Mechanical and Microstructural Evaluation of Plastically Deformed Brass

The mechanical properties as well as microstructure of Cu-30.6 wt% Zn alloys containing 0.01 wt% lead has been investigated. The brass alloy was cast and then cold rolled at various percentage reductions: 20%, 25%, 30%, 35% and 40% followed by stress relieving annealed at 450 ̊C. Hardness, ultimate tensile strength (UTS) and impact toughness values were evaluated at every stage of reduction. It was discovered that the strength, hardness and impact energy all increased with increased percentage reduction. Finally, the optical microscope of the samples was done and the result achieved correlated with the mechanical properties.


INTRODUCTION
Copper and its alloys are widely used in various fields of applications due to their excellent mechanical and functional properties and this has made it attractive to industries and researchers. It has been reported that addition of elements such as zinc, silicon, iron, lead, aluminum, manganese to copper have tremendous effects in improving the mechanical properties (Vilarinho et al.,2005;Garcia et al.,2010). However, the functional properties such as electrical and thermal conductivities reduces when copper is alloyed with other elements due to increase in thermal and interfacial resistance.
Brass has attractive combination of properties such as good corrosion resistance, good wear resistance, high thermal and electrical conductivities which are required in many components of automotive, electrical, valves and fittings (Garcia et al., 2010). In all of these components, the brass is usually shaped into semi-finished product by means of extrusion, rolling, stamping, cold forming or machining. To enhance the machinability of brass, 1-3% lead is usually added to reduce chip fracturing, cutting force, tool wear rate, improve machining rate ,surface finish and ease of (Bursikova et al.,2002;Kumar et al.,2007). The machinability of brass is improved by addition of lead, sulfur, tellurium and zinc (Tahaet al., 2012) while the machinability deteriorate when tin and iron are added (ASM, 1979;Grzegorczyk et al., 2012). Brass containing 2wt.% lead have improved machinability with load by acting as a microscopic chip breaker and tool lubricant, thereby increase the brittleness of the alloy. The solubility of lead in brass alloys is very low and for that reason it is found in microstructure as dispersed globules within the material. It also acts as a lubricant thereby decreasing the frictional coefficient between the tool and the material, creating discontinuity that enhances chip formation, reducing cutting force and tool wear rate (Whiting et al.,1999;Saigal et al.,1996).
The global environmental concern on the lead level release in drinking water has call for the need for alternative source for the replacement of lead in leaded brasses. And this has provided an impetus for the development of lead-free brass alloys (Fontaine and Keast, 2006). Element such as bismuth next to lead in the periodic table and with almost the same properties is expected to play the same role of lead in the brass alloys without adverse health effects. Selenium boosts the effect of bismuth, allowing Bismuth concentrations to be used. However, the ductility of various brass containing bismuth and selenium is lower than the conventional leaded brass (Peters, 1997). According to Michels (2002) the presence of Bismuth at the grain boundaries of copper can cause embrittlement of the copper leading to reduction in ductility of the free-leaded brass alloys. However, Whiting et al.(1999), reported that the presence of tin can prevent the segregation of bismuth to the grain boundaries but it is not clear as regards to how the presence of tin prevents the segregation of bismuth.
The deformation characteristics of brass has been studied by Verma et al.(2013) .The authors affirmed that higher copper content (larger than 60%) are needed to produce products by cold working so as to have enough formability. Higher zinc content in the brass promotes brittleness and optimal mechanical properties are displayed in brass containing 30% zinc, which is characterized by considerable plastic properties together with high tensile strength and hardness (Nowosielski, 2001).
The recrystallization annealing of CuZn3O was studied by Ozgowicz et al. (2008) and the increasing temperature of recrystallization annealing within the temperature range of 300-650ᵒC significantly enhanced the formability of the brass. The plasticity increased distinctly in this brass after annealing in the range of temperatures from 300ᵒC-400ᵒC. Also the recrystallization annealing brass with rolling reduction of 15.8% within the temperature range of 450ᵒC to 650ᵒC results in a drop of the tensile strength by about 50MPa and yield strength to about 100MPa (Ozgowicz,2008). The hardness of cold rolled brass decreases with decreasing degree of the cold-rolling reduction and the rising temperature of annealing. The brass CuZn30 after cold plastic deformation and recrystallization annealing was found to have fine-gained microstructure of α-solution with characteristics twin crystals due to annealing (Nowosielski et al., 2006;Whiting et al., 1995). Stack fault energy (SFE) plays an important role in the plastic deformation of metal alloys. Brass which has FCC structure exhibit low SFE (Danaf et al.,2000) and the stack fault energy decrease rapidly when the zinc content of brass increases. The optical microscopy investigation of the shear deformed samples in 70/30 brass showed an absence of any micro scale shear bands. Besides, the amount of deformed twinning in the shear band samples was found to be comparable with that in simple compression. Thus twinning and low stack fault energy play a vital role in texture evolution as the low stack fault energy promotes the condition for twinning and twinning in turn promotes increased strain hardening.
Plastic deformation involves the breaking of limited number of atomic bonds by the movement of dislocations. However, the movement of dislocations allows atoms in crystal planes to slip pass one another at much lower stress levels (Konieczny and Rdzawski, 2011). Several methods of plastic deformations have been invented with the aim of imposing an extremely high plastic strain on the material which results in structural refinement and strength increase without changing the dimensions of the specimens (Shaarbaf and Toroghinejad, 2008). The microstructure of a material influences the properties of the material and variation in microstructure can significantly change the strength, toughness, hardness, ductility and even thermal and electrical properties (Konieczny and Rdzawski, 2012). To change the microstructure of a material, a combination of mechanical deformation and heat treatment known as thermomechanical heat treatment may be used. So far, little work has been done on brass containing relatively low percentage of lead and the microstructure influence on the mechanical properties is still a subject of debate as some researchers established that increase reduction in cold rolling reduces the strength and hardness of the brass containing low weight percent of lead.
In this study, as-cast Cu-30.6wt%Zn alloy containing 0.01wt% lead was subjected to mechanical deformation cold rolling in the reduction range of 20 to 40% and then heat treated to a temperature of 450ᵒC. The mechanical properties as well as microstructural changes were evaluated.

