2.1 Preamble
Copper and its alloys may undergo work hardening in the manufacturing process of engineering products or due to operational and environmental conditions in their life span resulting some effect on the microstructure. Besides, thermal ageing may happen as a consequence of environmental or operational conditions of the products being used, which may have certain impacts on the desired properties of the new products while the scraped copper is used. Sometimes, inclusion of impurities like soldering elements, such as, tin, lead, etc. together with the occurrence of work hardening and thermal ageing may demonstrate improvement in some properties, which may be useful in the specified applications. Again, their effect on few properties may be significantly adverse for desired applications. As a result, the functional ability of products made of old/scraped copper in respect of desired properties may even cause the product to be rejected by the users. Considering these issues, the study materials have been selected, collected and prepared. Accordingly, suitable methods have been carried out to characterize the important properties under work hardening and thermal ageing conditions to explore the reuse potentials of solder affected copper.
2.2 Materials for Study
Copper wires and machine parts after extensive use of about 50 years or more have been taken as the material of focus to carry out the present study. In this context, about 80 kg of waste copper wires and machine parts were collected from different sources such as renovated buildings, factories, ship breakings, national grids, etc. and melted in the furnace. Once casted materials are ready, the chemical composition was examined and it has been observed that the old copper collected from different sources contains not only pure copper metal but also a little percentage of lead and tin along with a negligible amount of few other elements. Table 2.1 shows the range of elements found in the chemical composition tests of old copper wires collected from different sources.
Table 2.1 The range of elements in chemical composition of melted old copper wires collected from different sources (mass fraction %)
Element Composition Range
Copper (Cu) 97 – 99 %
Tin (Sn) 0.8 – 1.5 %
Lead (Pb) 0.8 – 1.4 %
Silicon (Si) 0.0 – 0.18%
Phosphorus (P) 0.0 – 0.27%
Iron (Fe) 0.0 – 0.06%
In the present study, in addition to the SnPb-solder affected copper, three complementary sample materials, such as, copper ingots [Cu 99.99%], copper-tin alloy [Sn 1.2%] and copper-lead alloy [Pb 1.2%] are taken into consideration to ascertain the influence of individual solder elements on the properties of solder affected copper.
Here, for identification purposes, pure copper is named as material-I, high Cu-Sn alloy as material-II and high Cu-Pb alloy as material-III. For the same identification purpose, the main alloy under investigation, that is the solder-affected old Cu, is named as material-IV.
2.3 Sample Preparations 2.3.1 Material casting
Here, pure Cu was collected from commercial sources as bulk copper ingots (150 kg). Material-II (Cu-Sn alloy) and material-III (Cu-Pb alloy) were developed through casting in the foundry workshop of BUET, where the amount of tin and lead were taken as similar to the amount found available in old/scraped copper wires (please see table 2.1). So, 0.5 kg tin was used with 50 kg copper to carry out casting of material-II and similarly 0.5 kg lead was used with another 50 kg copper to develop material-III by casting.
For the mentioned casting purpose, steel moulds of size 350mm×40mm×25mm were prepared, coated inside with a film of water-clay and preheated at the temperature of 200°C. The moulds were then placed in sand box to pour the melted copper alloys. The melting was carried out using a clay-graphite crucible in a natural gas fired furnace under suitable flux cover while the final temperature of the melt was maintained at about 1300°C. Since the milting temperatures of tin and lead are 328℃ and 232℃ respectively, they were added while copper is fully melted for the development of both material-II (Cu-Sn alloy) and material-III (Cu-Pb alloy) as shown in fig. 2.1. The melts were then allowed to be homogenized under stirring at 1200°C and poured in that preheated moulds to have the cast materials. It may be mentioned that material-IV, i.e., solder affected copper was also melted in the same furnace and casted in the same moulds. The cast materials were then machined to skin out the oxide layer from the surface and made ready for chemical composition test and other purposes.
Fig 2.1 Casting of sample materials in BUET Foundry Workshop: (a) moulds in sand, (b) furnace running, (c) temperature monitoring.
2.3.2 Composition test
After developing all four materials, i.e. material-I (pure Cu), material-II (Cu-Sn alloy), material-III (Cu-Pb alloy) and material-III (solder affected Cu), flat bars of dimensions 3001512 in millimeter from each type of materials were prepared through cutting and machining, keeping the effect on surface/edge grains minimal. These flat bars were at first homogenized for eight hours at a temperature of 500°C for the reduction of chemical segregation and then solution treated for two hours at a temperature of 700°C to improve their workability. Thereafter, their chemical compositions were examined using X-Ray Fluorescence (XRF) spectrometer (model: Olympus DPO-2000-CC). The results found in the composition tests of four sample materials are presented in table 2.2.
