Study On The Processing Technology And Quality Defects Of ACR Copper Tubes

Air Conditioner and Refrigerator copper tubes are generally used for industrial condenser,evaporator and important external connecting tubes of heat exchanging system,which the diameter is equal or greater than 16 mm.The casting-rolling method possesses the advantages of low cost of initial investment,low production cost and high production efficiency.Therefore,in the premise of assuring the quality of products,many enterprises consider that the medium and large diameter ACR copper tubes were attempted to be produced by casting-rolling method instead of the conventional extrusion method,in order to solve the problem of the limitation of the length and output of copper tubes produced by traditional extrusion process.This thesis mainly studies the influence of the two producing methods on the quality of ACR copper tubes and the solutions of the quality defects in the actual machining processes of ACR copper tubes.Two kinds of ACR copper tubes with final dimensions of ?19×1 mm were produced by one of copper enterprises.The production of each step ACR copper tubes which produced by casting-rolling and extrusion method were separately tracked and sampled in the production line.The forming quality of ACR copper tubes that manufactured by two producing methods were evaluated comprehensively by comparing and analyzing its chemical composition,microstructure,mechanical properties,surface quality and dimensional precision.The results show that the chemical composition,microstructure evolution and mechanical properties of ACR copper tubes produced by casting-rolling method were similar to the traditional extrusion method.In addition,the microstructure was more uniform and the grains were finer of the tubes formed by three-roller rolling process.With the increasing of drawing passes,the variation range of the ductility and micro hardness of cast rolling copper tubes were smaller,and the mechanical stability of finished tubes were improved by casting-rolling method.The surface brightness of ACR copper tubes manufactured by casting-rolling method is high,the roughness is low and its average residual stress was much less than the extrusion tubes.Furthermore,the size precision of the casting-rolling ACR copper tubes is higher and casting-rolling method could significantly promote the rate of qualified products.The surface cracks and jump lathe ring defects that appeared in the machining processes of the ACR copper tubes were analyzed by experimental methods,theoretical analysis and finite element simulation.The results indicated that the forming mechanism of surface cracks begin with inter-granular fracture(hot crack)and then followed by trans-crystalline fracture(cold crack).The main reasons of the development of two kind of cracks were that the crystallizer was blocked and harmful gas could not be vented timely in the liquid copper,which would become crack source on the surface of continuous casting tubes.Moreover,the inner surface of the crystallizer was rough,which could increase drawing resistance,and the crack formed at the surface of tubes,then cracks expand to its interior in the process of cooling.The jump lathe ring on the surface of copper tubes was actually non-uniform wall thickness defect,its formed by the plug moved out of the front limit position or rear limit position,so that causing the floating plug drawing system instability.When the plug position is over the front limit position,the copper tubes was excessively extruded by drawing die and plug,forming localized necking zones;When the plug position is over rear limit position,the plug can't contact the tube,hence the thickness of the tubes were increased and there will be convex zones at the tube surface.The optimum drawing process parameters were obtained by the finite element simulation: friction coefficient ?=0.05,drawing velocity ?=90m/min,transition arc radius R=5mm,drawing external die taper angle ?= 15 °and plug taper angle ?= 10°.