure and Mechanical Properties of Aluminium Alloy Welded by MIG and TIG welding Processes, Rama University Journal, 2007, 85-93

Key to the success of FSW? WM TMAZ HAZ Nugget here in Fig. 1 TMAZ and the fine grained nugget zone are shown. Such microstructure morphology closely corresponds to the microstructure in specimens without tunnel-like defect (arrow showing the nugget) In Fig. 2, HAZ and weld metal microstructure obtained by oxy-fuel welding. It can be seen that a coarsened HAZ and a typical cast – like microstructure of the weld metal are obtained. TMAZ/nugget in FSW welded specimen, the arrow indicated the nugget Oxy-fuel welded specimen HAZ/weld metal zone with a typical dendrite microstructure

Summary The results can be summarized as follows: FSW welds with crack – type tunnel defects exhibit the lowest tensile properties, similar to OSW welded specimens welded with consumables containing CRMs. FSW welds with triangular or polygonal tunnels have the highest efficiencies, similar to MIG and TIG welded specimens welded with consumables containing CRMs. . Welds with tunneling defect can be effectively applied only in static loading conditions.

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