Theoretical Formulation
4.2 Micro plasma arc welding
4.2.2 MPAW of steels
Similar procedure is adopted to weld 500 μm of SS304 and mild steel individually. First bead on plate was carried out to find the optimized parameter and the butt weld is carried out.
The welding parameters are chosen in three different speeds (2.75 mm/s, 4.2 mm/s, 5.26 mm/s) and current ranges of 7 - 20A with optimum combination of other fixed parameters (Table 4.5) similar to the one used with Ti6Al4V alloy. The fixture used here is different from the one used
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in Ti6Al4V alloy welding. Also, no purging gas is used in the process. The schematic diagram of experimental set up is shown in Fig. 4.24. Fig. 4.25 shows the process map of the feasible range of most significant parameters i.e. welding speed and current to get a successful weld joint for all the materials. The suitable combination of welding current and speed is required to produce stable and defect-free weld. When the welding speed is increased, the proper welding current range shifts towards higher side. Only heating of the material takes place at low heat input while beyond a certain limit, burn out of the material occurs. Thus heat input can be considered to be one of the most important parameters in determining feasible range of welding condition. By comparison of the weld zone it can be concluded that the feasible parameter range of the mild steel is more i.e. for similar condition, more current can be employed to obtain a successful joint.
This is due to higher conductivity of mild steel which causes the deposited heat to diffused away more rapidly. On the other side, the welding current is usually lower as compared to the welding of low carbon steel since the melting point of SS304 and electrical resistivity is less than low carbon steel. The thermal and mechanical properties of these two materials at room temperature are listed in Table 4.6.
Fig. 4.24 Schematic diagram of experimental set up.
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Table 4.5 MPAW process parameters used for two materials.
Welding Parameters Values Welding current (A) 10 - 13A Welding speed (mm/s) 2.75, 4.2, 6.67 Copper Nozzle diameter (mm) 1.2
Electrode diameter (mm) 1.2 Plasma gas flow rate (lpm) 6 Nozzle to plate distance (mm) 2 Shielding gas flow rate (lpm) 0.4
Pre Flow (s) 4
Post Flow (s) 4
Torch Position Vertical
Table 4.6 Constant material properties at ambient temperature [Mills, 2002].
Properties Stainless steel Low carbon steel
Density (Kg/m3) 8000 7850
Thermal conductivity (W/mK) 16.2 43
Expansion coefficient (µm/mK) 17.2 12
Specific Heat (J/kgK) 530 510.78
Latent heat of fusion (kJ/kg) 260 270
Diffusivity (µm2/s) 4.2 11.72
Meltingtemperature (K) 1723 1773
Solidus temperature (K) 1673 1723
Ultimate tensile strength (MPa) 754.7 330
Yield strength (MPa) 256 245
Young's Modulus (GPa) 200 200
Ductility(%) 60 36
Hardness (VHN) 176 108
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Fig. 4.25 Influence of welding speed and arc current: (a) SS304; (b) Low carbon Steel.
Figure 4.26(a) - (b) shows the weld bead produced for a full plate welding by bead on welding and butt welding respectively corresponding to a welding condition of 12 A and 4.2 mm/s. It is observed that the weld bead decreases slightly during butt welding of two sheets with similar process parameters. This is due to presence of slight gap between two sheets for actual butt joint as compared to bead-on-plate welding which may lead to the escape of heat. Figure 4.27 shows the weld macrographs obtained by butt welding of SS304 corresponding to certain welding conditions as summarized in Table 4.7. It is seen that the weld dimension increases with heat input.
Fig. 4.26 Weld bead corresponding to a welding condition of 12 A and 4.2 mm/s (a) bead on plate; (b) butt welding.
It is well known fact that the welded microstructure depends on the heat-input in case of 304 stainless steel. Figure 4.28 shows the micrograph of fusion zone, HAZ and base metal for two different welding conditions. The parent material, HAZ and FZ could be discriminated easily. Dendrites can be observed extending from the fusion boundary to the weld centreline (Fig.
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4.28b). Figure 4.29 shows the optical micrograph of fusion zones at three different heat inputs.
The low heat input can be attributed to smaller dendrite sizes and lesser inter- dendritic spacing in the fusion zone. Since the cooling rate was moderate, δ-ferrite with skeletal morphology was formed in solidified weld metal for the condition of W2 (Fig. 4.29b). It is observed from Fig.
4.29 that a higher heat input renders the dendrite structure coarser due to a decrease in the cooling rate.
Fig. 4.27 Weld macrographs corresponding to the welding conditions of Table 4.7.
Table 4.7 Process parameters for SS304 butt welding.
Fig. 4.28 Grain refinement in three zones: (a) W1; (b) W2.
Welding Condition
Current (A) Welding speed (mm/s)
Heat input per unit length (J/mm)
W1 11 5.26 52.28
W2 10 4.2 59.52
W3 11 4.2 65.47
W4 12 4.2 71.42
W5 14 4.2 83.33
W6 10 2.75 90.90
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Fig. 4.29 Microstructure of fusion zone: (a) W1; (b) W2; (c) W3.
The mechanical properties determined for the weldment are tensile test and hardness test.
Tensile test are carried out on a computer controlled universal testing machine as per ASTM standards (E-8). The tested tensile specimens are fractured in the base metal. The ductile fracture is observed in the tensile tested specimens. The tested welded specimens tensile strength (UTS) measured values are in the range of 600-760 MPa, whereas the base metal has shown tensile stress of 754 MPa. The weld joint efficiencies of over 90% are observed in majority of the samples and are comparable to the base metal strength. The details of tensile stress and yield stress measurements are given in Fig. 4.30b.
