Study of Mechanical and Thermal properties of Copper alloy weldment welded by resistance spot welding 1 Nitish Ranjan, P.G. Scholar, Department of Production Engineering, BIT Sindri, Dhanbad 2 Abdul Razaque Ansari, Assistant Professor, Department of Production Engineering, BIT Sindri,Dhanbad ABSTRACT In this study the mechanical and Thermal behavior of resistance spot welds (RSW) done on copper alloy C106/CW024, sheets, welded at different welding parameters, is examined. Tensile shearing strength of the welded Copper alloy C106/CW024, is investigate and hardness evaluations were carried out in order to determine the influence of welding parameters on the quality of the welds. The welded joints were subjected to static tensile-shear tests so as to work out their strength and failure mode. Thermal modeling of spot-welded joint to study the temperature variation of the surface of the sheet. The increase in weld current and duration increased the nugget size and the weld strength. Away from a critical nugget diameter the failure mode changed from interfacial to pullout. Taking into consideration the sheet thickness and therefore the mechanical properties of the weld, an easy model is proposed to predict the critical nugget diameter required to supply pull-out failure mode in under corresponding welds in copper alloy C106/CW024. Keywords: Copper alloys C106/CW024, Resistance spot welding, Welding force, welding current, welding time, Rockwell Hardness, FE model I.INTRODUCTION Resistance spot welding is getting significant importance in car, bus and railway bodies etc. due to automatic and fast process. The major factors controlling this process are current, time, electrode force, contact resistance, property of electrode material, sheet materials, surface condition etc. the quality is best judged by nugget size and joint strength. This study presents a systematic approach to determine effect of process parameters (electrode force, weld time and welding current) on tensile shear strength of resistance spot weld joint, hardness measurement, Types of Failure of the Welding Joint, Investigation of nugget diameter for the set of experiment and Thermal modelling of spot-welded joint to study the temperature variation of the surface of the sheet A general introduction for principle, working and parameters of spot welding is given below. Resistance Spot Welding (RSW) is among the oldest of the electric
Transcript
Study of Mechanical and Thermal properties of Copper alloy weldment welded by resistance spot
welding1Nitish Ranjan, P.G. Scholar, Department of Production Engineering, BIT Sindri, Dhanbad
2Abdul Razaque Ansari, Assistant Professor, Department of Production Engineering, BIT Sindri,Dhanbad
ABSTRACT
In this study the mechanical and Thermal behavior of resistance spot welds (RSW) done on copper alloy C106/CW024, sheets, welded at different welding parameters, is examined. Tensile shearing strength of the welded Copper alloy C106/CW024, is investigate and hardness evaluations were carried out in order to determine the influence of welding parameters on the quality of the welds. The welded joints were subjected to static tensile-shear tests so as to work out their strength and failure mode. Thermal modeling of spot-welded joint to study the temperature variation of the surface of the sheet. The increase in weld current and duration increased the nugget size and the weld strength. Away from a critical nugget diameter the failure mode changed from interfacial to pullout. Taking into consideration the sheet thickness and therefore the mechanical properties of the weld, an easy model is proposed to predict the critical nugget diameter required to supply pull-out failure mode in under corresponding welds in copper alloy C106/CW024.
Keywords: Copper alloys C106/CW024, Resistance spot welding, Welding force, welding current, welding time, Rockwell Hardness, FE model
I.INTRODUCTION
Resistance spot welding is getting significant importance in car, bus and railway bodies etc. due to automatic and fast process. The major factors controlling this process are current, time, electrode force, contact resistance, property of electrode material, sheet materials, surface condition etc. the quality is best judged by nugget size and joint strength. This study presents a systematic approach to determine effect of process parameters (electrode force, weld time and welding current) on tensile shear strength of resistance spot weld joint, hardness measurement, Types of Failure of the Welding Joint, Investigation of nugget diameter for the set of experiment and Thermal modelling of spot-welded joint to study the temperature variation of the surface of the sheet A general introduction for principle, working and parameters of spot welding is given below.