Material
The brass sample used was obtained from a local metal market in Owode Onirin, Lagos State Nigeria. The allow composition was measured and charged into a crucible furnace and melted. The molten metal was poured into a mold to produce 18mm diameter brass samples. The as-cast cylindrical specimen of 150mm length and 18mm diameter were cold-rolled with reduction range of 20-40% (that is, 20%, 25%, 30%, 35%, and 40%). The cold-rolled samples were thereafter annealed at 450°C.

Tensile Testing
Tensile tests were carried out using a universal tensile testing machine with a capacity of 25kN load at the Federal Institute of Industrial Research, Oshodi (FIIRO), Lagos, Nigeria. Specimens for tensile test were machined to a standard shape with a gauge length of 40mm, total length of 60mm, gauge diameter of 5mm and width of grip section of 10mm. The tensile test which is a destructive test was applied to the test specimens with increasing tensile force until the material fractured and corresponding readings were recorded. Strain rate of the tensometer was 40mm/min.

Hardness Testing
Brinell hardness testing was conducted on samples at FIIRO. A load of 5000N was applied on the specimen for 10 seconds dwell time using 10 mm ball indenter and the indentation diameter was measured using a digital micrometer screw gauge. The average value was calculated from three measurements considered from different areas on sectioned specimen surfaces. Hardness values was estimated from the diameter of indenter, diameter of indentation and magnitude of applied load by the formula below,

√
Where HB is the Brinell hardness number in kg/mm 2 , D = diameter of indentation in mm, d= diameter of indenter in mm, P= applied load in Kg.

Heat Treatment
Stress relief annealing was conducted on the samples by heating the as-cast samples to 450°C in a muffle furnace and allowing them to cool to room temperature inside the furnace.

