4.1 Preamble
Since the reuse of old/scraped copper is important for the technological development as well as economic progress of the society, tensile properties of such old copper need to be characterized with utmost priority. While doing the characterization of tensile behavior, the inclusion of solder elements such as tin, lead etc. again remains as the matter of contention along with work hardening and thermal ageing. It is because the solder materials like lead, tin, etc. might be very little in the old/scraped copper, but the effect on the tensile properties is substantial. Moreover, Cu having a cubic crystal structure offers a good level of ductility and malleability, but the work hardening causes the dislocations into the structure, which ultimately causes copper to assume different level of strength. As a whole, work hardening and thermal ageing in combination may reveal enhancement in some properties or decline in few cases resulting the new product made of old copper may be useful or again may be detrimental to desired properties [10,24]. Therefore, this chapter addresses the investigation on the changes in tensile behavior of copper with a very minute inclusion of SnPb-solder under the influence of work hardening and thermal ageing.
4.2 Experimental Details
The work hardened sample materials were sized up for standard specimens according to ASTM E8 guidelines for tensile tests and divided into different groups to carry out the series of experiments at the work hardening levels of 0%, 12.5%, 25%, 37.5%, 50% and 75%. The samples of one group were kept without heat treatment to observe the of work hardening effect on tensile behavior and the samples of another group were heat treated isochronally at various temperatures, such as, 25oC, 100oC, 150oC, 200oC, 250oC, 300oC, 350oC, 400oC, and 450oC for a period of one hour. Tensile tests were then carried out to perceive the effects of ageing temperature on ultimate tensile strength, yield strength, ultimate elongation, elastic modulus, etc.
Shimadzu Hydraulic Universal Testing Machine (Model: UH-F1000 kN X) was employed to perform the tensile tests for all four sample materials as shown in fig.4.1(a).
The samples are of dog bone shape of size 100x6x3 mm with gauge length (GL)=25mm as shown in fig 4.1(b). For the first round tests, crosshead speed was maintained as constant (1.5 mm/s). Then strain rate effect was observed varying the crosshead speed based on minimum to maximum limit of the machine, i.e., 0.2 mm/s, 1.5 mm/s, 7.5 mm/s, 15 mm/s, 30 mm/s, 60 mm/s and 100 mm/s, which are corresponding strain rates of of 1.33×10-4 s-1, 1.00×10-3 s-1, 5.00×10-3 s-1, 1.00×10-2 s-1, 2.00×10-2 s-1, 4.00×10-2 s-1and 6.67×10-2 s-1 respectively.
Microstructures of the same samples were examined using optical electronic microscope (OEM) of model BW-S500. In addition, the surface morphologies of work hardened (75% deformation level) before and after tensile failure of sample materials were investigated using a field emission scanning electron microscope (FE-SEM) of model JEOL JSM-7600F with the magnification of 500, 1000, 2000 and 30000.
Fig. 4.1 (a) UTM used for tensile tests, (b) Test specimens.
4.3 Stress-Strain Behavior
Load deflection data obtained through the tensile tests have provided the stress-strain curves for sample materials as illustrated in fig. 4.2. It indicates that all four copper based sample materials are of ductile in nature with moderate tensile strength. Fig. 4.2(a) indicates stress-strain relations of sample materials at their as-cast condition and fig.
4.2(b) depicts the results of cold-rolled samples of the same materials. Both the figures have demonstrated initial steeper rise of the stress values against the strain with an
indication of moderately high elastic modulus values and then reduction of slope making the tangent modulus little less than elastic modulus. All test results have also confirmed prolong necking period as an inherent property of ductile materials with different non- linearity levels for four sample materials.
Amongst the four sample materials, material-I, i.e., pure Cu at its as-cast condition (Fig. 4.2(a) black line) has shown slow rise of stress after the initial up-rise while reaching to the ultimate tensile strength (~240 MPa) and then a very slow fall of stress from there for a considerable elongation range indicating to the point of a stress value about 170 MPa, where the stress-strain curve has got another change. Finally, the stress has fallen to a very small value (~50 MPa) against the strain value of about 10% before the fracture of material-I (pure Cu). On the other hand, material-II, -III and -IV, i.e., all three alloys have lesser elongation than that of material-I after reaching to the ultimate tensile strength as seen with red, green and blue lines, respectively. However, material- III has shown a different pattern after the ultimate tensile strength point and the stress- strain curve continues to be elongated and remains unbroken for a considerable period with the lowest stress value of about 175 MPa before its fracture.