Resistance Spot Welding (RSW) is among the oldest of the electric welding method that used in the industry and it is useful and accepted method in joining metal.
Figure 1: Resistance Spot Welding
Spot welding is widely used in welding carbon steel because they have higher electrical resistance and lower thermal conductivity than the electrode that made from copper. The Spot welding is commonly being used in automobile industry, where it is used to weld the sheet metal forming a car. Spot welders can also be completely automated, and many of the industrial robots found on assembly lines are spot welders. Spot welding also being used in the repair industries.Principle of Operation for Resistance Spot WeldingResistance Spot Welding (RSW) is included in the group of resistance welding processes that heat is used in joining the work parts of metal. Heat is generated from electrical resistance across the two work parts In Resistance Spot Welding two work part of metal are joined together by applying electric current and pressure in the zone to be weld and resistance welding is different From arc welding because it’s not required filler metal or fluxes added to the weld area during the welding process.
Spot welding operates based on four factors that are:1. Ampere current that passes through the work piece.2. Pressure that the electrodes applied on the work piece.3. Time duration for current flow through the work piece.4. The area of the electrode tip which contact with the work piece.During the welding process the amount of electric current is flow from the electrodes to the work pieces. The weld force is applied by leg pedal. Squeezing the electrode to the work pieces, the right amount of pressure that applied on the work pieces is very important in order to obtain the good quality of welds. During the welding process, the electric current is flow
Figure 2: Time Sequence of the Resistance Spot Welding Cycle: (1) Clamping Time, (2) Weld Time, (3) Hold Time,(4) Off Time
through electrode tips to the separate work pieces of metal to be joined.The resistance of the base metal to electrical current flow causes heat, the heat is limited to the area which the tip of the electrode and weld area contacts. While the welding force is continuously applied, the heat is generating. In the holding stage (where the pressure is still maintained), the current is switched off and the nugget is cooled under the pressure. The heat that generated in spot welding is basically depend on the electric current and the time being used and on the electrical resistance of material between electrodes. The amount of heat generated is a function of current, time and resistance between the electrode the heat that generates in resistance spot welding according to Joule’s law is expressed by the Equation.
H=I2RTwhere
H = Heat is generated in joulesI = Current (in amperes)
R = Resistance (in ohms)t = Time to current flow (in seconds)
Resistance Spot Welding ParametersThe spot welding process parameters have their own importance. A small change of one parameter will affect all the other parameters. These parameters will determine the quality of the welds. The appropriate combination of the spot welding parameter will produce strong joining and good quality of welding. Spot welding parameters include.1. Electrode force2. Diameter of the electrode contact surface3. Squeeze time4. Weld time5. Hold time6. Weld currentElectrode ForceThe purpose of the electrode force is to squeeze the parts to be weld and the primary purpose is to hold the parts to ensure the parts in intimate contact at the joining interface. When the electrode force is increased the heat energy will decrease, a high pressure that exerted on the weld joint will decrease the resistance at the point of contact between the electrode tips and the parts surface. This means that the higher electrode force requires a higher weld current. Weld spatter can be happen because the pressure on the tips is too light or when weld current becomes too high. Too heavy pressure will cause small spot weld. In other words when the pressure increases, the electrical current and subsequent heat are transfer to a wider area, the penetration and area of the weld will reduce.Diameter of the Electrode Contact SurfaceOne general criterion of resistance spot welding is that the weld shall have a