Residual stresses distribution in TIG weldments of AA 6061 T6 Aluminium alloy

Transcription

1 Residual stresses distribution in TIG weldments of AA 6061 T6 Aluminium alloy 1 Hemadri Naidu.T, 2 Chennakeshavalu K 1 Research Scholar, KSIT, VTU, Bangalore, Karnataka, , India. 2 Principal, EWIT,VTU, Bangalore, Karnataka, , India. ABSTRACT : AA 6061-T6 Aluminum alloy is medium high strength heat treatable wrought structural material used for general purpose to aerospace applications. Its good corrosion resistance coupled with excellent workability made this alloy work horse alloy among Aluminium alloys used for structural applications. AA 6061 possess good weldability with ER 4043 filler wire even though it has post anodisiation color mismatching. AA6061 weldments invariably fail in heat affected zone (HAZ) due to dissolution of metastable Mg 2 Si strengthening phases due to weld thermal energy. The process of dissolution of metastable Mg 2 S phases can be retarded by good thermal management. In practice, welding being a localized thermal process, residual stresses will be induced invariably into the integrated structure and their manifestation/distribution will be altered by the usage of weld process variants. In this research paper residual stresses are evaluated using X-ray diffraction method and sin 2 Ψ method is used in measuring the strains in TIG joints of AA 6061-T6 across and along in weldments both on crown and root sides, HAZ and unaffected base material. This paper delineates the residual stresses generated by juxtaposing the weld bead width and GTAW process variants of AC, un-pulsed DCSP and current pulsed DCSP. The residual stresses generated due to weld preparation like chemical cleaning and mechanical scraping are also addressed. As weld bead width increases it found inbuilt stresses increase. The variation in stress pattern on root side with welding process is highlighted. By adopting low heat input pulsed DCSP GTAW process the thermal stresses in the weldments reduced from +88 MPa to +65 MPa and an improvement of 26.13%. The reduction in heat input minimized the thermal stresses in the fusion zone and influence in Heat affected zone is relatively less. Keywords: Residual stresses, post anodisiation, heat affected zone, TIG welding, heat input, heat treatment, tensile testing, scattering, thermal shrinkage, plastic deformation. I. INTRODUCTION AA 6061-T6 Aluminum alloy is medium high strength heat treatable wrought structural material used for general purpose to aerospace applications. Its good corrosion resistance coupled with excellent workability made this alloy work horse alloy among Aluminium alloys used for structural applications. However, any fusion welding process induces the residual stresses in the structure. Materials or processes which might contribute to deterioration of hardware in service shall receive special consideration. NASA report [1] on general section on special material addressed the residual stresses. It states that deterioration processes which impair the life expectancy of parts include galvanic corrosion, stress corrosion etc. Precautionary measures to prevent deterioration shall include consideration of such controls as limitation of operating stresses, relief of residual stresses, application of protection coatings, and use of special heat treatment. Residual stresses in structures occur for many reasons during various stages of manufacturing and heat treatment stages, including rolling, forging, grinding, machining. Residual stresses play major role in performance of the welded structure. Residual stresses generated due to non-uniform heating during welding are often called thermal stresses [2]. One of the troublesome problems that accompany the construction of welded structures is residual stresses and distortion. A weldment is locally heated, due to non uniform temperature distribution. This non-uniform temperature distribution causes thermal stresses and resulting residual stresses. The subject of weld stresses, cracking, fatigue cracking, stress corrosion cracking and weld defects are so interrelated that it is almost impossible to treat them independently. Residual stresses and related distortions in welded structures have studied since 1930s. However, most of the studies are mainly focused on ferrous materials [3], of course due to their wider and higher tonnage of usage. Studies on aluminum alloys are relative less and more concentrated on the alloys that are prone for stress corrosion cracking. However considering wider usage of AA 6061 Aluminum alloy ranging from water bottles on war field to water tanks of the space vehicles [4], the study on thermal stresses gives better understanding and insight to welding process and for corrosion behavior. In the present investigation the residual stresses generated in AA 6061-T6 Aluminum alloys welded using various processes and parameters are studied. The effect of weld bead width and pre-weld preparations on induced thermal stresses on the structure is elaborated. AA 6061 aluminum has the balanced amount of Mg and Si to form quasi-binary Al-Mg 2 Si with Mg to Si 1