Fig. 4.2(b) illustrates that the stress-strain behaviour of cold-rolled material-I, i.e., pure Cu is different from that of materials-II, -III and -IV, i.e., cold-rolled alloy samples.
The stress level of cold-rolled material-I has shown a fall of stress after reaching to the ultimate tensile strength for a considerable elongation range. But the curve has not gone down as observed for its as-cast condition. Cold-rolled materials-II, -III and -IV have shown less elongation range after achieving the ultimate strength to reach the lowest level of stress. In this respect, material-II and material-III are showing no significant changes in stress-strain behaviour after cold-rolling from their as-cast condition. However, material-III has not shown prolong elongation at the steady lowest stress value after cold rolling as seen for as-cast condition.
Fig. 4.2 Stress strain diagram of sample materials for tensile tests carried out at room temperature with constant strain rate of 10-3 s-1: (a) as cast and (b) 75% cold rolled.
The stress-strain curves of pure copper obtained in the present study have some agreements and disagreements with the findings of other researchers. The present results are thus compared with few findings of previous works of different researchers, such as, the results obtained by Khodaverdizadeh et al [46], Barmouz et al [47], Djavanroodi et al [48], Yang et al [49] and Copper Development Association [50] in few aspects.
Fig. 4.3 can be observed to look into the true stress-strain curves obtained by Khodaverdizadeh et al [46] for pure copper and friction joints of copper. The concerned graph (the line marked as BM) indicates initial steeper rising portion with moderately high elastic modulus value, then little lowering of slope for a considerable strain range with the less tangent modulus than that of elastic modulus and finally lowering of stress with notching effect for a prolong period. Here, the rising pattern of stress against strain in the curve is similar to that of present study. The yield strength and ultimate strength values are also nearer in both the studies. But the maximum strain value is about 0.45 which is quite larger than the value obtained in the present study (maximum strain ~0.1 only). It might be the cause of difference in strain rate of UTM during the tensile tests.
Since the strain rate is not found mentioned in the report of Khodaverdizadeh et al [46], the effect of strain rate difference is not possible to be quantitatively determined.
Moreover, the difference between true strain and engineering strain values also persists in this comparison.
The stress-strain curves obtained by Barmouz et al [47] for as-received copper samples and specimens produced without the presence of powder as well as with micro- and nano-sized SiC particles are presented in fig. 4.4. The curve of as-received copper in this figure indicates a gradual steeper increase in stress values against strain while reaching to the ultimate tensile strength at about 240 MPa and then a slow fall of stress from there for a considerable elongation period indicating stress value about 180 MPa.
Finally, the stress has fallen at high rate to a very small value of about 55 MPa before it reaches to the point of fracture with the strain value of about 0.18. Thus the results of Barmouz et al are quite similar to the results of the present study in all aspects such as pattern, ultimate tensile strength, stress and strain values at fracture point. This good agreement is really notable, and it has provided some bit of authenticity on the stress- strain behavior of pure copper obtained in the present study.
Fig 4.5 presents the true stress–strain curves for pure aluminum and copper obtained by Djavanroodi et al [48]. The curve of pure copper indicates gradual but steeper rise in initial portion with moderately high elastic modulus value, then little lowering of slope for a considerable strain range indicating less tangent modulus than elastic modulus and finally reaching to the ultimate strength with the value of about 260 MPa against the strain value of about 0.28. Here, the rising pattern of the stress against the strain is similar to that of the present study. The yield strength and ultimate strength values obtained by Djavanroodi et al are also nearer to that of the present work. But the notching effect with the fall of stress values before reaching to fracture point seems to be missing. It is observed from the graph indicating the occurrence of fracture point just after a little change in strain values without much change in the stress. It is not supposed to be happened for a pure copper sample. Since, the researcher has the this type of result, there might have obvious reasons for this situation. The probable reasons behind such situation would be due to number of factors including the approach/method of specimen preparation, heat treatment conditions, presence of any impurity in their materials, difference in strain rate during tensile tests, test environments, etc.