nugget diameter of 5×t1/2, “t” being the thickness of the steel sheet. Thus, a spot weld made in two sheets, each 1mm in thickness, would generate a nugget 5 mm in diameter according to the 5 × t1/2-rules. Diameter of the electrode contact surface should be slightly larger than the nugget diameter.Squeeze TimeSqueeze Time is the time interval between the initial application of the electrode force on the work and the first application of current. Squeeze time is necessary to delay the weld current until the electrode force has attained the desired level.Weld TimeWeld time is the time during which welding current is applied to the metal sheets. The weld time is measured and adjusted in cycles of line voltage as are all timing functions. One cycleis 1/50 of a second in a 50 Hz power system. As the weld time is, more or less, related to what is required for the weld spot, it is difficult to give an exact value of the optimum weld time.Hold time (Cooling-Time)Hold time is the time, after the welding, when the electrodes are still applied to the sheet to chill the weld. Considered from a welding technical point of view, the hold time is the most interesting welding parameter. Hold time is necessary to allow the weld nugget to solidify before releasing the welded parts, but it must not be to long as this may cause the heat in the weld spot to spread to the electrode and heat it. The electrode will then get more exposed to wear. Further, if the hold time is too long and the carbon content of the material is high (more than 0.1%), there is a risk the weld will become brittle. When welding galvanized carbon steel a longer hold time is recommended.Weld CurrentThe amount of weld current is controlled by two things:• The setting of the transformer tap switch determines the maximum amount of weld current available.• The percent of current control determines the percent of the available current to be used for making the weld. Normally low percent current settings are not recommended because it may harm the quality of the weld. The weld current should be kept as low as possible. When determining the current to be used, the current is steadily increased until weld spatter occurs between the metal sheets. This indicates that the correct weld current has been reached. The
temperature rises rapidly at the joined portion of the metal where the resistance is greatest if the current becomes too great internal spatter will result.LITERATURE REVIEWLuo et al. [1] proposed the application of a low-current pre-heating treatment to suppress the effect of oxide layer. As shown in fig.3, a pre-heating current of 8 kA for a pre-heating time of 50ms resulted in significant reduction in contact resistance at the faying interface of AA5052 Cu alloys and also improved its distribution. Consequently, the joint quality and consistency improved significantly [1].
Figure 3: The effect of pre-heating on the contact resistance at the faying surface of AA5052 Cu alloys (pre-heating current of 8 kA, pre-heating time of 50 ms)
Wu et al.[2] welded 2-mm AA6111-T4 Cu alloy under different welding conditions to produce two distinct target
button diameters of less than 4 t1/2 (5.7 mm) and equal to 5 t1/2 (7.9 mm), all of which failed in IF mode during TS
test, with an average failure load of 3.3 and 4.9 KN [2]. IF, PIF, and PF modes were observed during TS of 1-mm
6082-T6 Cu alloy spot welds, depending on the nugget diameter. IF mode occurred for nugget diameters up to 5.1
mm while PF mode occurred for nugget diameters above 5.6 mm. For nugget sizes between 5.1 and 5.6 mm, both
failure modes occurred. Furthermore, the authors derived the following equation to predict the critical nugget
diameter required to ensure pullout failure mode during TS test of heat treatable Cu alloys spot welds.
Kang et al. [3] reported nugget pullout failure mode, with a maximum tensile shear load of 6.1 KN, when conducting
TS test on dissimilar resistance spot weld between 2-mm AA5754 Cu alloy and 3-mm Aural2 die casting alloy. The
fracture initiated at the notch root and propagated around the nugget on the Aural2 side of the weld[3]. Studies on the
TS performance of three-sheet Cu resistance spot welds have shown that the peak load increases with increased
button diameter and that the failure mode changes from IF to PIF to PF at certain critical nugget diameters.