2 ratio of 1.73:1. AA 6061 is strictly binary Al-Mg 2 Si alloy with 1.4% Mg 2 Si. It contains minor addition of 0.3% copper to improve strength. Even though AA 6061 can t be welded autogenously, it possess good weldability with ER 4043 filler wire. Where color matching is important normally AA 5356 are preferred. For some applications 4047 and 4643 are used among 4000 series and ER 5183, 5554, 5556 and 5654 are employed among 5000 series [5]. Section 2 follows basic experimental setup. Section 3 explains post weld inspection. Section 4 explains about measurements of residual stresses and section 5 follows the results and discussions. II. EXPERIMENTAL SETUP In present work, 3.2 mm thick 300*400 mm length coupons of chemical milled quality AA 6061 alloy in T6 temper condition imported from M/S ALCAN is welded using mechanized GTAW process employing ER 4043 filler wire of 1.2 mm diameter in the form of pools of 7.0 kg. The chemical composition of AA 6061 and ER 4043 are given in table 1. Table 1. Chemical composition ( in wt%) of AA T6 and filler wire Materials Si Mg Cu Fe Mn Zn Ti Cr Al AA Balance ER Balance Welding Process: with 60% back ground current. No pulsation is used for filler addition. To obtain various bead widths the AVC is varied for AC welds only. Stainless steel back up is employed with standard weld groove and aluminum support at bottom. The details of the weld parameters employed are shown in table 2. The weld root is not grounded for residual stresses; however for tensile testing both crown and root are milled to parent metal thickness to meet the standards weld parameters for AC and DCSP welds with filler wire 4043 are shown in table Table 2. Weld parameters for AC and DCSP welds with filler wire The gas tungsten arc welding (GTAW) process is selected and used in present study due to its inherent advantages and process flexibility. GTAW process is known for its sound welds in aluminum alloys. M/S Hobart Brothers make 250 A automatic welding machine is used. Welding is performed using both balanced wave AC and DCSP cycles. DCSP welds are carried out using with and without current pulsation. The current pulsation of 10ms on and off period each is used Process Current Voltage AVC Weld speed Pulse cycle Heat input Remarks A V V mm/min (ms) J/mm AC GTAW Filler feed rate is 2200mm/ min DCSP GTAW Filler feed rate is 2200mm/ min PULSED DCSP GTAW On 10-Off 325 Filler feed rate is 2400mm/ min 60% back ground current Closed square groove joint configuration is adopted with intentional chamfering of the sharp edges. The sharping of the edges is done to avoid lack of side wall fusion and pseudo-indications during NDT. The 10 mm on either side of the coupons are mechanically scraped in unidirectional using scraping tool. The scraped region is inspected under UV light to find out any foreign material. The scraped region is covered with aluminum foil till the time of welding. Welding is carried out within four hours of scraping. Post Weld Inspection The weld bead geometry is measured in using vernier and dial gauge and reported in table 3. The weld root is not grounded for residual stresses measurement; however for tensile testing both crown and root are milled to parent metal thickness to meet the standards. After welding the coupons are Die Penetration Tested (DPT) using MANA PLUX and X-ray radiography Kraft Cramer machine. Minor porosities are observed at isolated locations. The porosity locations are identified with permanent marker pen, not by mechanical means to avoid any undue stresses. The spots for residual stresses measurements are marked and ensured that no weld defects or any dents on base material. If any, in the vicinity of the spot, at least 2 times the thickness of the coupon i.e 6.4 mm. Weld bead geometry for AC welds with different AVC s are shown in table 3. III. RESIDUAL STRESSES MEASUREMENT Residual stresses, also referred to as internal stresses, inherent stresses, and locked in stresses, can be measured using many techniques. In the present work X- ray diffraction technique is adopted. The details of the equipment is shown in figure 1. Cr based target is used and sin 2 Ψ method is used in measuring the strains. Being the small depth of X-ray penetration, the sampled region can be assumed to be plane stress. For plane 2