Microstructural Analysis
Samples for microstural analysis were obtained by cutting and sectioning of the heattreated materials. Sectioned surfaces were ground, polished, with the aid of powder abrasives and etched at the Metallurgy laboratory of University of Lagos, Lagos, Nigeria using the grinding/polishing machine. Polishing was performed using the standard procedure while the mixture of 5 g FeCl 3 and 50 ml HCl in 100 ml distilled water was used for etching. The identification of the heterogeneous disperse phases was carried out using the optical metallurgical microscope. Table 1.0 shows the weight percentages of the individual elements which make up the brass alloy. It clearly indicated that the brass contains 30% of zinc which are normally found in brass commercial alloy. The percentage composition of lead in the alloy is relatively low as it is less than 1% and that implies that the machinability of the alloy will be low as compared to those with relatively high lead content. However, the brass alloy will be environmentally friendly and has global utilization.  Figure 1.0 shows the hardness of rolled and stress relieved brass as a function of percentage reduction. It was found that the hardness of the brass increases with increase in the percentage reduction. Also, the rolled brass has higher hardness value than the rolled and stress relieved with the same amount of reduction. This is expected as percentage reduction involved cold working of the metal which generate dislocations within the crystal of the material. The dislocation density increases as the percentage reduction increases and the pile up as well as interaction of the dislocations during deformation results in increasing hardness of the brass alloy. However, stress relieving as thermal reduction in stress involves the rearrangement of dislocations which occurred as a result of mechanical working thereby reducing the dislocation density. It should be noted that the residual stresses, which occurred during cold working of the metal increase the total internal energy of the metallic structure and also the presence of dislocations increases the total internal energy of the structure. This hardness trend is in line with the observation of Askeland and Phule, (2006), that the higher the extent of cold working, the higher would be the level of internal energy of the material. Moreover, mechanical rolling involves application of compressive stresses at the surface of the material while the center of the material is under tensile. The higher the level of compressive stresses at the surface, the more the hardness of the materials. Again, particle size reduction due to mechanical rolling also contributed to the increase in hardness of the brass alloy as the percentage reduction increases. As reported by Nowosielski, (2001) and Song et al. (2011), higher zinc content (30%) in combination with higher degree of mechanical deformation can significantly increase the mechanical properties of brass. Thus, increase in dislocation density, high zinc content and particle or grain size reduction due to rolling all contributed to the high hardness value of the deformed brass.  Figure 2.0 shows the impact energy of the rolled and stress relieved brass as a function of percentage reduction. The impact energy was found to increase steadily with the deformation until 35% reduction. Further increase in percentage reduction from 35% to 40% reduction causes sharp increase in impact energy from 9.5J to 21J in the case of rolled brass. This is expected since the total energy increase with increase with strain hardening by cold working (Verma et al., 2013. However, the impact energy of rolled brass is lower than that of rolled and stress relieved brass as shown in Figure 2.0. This is because the stress relieve heat treatment would have eliminated the work hardening effect on the rolled brass (Askelend and phule, 2006).  Figure 3.0 shows the ultimate tensile strength as a function of percentage area reduction of the brass. The ultimate tensile strength was found to increase linearly with percent area reduction from 20 to 25% and thereafter the ultimate tensile strength had lower slope until 30% reduction was reached where it increases with further % reduction. Also the rolled and stress-relieved brass had lower ultimate tensile strengths as compared to the brass that was plastically deformed by rolling without stress relieving. It was estimated from Figure 3.0 that at 25% reduction, the ultimate tensile strength of rolled brass is about 38% higher than the rolled and stress relieved brass. However the trend of strength increase in the two brass conditions (rolled and stress relieved) are similar as the ultimate tensile strength increases with percentage reduction.  Figure 4.0 shows the optical micrographs of the cast brass alloys at various percentage reduction of 20%, 25%, 30%, 35% and 40% respectively. The primary alpha phase (α) is the zinc while the Beta phase (β) represents the copper phase in the alloy as shown in Figure 4.0 (A). It was observed from Figure 4.0 that, with increase in percentage cold roll of the brass, the grain sizes are increased and they also become smaller thereby making it hard for the grains to slide over another which has a great effect on the tensile strength as well as the hardness of the brass. Figure 4.0 (A), shows an optical micrograph of the control sample of brass and it may be seen that more-or-less rounded particles of zinc are crystallized, which are surrounded by fine eutectic copper (brighter parts) together with black spots which could be the defects introduced during casting of the alloys. Here, copper has networked structure. It is seen that as the percentage reduction increases, the figures show more refinement of the eutectic copper particles. It may be seen that the degree of refinement of the eutectic copper increased as the percentage cold roll increases. Figure 4.0 (B) shows the optical micrograph of the brass at 20% reduction which indicated an increase in defects due to mechanical deformation (rolling). Defect such as vacancies and dislocations are introduced as a result of thermo-mechanical processing and this has influence on the mechanical properties of the material. As deformation increased further, Figure 4.0 (C), more defects are introduced which became conspicuous with orientation of the grains. The defects persisted even up to 30% reduction as shown in Figure 4.0 (D). In Figure 4.0 (E), the concentration of defects was observed to reduce considerably as a result of randomization of the microstructure during rolling up to 35% reduction. Again new phase β' evolved from the primary phase (α and β) as a result of simultaneous rolling and heat treatment up to 450ᵒC which lead to two phenomenon of recovery and recrystallization. The grain sizes was also found to reduce after the recrystallization of the new phase.

Table1.0 Weight percentages of different elements present in the brass alloy
In Figure 4.0 (F), the defects were almost completely eliminated with the new phase concentration increase substantially. This clearly indicates that the new phase contributed to increase in hardness and strength as indicated in Figure 1.0 and 3.0 respectively. Therefore, precipitation strengthening could be the phenomenon accounting for the increase hardness and strength after 30% reduction rolling and stress relieving. (Askelend and Phule, 2006).

CONCLUSION
The improvement on the hardness and tensile properties of the brass after cold rolling was due to strain hardening by the cold working at the expense of ductility of the material. Annealing the cold rolled brass at 450ᵒC was found to reduce the dislocation density as a result of annihilation and polygonization of dislocations and therefore, the cold-rolled samples displayed higher hardness and tensile strength than the cold-rolled plus stress-relieved samples. At 40% reduction, the hardness of cold-rolled sample was 28HRC while that of cold-rolled plus stress-relieved sample was 11HRC. Also the strength of the cold-rolled sample at 40% was 268.18N/mm 2 while that of the stress-relieved was 176.62N/mm 2 . However, the impact energy increased as the cold-rolled samples were stress-relieved due to reduction in dislocation density.