Generally, a significant reduction in hardness has been observed in the HAZ and FZ of resistance spot welds of heat treatable, 6xxx series Cu alloys [4, 5, 6, 7, 8]. This is attributed to the dissolution of strengthening precipitates, especially in the T6 state [4, 6, 7] and to the destruction of work hardening [5]. Figure 4 shows the hardness profile across the BM, columnar crystal zone (CCZ), transition zone (TZ), and equiaxed crystal zone (ECZ) of 6061-T6 Cu alloy resistance spot weld. As shown in the figure, the decrease in hardness is concentrated mainly in the nugget, where the melting of the BM has led to the total dissolution of the strengthening precipitates [4, 6, 9]. A similar observation was made for 6082-T6 [6], AA6111-T4 [5], and AA6022- T4 [8] Cu alloys resistance spot welds. For 6061-T6 Cu alloy, the average microhardness value of the BM, CGZ, TZ, and EGZ was found to be 96, 70, 64, and 59 HV, respectively. It was found that under EMS, the average hardness of EGZ increased from 59 HV to about 64 HV, due to microstructure refinement [10]. For AA6111-T4 Cu alloy, the nugget center was found to be approximately 35% softer than the sheet surface [5].
Figure 4: Hardness profile of 6061-T6 Cu alloy resistance spot weld [11]
Sahota et al., 2013[12] The process parameters affect as an increase in weld current, weld time and electrode force results in an increase in weld nugget diameter and width. An increase in weld current, weld time and electrode force results in an increase in electrode indentation. So the parameters used should provide the high strength.Lin et al.2003 [13]When the spot welded joint is subjected to pure opening condition then it fails in the direction of load applied and when it subjected to combined opening and shearloading conditions it fails inclined from the surface of weld nugget.Chetan Patel, Dhaval Patel et al.2012[14] Therefore the spot welded joint should be strong in both in pure opening condition and in combined opening and shear loading conditions.Though the process parameters of the spot welding also affect the mechanical behavior of welded joint in the loading conditions so the parameters should also be suitable for the higher strength of joint.Types of Failure of the Welding JointThere are two fracture modes of the spot welding joint have analyzed, they are• Interfacial mode (or nugget fracture):fracture of the weld nugget through the planeof the weld. The dominant failure mode for small diameter spot welds.• Nugget pullout mode (or sheet fracture): fracture of the sheet around the weld; the nugget remains intact. Dominant for large diameter spot welds. Spot welds for automotive applicationsshould have a sufficiently large diameter, so that nugget pullout mode is the dominant failure mode. Interfacial mode is unacceptable due to its low load carrying and energy absorption capability (Stijn Donders et al., 2005)[15]II Experimental procedure Material selection:Copper alloy C106/CW024, sheets of 1.6 mm thick were used in this study. The nominal chemical composition and basic mechanical properties of this alloy are given in Table 1 and table 2. It is a non-heat treatable Cu-Mg alloy with relatively good mechanical properties and corrosion resistance commonly used in defence and shipbuilding industries. This alloy has a relatively large Mg content to promote solid solution strengthening and to increase the rate of work hardening, the most important strengthening mechanisms in this alloy. Resistance spot welding lap joints were done on specimens of 200 mm x 25 mm x 1 mm in size. Fig.6 shows the geometry and dimensions of the welded specimens. Sheet surfaces were randomly abraded with emery paper – P400 grade and afterwards cleaned with acetone.
Figure 5: Configuration of test specimens (in mm) tensile/shear test specimen
Table 1:Chemical composition for Copper Alloy C106/CW024,
The two configuration of specimens were chosen for spot. The one been specially made for testing of shear tensile strength as shown in figure 6 and other is for studying the nuggets size, hardness and microstructure of the spot as shown by CAD model in figure 7.
