Effect of base metal anisotropy on the spot weld characteristics of Interstitial Free (IF) steels

Transcription

1 Effect of base metal anisotropy on the spot weld characteristics of Interstitial Free (IF) steels K.Hariharan, and G.Balachandran* Advanced Engineering, Ashok Leyland Ltd., Chennai , India. *- Presently with Kalyani Carpenter Special Steels Ltd, Pune , India Abstract An isotropic weld nugget is generally assumed in modeling spot weld behaviour. The present periments were carried out on specimens along and the influence of the base metal anisotropy on the microstructure and hardness profile was established. Lap-s was carried out to establish the influence of anisotropy in the weld failure strength. It has been inferred that the base metal anisotropy has considerable influence on the width and hardness of heat affected zone (HAZ). The base metal anisotropy correlates with the experimental data. Key words: Spot welding, anisotropy, heat affected zone, yield criterion 1

2 Introduction, where the material resistance to current flow offers the required heat input for welding. Research has been carried out to understand the influence of weld parameters, viz current density (Ozyurek, 2008), electrode force (Tang et al., 2002 Aslanlar et al., 2008 can be related to the weld parameters (Kim and Eagar, 1988 microstructure and material (Marashi et al., 2008). Advanced high strength steels (AHSS) are (Jiang et al., 2007). Several work have been carried out to study the weld behaviour of steels like dual phase (DP) steels (Hayat et al., 2007) and TRIP) steels (Kim et al., (2005)), for steels. Marya and Gayden, (2005) studied the influence of different weld parameters on the weld fracture behaviour of DP steels. They observed that the weld microstructure and fracture Ma et al., (2008) correlated the weld DP steels and observed In metals (Hasanbasoglu and Kacar, 2007) and different thicknesses. Sun et al., (2004) joined aluminium and steel using a cold- Joining dissimilar metals is challenging owing to the difference in the chemical and mechanical (Vural et al., 2006). Marashi et al., (2008) noted that the spot weld joining of dissimilar spot welds results in asymmetric nugget hardness and the hardness at 2

3 between the two base metals. Mode assembly behaviour. Kim and Eagar, (1988) studied the influence of weld parameters on the weld lobe shape using a heat balance model. Wei and Ho, (1990) three dimensional nugget growth during a spot weld process using an unsteady axisymmetric heat Khan et al., (1999) used a coupled thermal-electrical-mechanical analysis to study the spot weld process, where in parameters; also studied. Feulvarch et al., (2004) proposed a general electro thermal model for spot weld between the contact surfaces as Nodeh et al., (2008) used an electro thermo mechanical modelling of spot weld process and the predicted residual stresses were in agreement with the measured residual stress. Chao, (2003) expression for failure load. Mukhopadhyaya et al., (2009) studied the spot weld strength using the Chao, (2003) model. They suggested that stress at failure is independent of weld input parameters and hence failure stress can be used to assess the spot weld strength instead of peak load at failure. Most of the above research assume axisymmetrical nugget for modeling (Lin et al., 2003). 3

4 used for subsequent annealing induces a (Pero-Sanz et al., 1999).. In a such as floor panel, roof, pillars etc. are joined of design for failure. The weld nugget for stresses. These thermal stresses are opposed by base metal surrounding the HAZ zone. The offered by base metal against the thermal load is expected to exhibit anisotropy a In the present work, the influence of anisotropy of low carbon steel on the spot weld Experimental work The present study is carried out performed in 1.2 mm thick commercially available (from material is shown in Table 1. Table 1 Wt (%) IF C Mn S P Al Si Others Nb Ti-0.033

5 V Tensile testing of base metal per the along rolling (0o), diagonal (45o) and perpendicular (90o constant cross head speed of 0.5mm/min. Extensometer of guage length 25mm was used for precise measurement of the yield point (Banabic, (2010)) rried out as per calculated using the formula, R w ln w f wo t ln l o wo l f w f Where w and t refers to the width and thickness strain; wf and wo refers to the final and lf and lo The mec imated from an average of three samples. Microstructure machine make: Zwick/Roell) at a load of

6 grams. e fusion zone towards base metal. Lap shear tests of spot welds in a servo- -head speed of 5mm/min. The load- Results and discussion Typical stress1 Figure - in IF steel. Figure 1: Stress- 6