(a) (b) (c)
Table 2.2 Chemical composition of the materials under study (mass fraction %)
Materials /Elements
Material-I (Pure Cu)
Material-II (Cu-Sn Alloy)
Material-III (Cu-Pb Alloy)
Material-IV (Cu-Sn-Pb Alloy)
Cu 99.9862 98.4555 98.4334 97.1132
Sn - 1.1335 - 1.2572
Pb - - 1.1973 1.1949
Si 0.0026 0.1415 0.1129 0.1726
P 0.0112 0.2695 0.2564 0.2621
2.3.3 Cold-Rolled Work Hardening
When a metal is plastically deformed at temperatures that are low relative to its recrystallization, i.e., less than one-half of the melting point measured on an absolute scale, it is said to be cold worked. For pure copper, the melting point is 1084°C, thereby it is expected to have the completion of recrystallization at about 370°C. Most of the energy expended in cold work appears in the form of heat, but a finite fraction is stored in the metal as strain energy associated with various lattice defects created by the deformation. Cold working is also known to increase greatly the number of dislocations in a metal. Since each dislocation represents a crystal defect with an associated lattice strain, increasing the dislocation density increases the strain energy of the metal.
There are number of means and methods for the cold work of metals. However, the present study has taken the cold-roll method to carry out the cold work of sample materials. For this purpose roller machine available in BUET was employed and flat bars of size 300mm15mm12mm were used to pass through the roller machine. The flat bars before rolling, the rolling machine and sample materials after rolling are shown in fig.
2.2.
Fig. 2.2 Cold rolling operations: (a) flat bars before rolling, (b) the rolling machine and (c) sample materials after rolling.
In this regard following checks were followed to carry out rolling operations:
i. The surfaces of the work pieces were made completely free of scale and dirt.
ii. The roll gap was set perfectly parallel to ensure breadthwise uniform decrease in thickness.
iii. Roller machine was operated at the minimum speed of rotation with the feed rate of maximum 10 mm/s so that excessive heat generation could be avoided.
iv. Minimum roll feed was maintained throughout. As such feed was always less than 0.1 mm for each roll pass.
v. Homogeneous flow of work piece was maintained keeping steady feeding of flat bar as well as constant speed of roller machine.
vi. Intermittent breaks of rolling were given to avoid the rollers to be heated up.
(a) (c)
(b)
vii. Work piece temperature was measured very frequently during rolling operations to ensure the thermal level nearer to ambient condition.
Basically, the maintenance of work piece temperature during work hardening was very crucial to keep microstructural integrity of each work piece. Therefore, the applied feed was bare minimum against each roll pass along with number of pauses in between the roll passes. By this way, the bar thickness was reduced slowly and gradually with the increase in pass number of the cold rolling process. Samples were divided for different cold-rolled work hardening level ranging from zero to 75% with the incremental work hardening step of 5% and collected after attaining the desired work hardening level accordingly. In addition, a large number of samples were collected at four major work hardening levels, such as, zero (as-cast condition), 25%, 50% and 75% cold-rolled deformation level to observe the effect of isothermal and isochronal ageing on the electro-mechanical properties at these work hardening conditions. Here the cold work percent is calculated using the following expression.
% cold work = =
Where
𝐴 = Original crossectional area of bar
𝐴 = Final crossectional area of bar after cold work
𝑡 = Original thickness of bar (Keeping breadth unchanged)
𝑡 = Final thickness of bar after cold work (Keeping breadth unchanged)
2.3.4 Specimen Development
After cold-rolled work hardening, all four study materials have been cut into pieces in suitable sizes to carry out series of experiments for various tests. The specimen were mainly of four groups and their number was to meet the requirements all tests with few extra for contingency as shown in table 2.3.
Table 2.3 Specimen sizes and groups
Sample Groups
Sample size Purpose No. of samples for each test
Total Samples
Group-i (Dog bone)
6x3 mm with GL=25mm
ASTM E8 standard to investigate tensile properties
24 384+
Group-ii (Square piece)
15x15x3 mm For conductivity, hardness, thermal stability,
microstructure, etc.
48 768+
Group-iii (Flat bar)
60x10x3 mm For corrosion test 24 384+
Group-iv (Cylinder)
12 mm length and 5 mm diameter
ASTM G99-05 standard for wear test
24 384+
2.3.5 Thermal Ageing
Thermal ageing was done using a box muffle type laboratory furnace of model: KJ- M1700-12LZ as shown in fig. 2.3. The total prepared samples were divided into three categories, such as, for thermally untreated (as cast), for isochronally treated and for isothermally treated. Samples of the first category were kept free from heat treatment to carry the series of experiments to observe the individual effects of alloying elements and work hardening. Samples of the second category were heat treated isochronally in the laboratory furnace at various temperatures, such as, 25oC, 100oC, 150oC, 200oC, 250oC,
300oC, 350oC, 400oC, 450oC and 500oC for a period of one hour. Experiments were carried out with them to observe the effect of ageing temperature variation on tensile behavior, micro-hardness, electrical conductivity, microstructure, etc. Following the experimental results of tensile strength and micro-hardness obtained for isochronally aged samples, the remaining samples as the third category were heat treated isothermally at 150oC, 200oC and 250oC corresponding to the higher strength and micro-hardness values with the variation of ageing period from 15 minutes to 240 minutes.