Experimental setup and procedure:
In the proposed investigation, Copper C106/CW024, Alloy sheet of 1.6mm is welded using 15 kVA Pneumatically operated resistance spot welding machine made ELECTROWELD having model no SP15PR.The machine employs type C18200 electrodes having an end diameter of 7 mm, an electrical conductivity of 0.463 m/X mm2 and a tensile strength of 310 N/mm2. The throat depth 460mm, nominal throat clearance 220 mm. maximum available short circuit current 8kA. The resistance spot welding machine is shown in figure 9
Figure 7:Weld configuration of specimens for shear tensile strength
Figure 6: Weld configuration full overlapped of the specimen
Figure 8: Resistance spot welding machine setup during experiment
The parameters considered for in the present study consist of welding current (kA), welding time(cycle) and electrode force (kg/cm3) in three level. The experiment will be based on a 33 full factorial design with three levels of each factors. Two Spots were generate for similar setting of parameters. The total number of Spot-welded joint would be54. The one set of 27 spots welding containing specimen has to under goes mechanical testing (Shear-tensile Testing) and other set has to undergoes metallurgical Characterization’s (Like Visual Inspection nuggets from top and Cross- section, Macrostructure of Nuggets, and Hardness test). The two different joint configuration are shown in figure 10 and 11. Parameters along with level are mention in the table3:
Figure 9 : Weld joint configuration- 1 after welding
Figure 3
Table 3 Parameters along with level are :
Parameters Level 1 Level 2 Level 3Welding current (kA) 8 9 10Welding time (cycle) 5 7 9Forge(electrode force)(kg/cm3) 6 8 10The set of parameters that produced 54 number of spot are varies according to a sequence and is shown in the table 4.
The parameter which are remained constant are given in the table 5
Figure 10: Weld joint configuration- 2 after welding
Sl. No Parameters Value1 Squeeze time (cycles) 52 Electrode Diameter (mm) 73 Hold time (Cycle) 44. Current frequency (Hz) 505. Sheet thickness(mm) 1.6
Hardness test specification
All spot-welded specimens are eventually subjected to macroscopic examination for measuring the nugget size. The Rockwell hardness test were conducted on each set of spot to observe the hardness at three locations at center of nugget and its vicinity Hardness are calculated at center, at a distance of 1mm from center and the base material at a distance of 2 mm from the center of the series of nuggets. Surface hardness measurements were carried out using Rockwell hardness testing machine under the 100 kg load with 0.1 mm interval. Automatic weight selection with automatic zero setting dial gauge. Rockwell test minor load is 10 Kgf & major loads are 60, 100,150 Kgf. Each sample are plotted by 1/16” Ball Indentor. The hardness of 27 nuggets are measured in Rockwell hardness in B scalei.e. HRB scale.
Figure 11: Rockwell hardness testing machine
Thermal modelling of spot-welded joint
Finite element simulation of resistance spot welding were done to obtained the thermal modelling of spot-welded joint and study the temperature variation of the surface of the sheet were done all the 27 set of spot. In resistance spot welding process, a large magnitude of electrical current passes through the sheet–electrode system for a short time through a restricted zone resulting in rapid heating. In the course of the welding process, a localized zone of solid material at the sheet to sheet interface moves to the liquid phase and gets solidified to produce the weld joint. Hence, the problem of heat flow in resistance spot welding is a case of heat conduction associated with melting and subsequent solidification.
Nature of heat flow is in case of RSW is 3D by applying both electrical current and compressive force by electrode. The Analysis involves the flow of heat at contact surface due to the produced resistance when large value of current is applied. In the Present analysis compressive force applied by electrode which also assist in spot formation were kept negligible. The effect of heat produce because of resistance at the contact were considered only. The thermal analysis is conceived as an axisymmetric case of transient heat conduction in the present work.
The governing equation for transient heat conduction in cylindrical coordinate system considering no variation of temperature with respect to θ axis can be stated as:
Where Q is the heat generation, r and z are radial and axial coordinates and k, C, and ρ are the temperature-dependent thermal conductivity, specific heat, and density of the material, respectively. The exterior surfaces of the electrodes and sheets lose heat to air by convection. Hence, the heat losses through the outer surfaces of both the electrodes, and the sheet are considered by accounting the convective heat transfer coefficient. .
The governing equation for a steady-state current conduction through the sheet electrode system can be given in terms of electrical potential, ϕ, as
where ρ is the electrical resistivity.