7 The mechanical along the three given in Table 2. Table 2 Orientation (angle) YS (MPa) UTS(MPa) Elongation(%) n R Microstructure and hardness distribution The macrostructure of the weld was influenced by anisotropy. The nugget width was almost constant, however the heat affected zone width exhibited considerable difference with Figure 2). The varia ly related to the Figure 2 The microstructure (Figure 3) con Bhadeshia and Honeycombe, (2006)) along with allotriomorphic ferrite. The fusion zone (Figure 4) is contact with the electrode exhibited equiaxed grains ( Figure 5) due to rapid cooling at the electrode- sheet interface. Heat affected zone (HAZ) Bayraktar et al., 7

8 (2007) growth to a higher value of local thermal gradient when compared to the displacement rate of observed in the HAZ zone could be due to higher amplitude of thermal gradient, which increases from the fusion zone towards base metal. Appreciable difference in microstructure was not Figure 3 playing fusion zone, HAZ and base metal 8

9 Figure 4: Fusion zone displaying allotriomorphic and W 9

10 Coarse equiaxed grains near electrode- base metal interface Figure 5: Equiaxed grains at electrode- base metal interface, below fusion zone The Figure 6. The different regions along which the hardness measurement were taken is shown in Figure 6 (b). Owing to the symmetry in microstructure of characterized. base metal to fusion zone. As discussed due to a high heat input. At a removes the texture present in base metal. However, it is observed that the hardness profile exhibit appreciable difference due to base metal anisotropy as it moves away from the fusion zone. The difference increases with distance from fusion zone with maximum difference near vely lower 10

11 This temperature induces thermal stresses on the surrounding material. The resistance offered by the surrounding material to the thermal stress depends on its strength. This is indicated by Figure 2) dimension where the HAZ width is maximum along ersa along a constant thermal load, the resistance to the thermal stress offered by indicator of the resistance to thermal stress. It is observed that the hardness of HAZ zone is This is in agreement w Table 2) with minimum yield strength deg deg deg (a)

12 (b) Figure 6: (a) Spot weld strength from lap shear tests The peak load to failure of spot weld is determined from lap shear tests. All the samples failed by nugget pull-out mode and failure occurred near HAZ zone. Anisotropy of sheet metals had a Figure 7. The peak load of failure P eak load of failure Figure

13 Mukhopadhyaya et al., (2009) proposed to use the stress at failure rather than peak load to failure, as unlike load, failure stress is independent of weld processing and material parameters. Chao, (2003) be used to determine the failure stress of spot weld from the nugget dimensions and peak load of failure. Accordingly, the failure stress of spot weld is given as 4 Pf f where dt f - stress at failure, Pf -peak load to failure, 'd' - nugget diameter and 't' thickness yield criterion. Several anisotropic yield criteria have been proposed in the past. Banabic, (2010) has reviewed in detail the developments of phenomenological anisotropic yield criteria. In the uniaxial stress using Hill48 criterion (Hill, (1948)) is given as k( ) 2 N cos 2 sin 2 x where N 1 R0 R0 (cos 2 sin 2 ) cos 4 ( ) R S and S 2 1/ 2 S sin 4 R0 R90 (Figure 8) the choice of yield criterion (Hariharan et al., (2010)). Appropriate choice of anisotropic yield criterio 13

14 Figure 8 The influence of anisotropy in spot weld is not considered in most of the structural analysis such as the used in the sheet metal assembly design. Similarly the Conclusion Based on the present study, following conclusions are made 1. The base metal anisotropy of the sheets being welded influences the spot weld characteristics. 2. The heat affected zone width is influenced by the anisotropy of base metal and is hardness profile of HAZ close to base metal exhibited considerable difference 14

15 between of sheet metal, though the nugget hardness is not influenced 3. monotonic yield strength. Hill48 criterion was ab of weld 4. spot welds. Acknowledgment Authors would like to thank Ms.Kalaivani, Advanced Engg, Ashok Leyland, Mr.Pari of CPPS, Ashok Leyland and Mr.P.Balasubramoni, student, IIT Madras for their support in 15.