3 strain the slope of the measured lattice spacing with specimen tilt θ can be related to in-plane stress. Table 3. Weld Bead geometry for AC welds with different AVCs. Process Crown (mm) Root (mm) AVC Remarks Width Height Width Height V AC GTAW AVC is direct way of controlling arc length. It is automated voltage not arc voltage Figure 1. Residual Stress Measuring Instrument. Residual stresses measurements are carried out at CPRI (Central Power Research Institute), Bangalore. Residual stresses are measured on three different locations of the weldments-center of the weld, Heat Affected Zone (HAZ) and base metal. For consistence purpose HAZ spot is fixed at 6.0 mm from the fusion line on crown side. The reported values are average of the three values measured in same spot. The residual stresses are measured both in longitudinal and transverse directions. The mechanical grinding/polishing are avoided as this may induce compressive stresses. For this reason the material is well cleaned prior to welding. On the weld due to the ripples present, polishing is inadvertently required to facilitate accurate readings during X-ray diffraction. The weld surface is electro-chemically polished using ice cold methanol such that no additional stresses are induced. In order to avoid scattering of X- rays due to uneven surface the electro chemical polishing is carried out recursively at times more than five times till consistence values are obtained. This is more in AC welds due to ripples compared to DCSP welds. Stresses are measured both on the crown side and root side of the weldments and the location of measurement is near center of the weld. The measured values on welded coupons are reported in table 4. Residual stresses are also measured on parent metal in as received condition, after chemical cleaning and scrapping operations to have base values for analysis. Residual stresses for AC and DCSP welds in different zones are shown in table 5. The measured values on base material prior to welding are reported in table 6. Table 4. Residual stresses for AC and DCSP welds in crown and root. Process Center of the Weld Center of the Weld Remarks on crown (MPa) on root (MPa) AC GTAW Average of 5 readings DCSP GTAW Average of 5 readings PULSED DCSP GTAW Average of 5 readings Table 5. Residual stresses for AC and DCSP welds in different zone. Process Center of the Weld on HAZ ( Base metal ( Remarks crown (MPa) MPa) MPa) AC GTAW Average of 5 readings DCSP GTAW Average of 5 readings PULSED DCSP Average of 5 readings GTAW Table 6. Residual stresses on base material AA 6061-T6. Base material AA On the Remarks 6061-T6 surface (MPa) As received -25 Average of 3 condition readings After chemical cleaning -10 Average of 3 readings After scraping -30 Average of 3 readings 3

4 IV. RESULTS AND DISCUSSIONS All welds are X-ray radiographically inspected and sound weldments only selected for residual stress measurement. Even minor porosity areas are avoided such that the interaction of defects with residual stresses is not well established. Majority of the measurements are done by carrying out the measurement on crown side. For comparison purpose a few measurements are done on root side. The location of the measurement on HAZ is taken 6.0 mm away from fusion line for all welds to maintain uniformity. Residual stresses are stresses that remain in a body after all external stresses are removed. Residual stresses in a component are those which need not maintain equilibrium between the component and surrounding environment. They prevail in the absence of the external loads. The mechanism that results in residual stresses in the welding process starts with the deposition of molten weld metal which heats the immediately adjacent material. After the solidification of weld material, normal thermal shrinkage is resisted by the adjacent, cooler material. The development of residual stresses can be explained by considering heating and cooling under constraint [3], wherein the expansion and contraction of the weld metal and near base metal are restrained by the far way rigid base material. Hence, residual stresses can be minimized by minimizing the expansion and contraction. This can be achieved by adopting high heat density processes like electron beam welding (EBW), DCSP, GTAW or pulsed GTAW process. Residual stresses are always global in nature and within the elastic limit of the material. Once locked in stress crosses the yield point the material yields. This concept is exploited