FE model
The three model was developed in NX and ANSYS is used for simulation. The system-generated mess was obtained
having nodes 23350 and elements 6902. The generated mess is shown in figure 12
Figure 12: Generated mess for simulation
Tensile shear test:The tensile/shear and cross-tension specimens were machined according to the national standard GB/T 2651-1989, as shown in Fig.() . All the specimens are welded in lap configuration. The tensile/shear specimens were made by using two 100 mm x 25 mm sheets with a 25 mm x 25 mm overlap area. And the cross-tension specimens were made by using two 200 mm x 25 mm sheets with a 200 mm x25 mm overlap area. All the specimens were welded in the middle of the overlap area having single spot. The tensile/shear and cross-tension tests were performed using a CSS- 44100 electronic universal testing machine. The tensile/shear weld strength was obtained by individual specimens which is tested.
Figure 13: Configuration of test specimens (in mm) tensile/shear test specimen
RESULT AND DISCUSSION
Nugget size: Visual testing was conducted on 54 set of spots done on Copper sheet. It is observed that as the current value increases the diameter of nugget also increases. In experiment 1, 10 and 19 the current is increased from 8kA to 10 kA, as in RSW the current is the most influential parameter the increased value of current results in bigger size of nugget. Experiment no.1 , 2 and 3 when compared each other, the nugget diameter has slight variation as the current remains constant at 8kA but the slight variation has been seen because of increase in the electrode force from 6 to 10kg/cm 3 . There has been expulsion of material in experiment no 16; it would have been because of wrong setting of parameter.
Figure 14: nugget size for a set of experiment
Hardness: A significant reduction in hardness has been observed at center of nugget in when current was set at lower value and hardness value was near to base metal and slightly increases when current is increased to 10kA. The Hardness value obtained at the center say (freezing zone, FZ), 1mm away from FZ say (Heat effected Zone, HAZ) and base metal have been shown in table 6. A significant reduction in hardness observed is attributed to the dissolution of strengthening precipitates, especially in the T6 state and to the destruction of work hardening. The decrease in hardness is concentrated mainly in the nugget, where the melting of the BM has led to the total dissolution of the strengthening precipitates. The variation of hardness value at nugget center for 27 set of spots has been shown in figure16.Table 6 The Hardness value obtained at the center say (freezing zone, FZ), 1mm away from FZ say (Heat effected
Zone, HAZ) and base metal
Experiment no Nugget center (HRB) Nugget edge (HRB)
Figure15: Variation of hardness value at nugget center for 27 set of spots joint
FEA Result: Transient temperature fields developed during the resistance spot welding for different input process parameters are computed using Workbench module available in ANSYS (coupled thermo-electric analysis). The FE model is adopted for predicting the weld nugget diameter and thickness for a wide range of input process parameters, 27 factorial design would take totally nine combinations (three levels— heating time and welding current). The workpiece material considered for this study is Copper C106/CW024, Alloy. The copper electrode of radius, r=3.5 mm, indicating that the materials within this zone is melted and a formation of round nugget.
Figure 15 presents the temperature distribution on the electrode and Copper alloy sheets on the surface for a selected experiment no 27.
Heat Calculation: When current is passed through a conductor the electrical resistance of the conductor to current flow will cause heat to be generated. The secondary portion of a resistance spot welding circuit, including the parts to be welded, is Actually a series of resistances. The total additive value of this electrical resistance affects the current output of the resistance spot welding machine and the heat generation of the circuit. Although current value is the same in all parts of the electrical circuit, the resistance values may vary considerably at different points in the circuit. The heat generated is directly proportional to the resistance at any point in the circuit
The basic formula used here t calculate the theoretical heat is be stated:
H=I2RTWhere, Q is the heat generated in joule, I is the welding current and T is the welding time. The resistance is calculated using the linear relationship between the sheet thickness and average sheet resistance before the welding process. The values of resistance was found to be 0.0003368 ohm. The calculated value of heat is shown the table 7 . E(Experiment No) -1.2.3 , E-4.5.6, E-7.8.9, E-10.11.12, E-13.14.15, E-16.17.18, E-19.20.21, E-22.23.24 andE.25.26.27 are grouped together and steady state thermal modelling is done one ANSYS. The calculated value of heat is used in steady state thermal analysis.
