CAST lead brass alloy is widely used in sanitary ware, plumbing equipment, instrument, hydraulic valve and so on because of its good mechanical properties and machinability. Although the crystallization temperature range of cast lead brass is small, the filling ability is strong, and there are degassing and deoxidizing effects of zinc and so on, but because brass is easy to form thick columnar crystal in the solidification process, the casting structure is thick, and it is easy to crack in the casting process, it is one of the effective ways to improve the hot crack resistance and compactness by refining the microstructure of cast brass. At present, adding refiner is the main method to refine the as-cast microstructure of aluminum brass. The effect of Boron on the microstructure of aluminum brass has been studied, the effect of Boron and rare earth on the microstructure and dezincification of cast lead brass was also studied. The effect of rare earth and Boron refining on as-cast microstructure and dezincification corrosion property of cast lead brass was studied. 1. Test material and its process test material grade ZCuZn40Pb2 lead brass, chemical composition (mass fraction) is 36.4% Zn, 1.70% Pb, 0.62% , a 1, Cu allowance. Rare earth was added with 10% RE. Ai Master Alloy and Boron was added with cu-4.68% b master alloy. The alloy was melted in a graphite crucible in a well-type resistance furnace. The smelting process of lead brass is as follows: when the temperature rises to 1030 °C, Boron and rare earth refiner are added respectively by Bell Jar method, and the lead brass is fully stirred, and then kept for 10 minutes, the Metal Type Test Rod and the Metal Type Round Cup specimen with the preheating temperature of 350 °C are cast. The Round Cup method is the most simple and direct way to check whether the refining treatment is good or not in casting copper alloy enterprises. The concrete method is pouring round cup sample, pouring the unsolidified copper liquid in the center of the Round Cup sample after the shell is solidified, and obtaining the Shell Cup sample with solidified crystal face. Dezincification test of cast lead brass with corrosion solution of L% CUCL2: solution, solution temperature of (75 ± 10) °C, corrosion time of 24 H, removal after corrosion, cleaning and drying, cutting specimen from center, preparation of metallographic specimen, the effect of refining treatment on dezincification corrosion of cast lead brass was investigated by observing and measuring the depth of dezincification corrosion with microscope. The microstructure was observed by metallographic microscope. The corrosion solution was 5gfecl3 + 5mlhcl + 100ML aqueous solution. 2. Test Results and analysis 2.1 effect of refiners on the microstructure of lead brass fig. L shows the microstructure of lead brass treated with Boron and rare earth. It can be seen that the microstructure of unrefined permanent mold cast lead brass is very thick (fig. LA) . According to the analysis of the phase diagram of copper and zinc, the as-cast microstructure should be α phase (white) + β phase (black) + PB phase, and the PB phase should be distributed as black single particle on α phase or between α and β phase. After RE and B refinement, the α and β phases were refined obviously, and the thick lath α and β phases disappeared after b refinement, and became short lamellar or equiaxed grains (Fig. 1B) . Compared with as-cast microstructure (Fig. 1C) , the microstructure of RE refinement was obviously refined, and the β and α phases were changed from thick long Lath to short thin lath. Therefore, as-cast microstructure of cast brass can be improved obviously by RE and B refining treatment. Under this test condition, the refining effect of as-cast lead brass treated by Boron is better than that treated by rare earth.

2.2 The effect of refiners on the surface morphology and microstructure of shell-and-cup Specimens Fig. 2 is the surface morphology of shell-and-cup specimens before and after refining with Boron in the same furnace. It can be seen that the surface of the shell-and-cup sample cast by the unrefined copper solution is rough (Fig. 2A) , and that there are many small bumps on the crystal surface in the macroscopic view. Fig. 3A is the morphology of the unrefined crystal surface and its nearby structure, the structure of the rough surface is coarse columnar crystal, and the coarse little bump is the columnar dendrite left by the liquid/solid interface during the solidification of copper liquid, thus, the leading edge columnar crystals in the liquid are completely retained, forming a rough surface. Fig. 2B is the surface of the boron-refined Shell Cup sample. Compared with the unrefined sample, the surface is smooth without roughness and small bump. The microstructure of the surface optical microscope is shown in Fig. 3B. The Crystal surface is uneven, but the crystal surface is smooth, the microstructure of smooth crystalline surface is equiaxed crystallization, so refining treatment can change the solidification structure of cast lead brass.