Fig. 4.6 illustrates engineering stress–strain curves of the HT 50 µm copper wires with different gauge lengths, i.e., ten times, fifty times and hundred times the diameter of specimen as obtained by Yang et al [49]. All the stress-strain graphs in fig. 4.6 indicate gradual rise of stress with initial steeper portion resulting a moderately high elastic modulus value of copper, then a gradual lowering of slope with comparatively less tangent modulus than elastic modulus, which continues for a considerable strain range up to reaching to the ultimate tensile strength value of about 200 MPa against the strain value of about 0.15. The said stress-strain curves then have gone down with increase of strain values as an effect of notching. These graphs differ with the stress-strain curve of the present study by the high lowering trend of stress values immediately after ultimate strength point. Moreover, it is not clear about the point of fracture after ultimate strength in the graph of Yang et al [49].
The tensile behaviour of copper obtained Copper Development Association [50] is presented in fig. 4.7. It illustrates the stress–strain curves of annealed copper for the tensile tests carried out at different temperature conditions such as 4 K, 20 K, 76 K, 195 k and 295 K. Out of them, the curve at 295 K may be considered suitable to be compared
with the present study where tensile tests were carried out 25℃ and it is observed that the pattern of stress-strain curve in fig. 4.7 for 295 K is similar to that of the present study. The elastic modulus, tangent modulus and ultimate tensile strength values are also nearer to that of the present results. However, the strain at the point of fracture obtained Copper Development Association [50] is about 0.45 which is quite higher than that of the present result. The reasons for such occurrence might be for annealing process and other factors related to test conditions.
Now, as a whole it can be said that the stress-strain behavior of pure copper obtained in the present study goes with reasonable agreement for most of the tensile parameters.
Only few cases are observed where the parameters are found different from the present study. The values of tensile parameters that could be drawn out from the graphs of the referred researches [46-50] for pure copper at its as-received or as-cast conditions are presented in table 4.1. Ultimate tensile strength, elastic modulus, tangent modulus, etc.
values are nearer for most of the researches. Only contention lies with the maximum strain having deferent values ranging from 10% to 45%. However, three researches out of mentioned five have shown maximum strain of ranging 10% to 18% in which the present study falls into. Therefore, it can also be taken as reasonable data.
Table 4.1 Parameters of tensile results for different researches Parameter/ Study
Case Ultimate
Tensile Strength (MPa)
Elastic Modulus (GPa)
Tangent Modulus at 2%
strain (GPa)
Stress at Fracture Point (MPa)
Strain at Fracture Point (%)
Present Study 240 112 89 50 10
Khodaverdizadeh et al
330 100 75 60 45
Barmouz et al 240 100 90 55 18
Djavanroodi et al 260 102 85 240 28
Yang et al 200 102 88 - 15
Copper Development Association
220 105 90 150 43
Fig. 4.3 True stress versus true strain curves of pure copper base metal (BM) and friction stir welded (R600T25, R600T75 and R900T75) samples [46].
Fig. 4.4 Stress-strain curves of as-received copper and specimens produced without powder and with micro- and nano-sized SiC particles. SiC volume fractions in the
specimens with powder were 6 and 18% [47].
Fig. 4.5 The true stress–strain curve for pure aluminum and copper [48].
Fig. 4.6 Engineering stress–strain curves of the HT 50 µm copper wires with different gauge lengths: 10D, 50D and 100D (diameter of specimen) [49].
Fig. 4.7 Stress-strain curves of annealed copper [50].
4.4 Ultimate Tensile Strength (UTS) 4.4.1 Effect of Alloying and Cold Rolling
The UTS values obtained on average from readings of 8 repeated tensile tests carried out for four sample materials, i.e., material-I (pure Cu), material-II (Cu-Sn alloy), material-III (Cu-Pb alloy) and material-IV (Cu-Sn-Pb alloy) subjected to work hardening at 0%, 25%, 50% and 75% cold rolling along with thermal ageing at different temperatures have been illustrated in Fig. 4.8.