ab
c
de
f
g h i
Figure 13 : Temperature distribution plots during RSW process for nine different experiments:( a) E-1.2.3 (b) E-4.5.6 9(c) E- 7.8.9, (d) E-10.11.12 (e)E-13.14.15, (f) E-16.17.18 (g) E-19.20.21, (h) E-22.23.24 (i) E.25.26.27
Wel
ding
Cur
rent
/Wel
ding
tim
e/He
at
Table 8 Maximum temperature recorded after simulation for each set of experiment
no current (KA) (Second) Heat (Joule) Temperature(Degree C)
Welding current (KA) Welding Time (Second) Heat (Joule)
Figure14: Variation of welding current, welding time and heat for different set of experiment
Max.Welding TimeWeldingExperiment
7.36
Figure 15: Temperature distribution for set of experiment
Tensile-shear strength:Fracture modes of the spot welding joint, which has been observed in different specimen after conducting tensile shear strength. The two mode are observed as follows.• Interfacial mode (or nugget fracture): fracture of the weld nugget through the plane of the weld. The dominant failure mode for small diameter spot welds.• Nugget pullout mode (or sheet fracture): fracture of the sheet around the weld; the nugget remains intact. Dominant for large diameter spot-welds.
Welding current is the most influential parameter in RSW. Generally, a low-current results in low heat input, an under sized size, and poor penetration. Increasing the current leads to an increase in heat generation, nugget size, and tensile shear load. Because of their high thermal conductivity, high welding current and short welding time are required for RSW of Cu alloys. However, excessive welding current could lead to excessive heat generation, severe expulsion, poor joint quality, and appearance. Figure20 shows the effect of welding current on the strength of 1.6-mm C106/CW024, Cu alloy spot weld at various welding times and a constant electrode pressure of 6 kg/cm2. Over the range of current studied, at a given welding time, the tensile shear load increased with increase in welding current due to increase in nugget size. Maximum tensile shear load of about 2.75 kN was obtained at a welding current of 10 kA and 8 cycles welding time. In this due to same heat generation E(Experiment No) -1.2.3 , E-4.5.6, E-7.8.9, E-10.11.12, E-13.14.15, E-16.17.18, E-19.20.21, E-22.23.24 andE.25.26.27 are grouped together and tensile shear strength for these parametes is calculated as shown by Table 9 .
E - 1 . 2 . 3 E - 4 . 5 . 6 E - 7 . 8 . 9 E - 1 0 . 1 1 . 1 2E - 1 3 . 1 4 . 1 5E - 1 6 . 1 7 . 1 8E - 1 9 . 2 0 . 2 1E - 2 2 . 2 3 . 2 4E . 2 5 . 2 6 . 2 7
AXIS TITLE
450
400
350
300
250
200
150
100
50
0
Max. Temperature (Degree C)Welding Time (Second)Welding current (KA)
VARIATION OF WELDING CURRENT, WELDINGTIME AND HEAT
Table 9 tensile shear strength at diffenert welding current
variation of welding current welding time heat andTensile shear strength
141210
86420
Figure 16: shows the effect of welding current on the strength of 1.6-mm C106/CW024, Cu alloy spot weld at various welding times Although the nugget diameter and joint strength increase with increase in welding current, the electrode imprint/indentation also increases, even at constant electrode force. A spike in electrical current could create a large imprint and potential burn-through effect, especially on thins sheets, affecting joint quality and appearance. Thus, although it is favorable to select high welding currents for Cu alloys, the values should not be unnecessarily high.