in relieving the residual stresses by mechanical or thermal means [9]. Generation of residual stresses in weldments can be reduced by imposing opposite sign stresses just prior to welding near weld interface. The normally adopted practices are flaring up the joint or shrink fit the joint. Residual stresses in weldments after welding can be reduced by production of plastic deformation in proper amounts and distribution [5] after welding. The shot peening or malleting immediately after welding is invariably done in production practice for ferrous material. However it is not very effective for ductile materials like aluminum as imparting controlled deformation difficult. Any improper peening of weldments results in, in contrary, additional localized stresses. The method followed for Aluminium alloy welding in reducing the residual stresses is predominantly pre-weld flaring up or shrinks fit, rather following post weld operations. The post weld thermal treatment can t be adopted for Aluminium alloys as the temperature required for effective nullification of stresses is above the ageing temperature. Hence this results in over ageing of the base material, thereby reduces the resultant properties. In supper saturated alloys like AA 2219, this may result in selective precipitation and over ageing at fusion line and may result in drop in toughness [6]. Hence no post weld stress relieving followed in present investigation. The distribution of longitudinal residual stresses,σ x, can be approximated by the equation σ x (y) = σ m {1- y 2 /b 2 } Exp {-1/2 y 2 /b 2 } Equation. (1) Where σ m is the maximum residual stress and b is the width of the tension zone, normally equal to weld bead width. Normally transverse residual stresses σ y, are of relatively low intensity is produced in the middle part of the weld, where thermal contraction in the transverse direction is restrained by the cooler base material at the ends of the weld. If the lateral contraction of the weld is restrained by clamping and fixtures, further tensile stresses are complemented to the transverse stresses as the reaction stress. However, this external constraint has insignificant effect on longitudinal residual stresses σ x. from the Eq. 1, by reducing the weld width the residual stresses can be minimized. The weld bead width is depending upon the weld process and parameters. By adopting the high heat density electron beam welding (EBW), the weld bead width can reduced. However AA 6061 is prone for solidification cracking when welded autogenously, without filler addition. To alleviate hot cracking in weldments of AA 6061, Si rich ER 4043 is widely used. The variant of GTAW DCSP welding process results in narrower welds compared to AC process due to high heat density and narrower arc cone radius. The heat input can be further reduced by adopting pulsed DCSP welding [10]. As evident from table 2, the heat input is reduced from 585 J/ mm to 388 J/mm by adopting AC welding to DCSP welding process. It is further reduced to 325 J/mm by employing current pulsation with 10ms on and off periods each and 60% back ground current. The peak weld center tensile residual stresses are reduced from +80 MPa to +65MPa. This demonstrates the influence of the weld heat in put on the generation of residual stresses. The bead crown width is reduced from 8.0mm for AC welds to 6.8 mm for non-pulsed DCSP welds. By adopting pulsation the width is reduced marginally, however the reduction in heat input is considerable. For given process there will be limitation in reducing the bead width. The bead width is equally dictated by the surface tension and other buoyancy forces operated during welding. Surface tension is altered by the shielding gas used. In AC GTAW process Argon gas is used whereas Helium gas is used for DCSP GTAW process. Helium gas has influence in surface tension of the molten metal [2]. Helium gas can t be used for AC welding due to high ionization potential and hence arc instability during reverse cycle. The reduction in heat input reduces the temperature gradient and this aids in reducing the residual stresses. The reduced weld heat input resulted in lower Heat Affected Zone (HAZ). The reduction in the residual stresses in HAZ is very marginal as evident from table 5. 4

