Mechanical behavior of solder affected copper-bulk subjected to work-hardening and thermal treatment

Abstract

The reuse potential of solder affected scraped/waste copper is investigated in the present research under the influence of work hardening and thermal treatments. In fact, the urge of using old/scraped copper to manufacture new components necessitates useful characterization of various properties to explore its reuse potential. In this context, a number of physico-mechanical properties of interest, namely, hardness, strength, conductivity, thermal stability, corrosion resistance, wear resistance, microstructure, etc. are chosen to characterize the solder-affected copper materials as well as analyze their possible changes/improvements under the influence of work-hardening and thermal treatments. Since the old/waste copper usually contains little amount of lead and tin, three additional sample materials, such as, copper ingots, high copper-tin alloy, and high copper-lead alloy are taken into consideration to ascertain the influence of individual solder elements on the properties of the material in focus. It is observed that addition of trace amount of tin and lead can significantly influence micro-hardness, tensile strength, yield strength, fracture elongation, elastic modulus, conductivity, thermal capacity, etc. Micro-hardness and strength values of all four sample materials have been found to be increasing in curvilinear pattern with the rise of cold-rolling level up to 50~55% and then get flatten, and cold-rolled work-hardening level of ~35% is found to be a critical deformation level where the hardness/strength of copper-tin alloy supersedes pure copper and copper-tin-lead alloy supersedes copper-tin alloy. Thermal ageing has lowered hardness, ultimate strength, yield strength and elastic modulus, but increased fracture elongation at the elevated temperatures. Strain rate variation have affected all the tensile behavior of the sample materials. Wear tests haveshown that the presence of a little amount of tin has increased the hardness and improved the wear resistance of copper, while similar amount of lead in copper has reduced the hardness but increased the wear resistance. General perception of ‘the harder the wear resistant’ has been found to be matched partially. But copper-lead alloy shows high wear resistance even with lower hardness values. Coefficient of friction (COF) values have revealed non-linearly gradual increasing trend at the initial stage and after certain sliding distance COF values of all four sample materials have reached to some steady state level. The solder affected copper has shown its COF to be in between that of copper-lead alloy and copper-tin alloy with the maximum COF value of 0.533. The corrosion tests carried out in pH varied environments and sea water environment reveal thatthe solder material inclusion has increased the corrosion rate for 36 days immersion by about 14% and 60% respectively. However, corrosion rates have been found mitigated considerably after 75% cold rolling. Electrical conductivity of copper falls drastically with the addition of only about 1% tin or 1% lead, which increases little initially after cold-rolled deformation up to ~35% and start falling again with the rise of deformation levels.Shifting of endothermic peak has occurred and thereby recrystallization temperature of copper is transformed as an effect of alloying due to inclusion of tin-leadsolder in copper as well as work hardening as observed through differential scanning calorimeter. Micrographs obtained through optical microscope and SEM have confirmed that the changes in crystallinestructuresand formation of intermetallics due to inclusion of solder elements in copper are responsible for the variation of electro-mechanical properties in relation to cold rolling and thermal ageing. Therefore, the present study implies higher suitability of solder affected copper at the leftover state for applications in manufacturing a number of mechanical engineering products likemarine propellers, bush/liner of shaft bearings, valves, stern glands,fittings of fire-main lines, fire hydrants, cooling water lines, etc.