Fundamental Research And Preparation Of An Environmentally Friendly, Free-Cutting White Copper Alloy

Abstract:Traditional white copper alloy Cu-Ni-Zn-Pb has noble silvery brilliance, great mechanical property, wonderful machinability, fantastic corrosion resistance and high tarnish resistance property. It has been widely used as material for coins, ornaments and decorations, as well as in ocean, power, medical and chemical industrial. However, it contains Ni and Pb which are both harmful to environment and humans. The use of Ni and Pb has been strictly forbidden in most European and American countries, especially for Ni-contained product that has direct contact with skin. Therefore, it becomes urgent to develop a white color copper alloy that has excellent comprehensive performance but without Ni and Pb.In order to replace elements Pb and Ni as well as improve alloy corrosion resistance, this paper developed a silver alloy Cu-12Mn-13Zn-1Sn-1A1-0.3Si-0.1Ce (wt.%) mainly composed of Mn and Zn. A little Al, trace Si and rare-element Ce were also included. Related mechanism and performances were studied, including machinability, mechanical property, cutting property and corrosion resistance. The research contents and conclusions in this paper are summarized as follows.1. Without Ni and Pb, alloy Cu-12Mn-13Zn-1Sn-1A1-0.3Si-0.1Ce (wt.%) was designed and prepared so that it has very similar performance as traditional white copper alloy Cu-15Ni-24Zn-1.5Pb (wt.%) in working hardening rate, mechanical property, white chromaticity and tarnish resistance property. Compared with the tranditional alloy, it has better cold rolling deformation ability, cutting property and corrosion resistance in artificial sweat.2. The constitutive equation has been built for describing the hot deforming process of alloy Cu-12Mn-13Zn-1Sn-1A1-0.3Si-0.1Ce (wt.%), which is ?=e21.823[sinh(0.00825?)]4.348exp(-201.020/(RT)). Two processing graphs have also been built with strain of0.5and0.7, respectively. Two feasible hot machining regions620?-800?/0.01s-1-0.56s-1and670?-770?/0.01s-1-10s-1have been found. The most suitable condition for hot deforming the designed alloy is750?/10s-1.3. In order to improve the cutting property of free Ni&Pb white alloy, element Si was added into Cu-Mn-Zn. By studying the action mechanism, it has been found that the combination of Si and Mn can generate the second phase particles mainly composed of Mn3Si phase, and about1?m in size. As the content of Si in alloy approaches0.3wt.%, the content of well distributed Mn3Si phase that is larger than criteria dimension could be a lot more than the content of Pb in alloy Cu-15Ni-24Zn-1.5Pb (wt.%). As a result, the designed alloy obtains better surface roughness and requires smaller cutting force.4. Electrochemical test and X-ray Photoelectron Spectroscopy analysis were made to make research on the corrosion process of the Cu-12Mn-13Zn-1Sn-1A1-0.3Si-0.1Ce (wt.%) alloy and its product layer construction in artificial sweat. There was a passive film generated on the alloy surface, after the immersion in artificial sweat for some time. A protective and compact oxide film then came out with time, composed of Cu2O, A12O3and SiO2. The protective film grew thicker and compacter, deducing the alloy corrosion rate. Other electrochemical parameters changed regularly with immersion time, including the polarization resistance Rp, the film resistance Rfilm, the film capacitance Cfilm and CPE coefficient Q0.5. It revealed the evolution law of corrosion product of the Cu-12Mn-13Zn-1Sn-1A1-0.3Si-0.1Ce (wt.%) alloy after a immersion in NaCl solution and in NaCl spray. At the very beginning stage, the alloy immersed in solution was corroded badly, covered with lots of oxide and found generating chlorides. About60%alloy hasn't been oxidized in NaCl spray, and with no chlorides found. As the immersion time went on, corrosion product was generated under two conditions, both with inner and outside layers. The outside layer of immersion product is well covered with greyish-green graininess basic copper chloride. However, the composition of corrosion layer in NaCl spray varies with different dripping sedimentations. The layer with dripping settlement has similar content as immersion corrosion product, mainly composed of blue-green graininess basic copper chloride. The layer without dripping settlement is tan and mainly composed of metal oxide, like the product of air corrosion.