Vickers micro hardness tests are performed on a transverse section of the weld bead at 0.25mm below the top surface. The average hardness is found to be the highest in the fusion zone. Also hardness decreases with heat input. This can be attributed to the low heat input resulting in high cooling rate produced and hence increasing hardness in both HAZ and weld metal
.
Fig. 4.30 (a)Tensile stress–strain curve; (b) Joint strength of the joints.
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Fig. 4.31 Hardness values.
4.2.2.1 Comparison with low carbon steel
Figure 4.32 shows the weld bead obtained at similar welding condition for two materials.
It is observed that the weld bead of carbon steel is smaller for the same welding condition but the total heat affected zone is higher [Fatima et al., 2014]. Carbon steel has very high thermal conductivity which causes most of the heat to dissipate away. Whereas SS304 has lower thermal conductivity and the heat is highly concentrated in the weld zone causing it to form wider bead.
Fig. 4.33 shows the weld macrograph at 10A and 4.2 mm/s. The weld bead formed by carbon steel is more elongated and the weld width is smaller as the heat is more distributed. Due to the low heat input (approximately 52-110 J/mm of the investigated region) by the micro plasma arc, the depth/width ratio is low and is not enough to produce a keyhole. In general, an elliptical shape is obtained.
Fig. 4.32 Weld beads (bead on plate welding) obtained at 10 A, 4.2 mm/s: (a) SS304; (b) Low carbon Steel.
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Fig. 4.33 Weld beads (bead on plate welding) obtained at 10 A, 4.2 mm/s: (a) Low carbon steel;
(b) SS304.
Figure 4.34 shows the mechanical properties of the two materials at welding condition of 10 A, 4.2 mm/s. Transverse tensile strength of all the joints has been evaluated. The tensile testing was carried out in accordance with ASTM E-8 standard. Two specimens for tensile test are considered and the average values are considered. Tensile strength of SS304 welded joint is 784 MPa with 48% elongation while that of carbon steel welded joint is 363 MPa with 24%
elongation. Joint efficiency is also being calculated for both the welded joints which are 104.78 and 110.19%, respectively. The yield strength for both the materials is 350MPa (SS304) and 255MPa (carbon steel) at the given welding condition. It is observed that the work hardening rate is more for SS304 than carbon steel. The micro-hardness test is carried out with Vickers pyramid indenter by 50 g load for 15 s along the middle line of the cross section (Fig. 4.33). The average hardness values for both the materials are evaluated. The hardness for the base materials is found to be 176 and 108 for SS304 and low carbon steel. Various factors can contribute to the hardening like grain size, phase composition and metallic inclusions. The hardness is found to be highest at the weld zone for SS304 as shown in Fig. 4.34b. This may be due to the formation of hard martensitic structure at the weld zone. While for low carbon steel, the highest is in the HAZ zone (Fig. 4.34b).This may be due to coarser grain in the HAZ of low carbon steel weldment .
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Fig. 4.34 (a) Stress strain diagram; (b) Hardness of the materials obtained at 10 A, 4.2 mm/s.
The weld-induced distortion is mainly influenced by welding conditions, geometric shape and restrains conditions. In the present investigation, the comparative analysis shows the distortion dependency on the difference in mechanical properties. Both the materials are distorted in a convex concave distortion. Fig. 4.35 shows the bending distortion (displacement along Z-direction) for the two materials in the longitudinal direction along the weld line and transverse direction corresponding to a welding condition of 4.2 mm/s speed and 10A current. It is seen that although the pattern of deflection of these materials is the same, the deflection is higher for SS304. This is because the coefficient of thermal expansion of SS304 is more than low carbon steel and thus SS304 expands and contracts at a faster rate than carbon steel resulting in greater warping.
Fig. 4.35 Deflection comparison of the two materials at 4.2mm/s and 10A current (a) longitudinal direction along the weldline; (b) transverse direction at the weld stop edge.
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Fig. 4.36 Effect of heat input on weld dimensions.
The bead geometry which is specified by bead width, reinforcement, penetration, directly influences the load carrying capacity of the weldments. Fig. 4.36 revealed that both bead width and penetration of both the materials increases with increase with increase in heat input as greater amount of heat is available for forming the molten pool. However, at a critical value of heat input when full penetration of the SS304 weldment is obtained, the bead width decreases and thereafter the bead width increases. However, in case of low carbon steel, this effect is not obvious i.e. bead width increases with the increase in heat input for the entire range. Moreover the penetration increases more rapidly at lower heat input. Fig.4.37 shows the variation of mechanical properties with heat input. The ultimate tensile strength of the material first increases and then decreases with increase in heat input. The maximum UTS of 784MPa is obtained at 10A and 4.2mm/s (~ 59.5 J/mm) for stainless while 369MPa is obtained for low carbon steel at 12A and 42mm/s (~ 71.4). The lower strength at lower heat input is attributed to insufficient penetration. The value is found to be comparable to the base material in the investigated region indicating good strength of the weldment. The hardness value is found to be the highest at HAZ zone for all the cases in case of low carbon steel weldments. However, the hardness is highest in the weld zone for SS304. Fig. 4.37 reflects the average hardness values of HAZ zone for low carbon steel and weld zone for SS304 weldments. The hardness increases with increase in heat input. The deflection (Z-displacement) of the weld edge of the centreline is shown in Fig. 4.38.
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The deflection increases with increase in heat input and becomes steady with further increase in heat input. Thus, it can be conclude that the heat input has significant effect on mechanical and microstructural characteristics of the plasma micro welded joints.
Fig. 4.37 Effect of heat input on mechanical properties.
Fig. 4.38 Effect of heat input on deflection.