Fig. 2.3 Artificial ageing using a laboratory furnace in BUET.
2.4 Experimentals
Group wise prepared samples were used at different laboratories in BUET. A series of experiments were carried out to investigate the effects of trace element alloying with copper, cold work, thermal ageing etc. on number of electro-mechanical properties, such as, tensile strength, elastic modulus, micro-hardness, electrical conductivity, thermal capacity of the sample materials. The experimental details for each category test along with results obtained are described in the subsequent chapters in this report. However, a short description is given here for basic information.
In an attempt to investigate the reuse potential of old copper wires and components, material hardness was chosen as one of the primary mechanical properties to be known in advance. Square pieces of samples from all four materials were divided into different groups and sub-groups to carry out the series of experiments at different work hardening levels from zero to 75% and isochronal as well as isothermal ageing conditions. The
micro-hardness values were actually measured for all categories of samples whether untreated or thermally treated under each temperature of thermal ageing using PC interfaced Digital Micro Vickers Hardness Tester (TM HV-1000DTE) in which 1 kgf load was applied for 10 seconds. Fig. 3.1 in chapter 3 is showing the examination procedure of micro-hardness test. The samples used for hardness test were again used for electrical conductivity test.
Fig. 2.4 Conductivity examination using TMD-101 conductivity meter.
The electrical conductivity values were measured using digital conductivity meter (Model: TMD-101) with an accuracy of ±0.01% IACS (International Annealed Copper Standard). Fig. 2.4 is showing the measurement of electrical conductivity. The measurements of both micro-hardness and electrical conductivity were repeated at least twenty times for each condition of a sample so that the data can be considered as reliable and consistent.
Shimadzu Hydraulic Universal Testing Machine (Model: UH-F1000 kN X) was employed to perform the tensile tests for all four sample materials for each condition.
The samples are of dog bone shape of size 100mm6mm3mm with gauge length (GL) of 25mm following the ASTM E8 standard. For the first round tests, crosshead speed of 1.5 mm/s corresponding strain rate of 1.00×10-3 s-1 was maintained as constant. 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 to maintain the corresponding strain rates 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-1 and 6.67×10-2 s-1. All the test samples for tensile tests were prepared according to the standard specification of
ASTM E-8M. Fractographic observations of the surfaces fractured by tensile test have been carried out in a Field Emission Scanning Electron Microscope.
Friction and wear behavior of the solder affected copper alloy have been investigated using a pin-on-disk Tribometer with ASTM Standard G99-05 under dry sliding, fresh water wet sliding and sea water wet sliding conditions. Here, the counter surface for wear test was a rotating disc made of stainless steel. Wear rates were examined by measuring the loss of weight for different sliding distances using a weighing machine of model:
Sartorius Entris 224-1S having pan diameter of 90 mm with the maximum weighing capacity of 220 g and of 0.1mg precision. Moreover, the volume loss for every wear test run was counter checked by measuring the length of the pin using an ultrasonic thickness gauge of model: CG100 ABDL having measurement resolution of 0.01mm. Thereafter, the volume loss values and densities of the sample materials were used to obtain the wear loss values in µg unit. as well as loss of volume and coefficient of friction values were examined Optical and SEM study of the worn surfaces has also been done to identify the nature of wear as a function of alloying elements.
The calorimetric measurements were carried out as an observance of thermo-chem- ical changes of work hardened (75% deformation level) sample materials from their as- cast condition using differential scanning calorimeter (DSC) of model TA-DCS 2920.
The information on endothermic and exothermic process of the samples has been netted during physical transitions at a constant heating rate in a controlled ambience starting from room temperature (25oC) to 700oC. The specimen of 150 mg for each test was placed in a platinum crucible into the DSC furnace to measure the heat flow in varied temperatures and changes in heat capacity with the heating rate of 20oC per minute.
In the present study, corrosion behaviors of four sample materials were investigated in both fresh water and sea water environments using gravimetric method. Fresh water means the distilled water of pH7. Hydrochloric acid (HCl) was used with the distilled water to develop acidic solution and KOH was used to make alkaline solution for the variation of pH values of the solutions such as pH1, pH3, pH5, pH7, pH9, pH11 and pH13. To observe the corrosion behavior of sample materials, about 6000 liter of sea water was collected from three locations of the Bay of Bengal.
Microstructures were examined using optical electronic microscope (OEM) of model BW-S500. To examine the microstructures, the surfaces of all samples were at first polished SiC abrasive paper with grit number 180, 300, 600, 900, 1200, 1500 and 1800 in succession. After dry polishing, samples were wet polished using alumina paste in presence of water flow and washed with acetone. On completion of surface preparation, samples were etched in H2O2 solution. In addition optical microscopic observation, the surface morphologies of work hardened (75% deformation level) sample materials were investigated using a field emission scanning electron microscope (FE-SEM) of model JEOL JSM-7600F with the magnification of x30000. EDX spectra of samples were recorded by an electron dispersive spectrometer of model JEOL EX-37001 associated with the FE-SEM set up.