III.CONCLUSION:The influence of the weld parameters on the mechanical and thermal properties of resistance spot welds on an copper alloy C106/CW024, was studied. The conclusions obtained are summarisedas follows:The increase in the weld current and time increased the nugget diameter and hardened the microstructure, these transformations were accompanied by a significant reduction in hardness isnormally observed in the FZ and HZ of spot welds in heat treatable, C106/CW024, Cu alloys, dueto the dissolution of hardening precipitates. The reduction in hardness is concentrated mainly in the nugget center, because it experiences the highest temperature.A significant increase in the failure load in static shear lap tests was observed in welds done with increasing weld current and time, because of the augmentation of the nugget diameter beyond a critical nugget diameter the failure changes from interfacial mode to pullout mode.
REFERENCES[1] Luo Z, Ao S, Chao YJ, Cui X, Li Y, Lin Y (2015) Application of pre-heating to improve the consistency and quality in AA5052 resistance spot welding. J Mater Eng Perform 24(10):3881–3891[2] Wu S-n, Ghaffari B, Hetrick E, Li M, Z-h J, Liu Q (2014) Microstructure characterization and quasi-static failure behavior of resistance spot welds of AA6111-T4 Copper alloy. Trans Nonferrous Metals Soc China 24(12):3879– 3885[3] Kang J, Chen Y, Sigler D, Carlson B, Wilkinson DS (2015) Fatigue behavior of dissimilar Copper alloy spot welds. Procedia Eng 114:149–156[4] Shi Y, Guo H (2013) Fatigue performance and fatigue damage parameter estimation of spot welded joints of copper alloys 6111‐T4 and 5754. Fatigue Fract Eng Mater Struct 36(10):1081–1090[5] Wu S-n, Ghaffari B, Hetrick E, Li M, Z-h J, Liu Q (2014) Microstructure characterization and quasi-static failure behavior of resistance spot welds of AA6111-T4 Copper alloy. TransNonferrous Metals Soc China 24(12):3879–3885[6] Pereira A, Ferreira J, Loureiro A, Costa J, Bártolo P (2010) Effect of process parameters on the strength of resistance spot welds in 6082-T6 copper alloy. Mater Des 31(5):2454–2463[7] Afshari D, Sedighi M, Barsoum Z, Peng RL (2012) An approach in prediction of failure in resistance spot welded Copper 6061- T6 under quasi-static tensile test. Proc Inst Mech Eng B J Eng Manuf 226(6):1026–103[8] Kang JD, McDermid JR, Bruhis M (2013) Determination of the constitutive behaviour of AA6022-T4 copper alloy spot welds at large strains. Mater Sci Eng a-Struct 567:95–100[9] Hayat F (2012) Effect of aging treatment on themicrostructure and mechanical properties of the similar and dissimilar 6061-T6/7075-T651 RSW joints. Mater Sci Eng A 556:834–843[10] Li Y, Zhang Y, Bi J, Luo Z (2015) Impact of electromagnetic stirring upon weld quality of Al/Ti dissimilar materials resistance spot welding. Mater Des 83:577–586[11] Li Y, Zhang Y, Luo Z (2015) Microstructure andmechanical properties of Al/Ti joints welded by resistance spot welding. Sci Technol Weld Join 20(5):385–394[12] Sahota D S, Ramandeep Singh, Rajesh Sharma and Harpreet Singh (2013), “Study of Effect of Parameters on Resistance Spot Weld of ASS316 Material”, March[13] Lin S-H, Pan J, Tyan T and Prasad P (2003), “A General Failure Criterion for Spot Welds Under Combined Loading Conditions”, June[14] Chetan R Patel and Dhaval A Patel (2012), “Effect of Process Parameters on the Strength of Copper Alloy A5052 Sheets Joint Welded by Resistance Spot Welding with Cover Plates”, August[15] Stijn Donders, Marc Brughmans, Luc Hermans and Nick Tzannetakis (2005), “The Effect of Spot Weld Failure on Dynamic Vehicle Performance