2.3 the effect of refinement on the dezincification of cast brass Fig. 4 is the effect of refinement on the dezincification depth of cast lead brass. As a result of Dezincification, the microstructure of the surface layer is different from that of the inner white layer, which is caused by the corrosion of zinc on the surface in the corrosive environment, the concentration of zinc is decreased, the zinc-rich phase (β phase) in the microstructure is decreased, and the α phase is increased. The dezincification depth of unrefined lead brass is relatively large (Fig. 4A) . The average dezincification depth of unrefined lead brass is 17000um, followed by that of refined rare earth brass (Fig. 4B) . The average dezincification depth is 132.2um, the average depth of the zincification layer is only 57.2 um. The results show that as-cast microstructure can be refined and the dezincification corrosion of cast brass can be effectively restrained. In particular, the effect of Boron refining on zinc-free corrosion of cast brass is more obvious. There are two main mechanisms of dezincification corrosion of lead brass: one is dissolution-redeposition mechanism, the other is preferential dissolution mechanism of zinc, which may be the result of both of them. The preferred dissolution mechanism of zinc is that the zinc on the surface of the alloy dissolves preferentially from the brass, resulting in double vacancies which diffuse into the alloy and zinc alloy component diffuses to the surface until it dissolves. Dissolution. The mechanism of redeposition holds that the dezincification of brass includes two possibilities: one is that copper and zinc are dissolved at the same time on the anode, and when the concentration of copper ion in the solution reaches a certain level, the copper ion is reduced to metal copper and deposited on the surface, as an additional cathode, the dissolution of zinc in the alloy is accelerated; the other is that for a short initial period of time, zinc preferentially dissolves into the solution, and as the diffusion of zinc becomes difficult, copper and zinc will dissolve simultaneously, accompanied by the return deposition of copper. The effect of Boron refining on the dezincification of brass is mainly due to the addition of Boron into ZCuZn40Pb2 lead brass, which can fill in the grain boundary and double vacancies, increase the bonding force, obstruct the migration channels of grain boundary and double vacancies, and decrease the migration velocity of zinc, thus the corrosion rate of ZCuzn40Pb2 lead brass is reduced. Re refining can eliminate the impurities in the brass substrate. When ZCuZn40Pb2 brass contains impurity elements such as O and s, the potential difference between them and the substrate electrode is large, which forms the galvanic cell and accelerates the corrosion, rare earth can form a high melting point, low density rare earth compound with O and S, which is easy to float up to the slag, thus purifying the matrix, reducing the corrosion rate, rare earth added to brass, forming a very thin and dense rare earth oxide on its surface, it can prevent the diffusion of Zn and Cu atoms to the solution, reduce the dissolution rate, thus delay the corrosion and improve the dezincification corrosion performance of the alloy.

3. Conclusion (1) the as-cast structure of ZCuZn40Pb2 can be refined by RE or B refining treatment, and the thick long strip structure of brass can be changed into short thin strip or equiaxed crystal structure by refining treatment. The refining effect of B refining treatment is better than that of adding re refining treatment. The surface of the shell-cup sample can be changed from rough to smooth by refinement. The rough surface layer structure is columnar crystal and the smooth surface layer structure is equiaxed crystal. (2) ZCuZn40Pb2 alloy can inhibit the dezincification corrosion of as-cast brass by RE or B refinement. The ZCuZn40Pb2 alloy can inhibit the dezincification corrosion more obviously by B refinement than by RE refinement.

 Source: Copper Alloy Casting