5 The effect of bead width on residual stresses is studied on AC GTAW process only. It is easier to vary the arc length in AC welding compared to DCSP GTAW process. By increasing the arc length while keeping other parameters constant, there by heat input, the weld bead varied. Using the servomotor controlled Automated Voltage Control (AVC) the arc length is varied. Even though the heat input assumed to be constant by keeping the variable parameters constant, the radiation losses will be more. The various bead widths obtained are 8.0, 8.8, 9.3, 10.6 and 11.2 mm as depicted in table 7. Beyond 11.2 mm bead width lack of penetration is observed by increasing AVC. Even for 11.2mm bead width very narrow under bead observed, however X-ray radiography not revealed any lack of penetration. The variation of residual stresses with bead width is given table 3. The low average tensile residual stress of +80 MPa observed for 8.0 mm bead width against +98 MPa observed for 11.2 mm bead width. More scattering of values seen for 11.2 mm bead compared to lower bead widths. Table 7. Residual stresses on different weld bead widths of AC weld. Crown width (mm) Residual stresses (MPa) Remarks Average of 3 readings Average of 3 readings Average of 3 readings Average of 3 readings Average of 3 reading In all weldments higher stresses are observed on the crown side compared to root side, (table 4). In AC welds the difference is substantial. The variation is found to be 10MPa. However in DCSP welds in some locations even compressive nature stresses are observed on root side. This indicates that as the crown width to root width ratio increased the stress intensity and nature changes. Residual stresses are measured on the as received base material prior to welding and found to be compressive in nature in the order of -25 MPa. However prior to welding the coupons are treated as explained in section 3.0 to remove the surface contaminants and oxide layer. After surface treatment the stresses are reduced to -10 MPa. Prior to welding the areas adjust to weld line is scraped unidirectional to remove the tenacious oxide layer. Surface stresses are measured in the scraped area and found be compressive and is in the order of -30 MPa. These values are to be taken for comparative consideration only not as absolute values since accuracy in X-ray diffraction is ± 20 MPa [7]. It is to be noted that diffraction is inherently selective and limited by nonlinearity in sin 2 Ψor surface condition [8]. For some samples the peening/malleting operation is carried out immediately after welding. The peening operation induced compressive residual stresses in the weldments and as high as 110 MPa. It is interesting to note that there is no noticeable change in the HAZ region. Hence peening might alter only sub surface levels and plastic deformation not taken place across the thickness as AA 6061 being ductile material. As explained earlier, residual stresses bring elastic in nature, by imparting plastic deformation residual stresses can be relieved. Total sum of the stresses being zero the presence of compressive stresses indicates equal amount of total tensile component over an area. So more than the nature of stresses the distribution of residual stresses over the area is important. The reduction in heat input during welding reduced the residual stresses as evident in table 5. In all the cases the nature of distribution is same with tensile stresses in weld metal and compressive stresses in HAZ region. Reduced heat input reduced the stresses in general and has more effect on the root side. V. CONCLUSIONS The following observations were made during this research work as follows: 1. Tensile residual stresses are observed in the weldments with high peak stress in AC weldments compared to DCSP GTAW weldments. 2. Surface stresses are considerably changed while preparing surfaces for welding. 3. Crown side of weld show higher stresses compared to root side (Pulsed DCSP +65 MPa to +15 MPa). As the bead width to root width is reduced the stresses on root side are reduced. 4. Bead width has significant effect on residual stresses. Bead width can be lowered to reduce the residual stresses by adopting high heat density process. 5. Reduced weld heat input lowers the peak residual stresses and the nature and distribution are same. 6. DCSP current pulsed welding gives lower residual stresses compared to other processes by 15%. REFERENCES [1] NASA report no SE-R-0006 rev C General specification requirements for materials and process. [2] Welding fundamentals and processes, Volume 6A ASM International hand book, ISBN: , Oct 31, [3] Sindo Kou, Welding Metallurgy, Second Edition, Wiley- Inter-Science, A John Wiley & sons Inc Publication, ISBN , Oct [4] Svetsaren, the esab welding and cutting journal, vol. 59, no.1, 2004, [5] No Properties and Selection Non ferrous alloys and special purpose materials, Hand book Volume 2 ASM Hand book Volume 2, ISBN: ,Pub: Oct-1,

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