16 References stance spot welding. Materials and Design,29, Banabic, D., 2010.Sheet metal forming processes,springer-verlag. Bayraktar, E.,Kaplan, D.,Devillers, L.and Chevalier, J.P., 2007.Grain growth mechanism during teels. Journal of Materials Processing Technology,189, Bhadeshia, H.K.D.H.and Honeycombe, R.W.K., 2006.Steels Microstructure and Subjected to Tensile, Shear,or Combined Tensile/Shear Loads. Journal of Engineering Materials and Technology,125, Journal of Materials Processing Technology, , Journal of Material Forming,3, Hasanbasoglu, A.and Kacar, R., 2007.Resistance spot weldability of dissimilar materials(aisi 316L DIN EN steels). Materials and Design,28, Hayat, F.,Demir, B.and Acarer, M., 2007.Tensile shear stress and microstructure of lowcarbon dual-phase Mn- Metal Science and Heat Treatment,49,

17 Proceedings of the Royal Society of Lo,193, Advanced High Strength Steels for Automobiles. Journal of Iron and Steel Research,19,1-6. Khan, J.A. spot welding using coupled thermal-electric-mechanical model. Science and Technology in Welding and Joining,4, Kim, E.W.and Eagar, T.W.. Parametric analysis of resistance spot welding lobe curve, SAE welding of TRIP steel with response surface methodology. Inter Research,43, Lin, S.H.,Pan, J.,Tyan, T.and Prasad, P., 2003.A general failure criterion for spot welds under,40, Ma, C.,Chen, D.L.,Bhole, S.D.,Boudreau, G.,Lee, A.and Biro, E., 2008.Microstructure and -welded DP600 steel. Materials Science and Engineering A,485, Marashi, P.,Pouranvari, M.,Amirabdollahian, S.,Abedi, A.and Goodarzi, M., 2008.Microstructure and failure behavior of dissimilar resistance spot welds between low Materials Science and Engineering A,480, Marashi, P.,Pouranvari, M.,Amirabdollahian, S.,Abedi, A.and Goodarzi, M., 2008.Microstructure and failure behavior of dissimilar resistance spot welds between low 17

18 Materials Science and Engineering A,480, Marya, M.and Gayden, X.Q., 2005.Development of Requirements for Resistance Spot Welding Dual-Phase (DP600) Steels Part 1 The Causes of Interfacial Fracture. Welding Journal,84,172s-182s. Journal of materials processing technology,209, resistance spot welding, FE modeling and X- Journal of Materials Processing Technology,205, Ozyurek, D., 2008.An effect of weld current and weld atmosphere on the resistance spot Materials and Design,29, Pero-Sanz, J.,Ruiz- Textures for -Carbon and Extra Low- MATERIALS CHARACTERIZATION,43, Sun, X.,Stephens, E.V.,Khaleel, M.A.,Shao, H.and Kimchi, M., 2004.Resistance Spot Welding - From Process to Performance - Part I: Experimental Study. Welding Journal,83,188s-195s. Tang, H.,Hou, W.and Hu, S.J., 2002.Forging force in resistance spot welding. Proc Instn Mech Engrs Part B: J Engineering Manufacture,216, strength of the resistance spot welded joints of different steel sheets. Journal of Materials Processing Technology, 176,

19 Wei, P.S.and Ho, C.Y., 1990.Axisymmetric nugget growth during resistance spot welding. ASME Journal of Heat Transfer,112, List of Figures Figure 1: Stress-strain curve of base metal al ying fusion zone, HAZ and base metal Figure 5: Equiaxed grains at electrode- base metal interface, below fusion zone...10 Z List of tables

20 Stress-strain curve of base metal along different orientations

21

22

23 Fusion zone displaying allotriomorphic and Widmanstatten ferrite

24 Coarse equiaxed grains near electrode- base metal interface Equiaxed grains at electrode- base metal interface, below fusion zone

25 deg deg deg (a) (b) (a) in HAZ

26 P eak load of failure O rie nta tion (de g re e s) Variation of peak load of failure with orientation 90

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28 Wt (%) IF C Mn S P Al Si Others Nb Ti V-0.028

29 Mechanical properties of base metal Orientation (angle) YS (MPa) UTS(MPa) Elongation(%) n R