Here, the UTS values of material-I, -II, -III and -IV, at 0% cold rolling, i.e., as cast condition at room temperature have been found to be 242.44 MPa, 288.48 MPa, 235.85 MPa and 280.88 MPa, respectively. It indicates that the addition of merely about 1.13%
Sn with copper to develop material-II has increased the UTS by 18.99%, whereas the addition of about 1.97% Pb to develop material-III has decreased the UTS by 2.72%, and the addition of about 1.25% Sn and 1.19% Pb together to have material-IV has increased
the UTS by 15.85%. The UTS of commercial Sn is 230 MPa which is little less than that of Cu. But while these two elements, i.e., Cu and Sn are combined together to form Cu- 1%Sn alloy, the UTS has been increased significantly. The reasons behind such behaviour of solid solution alloy formations are intermetallic compounds (CuxSny), elastic misfit interaction (size effect) due to the resultant elastic distortion that interacts with the dislocations and different metallic phases within the crystal structure [51,35-37]. On the other hand, inclusion of Pb in the formation of Cu-1%Pb alloy has reduced the strength due to extremely low tensile strength of Pb (12-15 MPa) which is also exacerbated by its creep and fatigue behaviour. Moreover, Pb does not have any tendency to form intermetallic compounds with Cu [38-40]. As a result, the UTS value of material-III is the lowest and that of material-IV remains in between material-I and material-II at their as cast condition. Similar behavior for inclusion of Sn and Pb in Cu could have observed in the investigation of micro-hardness as revealed in chapter 3.
Once the samples have undergone work hardening, they have behaved differently on strength from their as cast condition. The UTS values have been increased with the increase of work hardening levels for all four sample materials. However, the incremental rates of UTS values are found to be dissimilar for different sample materials as shown in Fig. 4.8. The initial jump is observed from cast condition to work hardening level of 12.5% and 25% cold rolled conditions, and thereafter, the rising trends of UTS curves have become little less for all four sample materials. Nonetheless, after work hardening level of 75% cold rolling the average values of tensile strength for material-I, -II, -III and -IV have increased to 260.14 MPa, 307.41 MPa, 266.52 MPa and 313.32 MPa respectively, which are 1.073, 1.065, 1.13 and 1.115 times of their corresponding as cast strength. It is also interesting to note that material-III (Cu-Pb alloy) has lesser ultimate strength than of material-I (pure Cu) before cold-rolling condition. But the UTS value of material-III has crossed over that of material-I at the cold-rolled level of about 30-35%
and thereafter material-III has remained stronger than material-I at higher deformation levels. Again, the UTS of material-IV is also less than that of material-II before cold- rolling, but the UTS of material-IV overshoots that of material-II at the deformation level of about 35-40% and remains at higher position onwards as seen in Fig. 4.8.
It indicates that the minor presence of Pb has the effect on UTS values at higher cold- rolled work hardening levels which could increase the UTS values of material-III and IV
for 50% and 75% cold rolled conditions significantly. So, a deduction may be drawn that the effect of cold-rolling on strength of Cu in presence of Pb is quite different from that of Sn. The overtaking phenomenon at the cold-rolled work hardening level of about 30- 35% is similar to micro-hardness values examined after cold rolling, where the cold rolling level of ~30% was obtained as a critical transition work hardening level as mentioned in chapter 3 in the analysis of micro-hardness investigation.
Fig. 4.8 UTS variation of copper and solder-affected copper alloys against work hardening level at room temperature with constant strain rate of 10-3 s-1.
0 25 50 75
200 220 240 260 280 300 320
Ultimate Tensile Strength, MPa
Cold-rolled deformation, % Cu Cu-Sn Cu-Pb Cu-Sn-Pb
4.4.2 Effect of Thermal Ageing
Since mechanical properties of a metal or alloy depend on temperature and treatment, the thermal ageing that was done to observe the effect on UTS of work hardened copper based materials of 75% cold rolled condition is presented in Fig. 4.9. The UTS values of all four sample materials have been observed to be increased initially with the rise of ageing temperature as indicated black, red, green and blue lines for material-I, -II, -III and -IV, respectively. The four sample materials after thermal ageing at 100℃ have shown their UTS values as 263.5 MPa, 316.4 MPa, 278.7 MPa and 332.8 MPa, respectively. The UTS values of all four samples materials (material-I,- II, -III and -IV) have reached to the maximum after thermal ageing at 150℃. Similar pattern of tensile behaviour was also observed for thermal ageing with commercial copper after cold rolling [24].
Fig. 4.9 UTS variation of copper and solder-affected copper alloys subjected to work hardening of 75% cold rolling with respect to the ageing temperature, while samples were aged isochronally for 1 hour and UTM was operated at strain rate of 10-3 s-1.
0 50 100 150 200 250 300 350 400 450
120 140 160 180 200 220 240 260 280 300 320 340
Ultimate Tensile Strength, MPa
Ageing Temperature, oC
Cu Cu-Sn Cu-Pb Cu-Sn-Pb