| Aluminum alloy composite tubes are widely used in automotive to replace the traditional copper heat exchanger, so as to reduce the vehicle weight and save energy. Currently high frequency welding aluminum composite strip is mostly used for producing composite seamed tube, but weld affect the assembling of tube and fin, resulting in unstable quality of the heat exchanger products. Seamless composite tube with excellent performance can be produced by extruding composite ingot assembled by3003alloy ingot and4343extruded tube.OM, SEM, TEM, XRD, EDS were used to observe and analyze the microstructures and compositions of the unextruded ingot. The results show that the main phases in as-cast3003aluminum alloy are equiaxed α(Al) and eutectics, which contain bone shape-like αa-Al(MnFe)Si intermetallic compounds. After600℃/5hours homogenization, the second phase dissoluted and spheroidized, the grain boundaries became discontinuous; the supersaturated Mn solid solutions near the grain boundaries desoluted and nano-sheet α-AlMnSi phases precipitated.4343aluminum alloy demonstrates a typical dendritic structure, constituted with a large number of dendritic α(Al) and bone shape-like Al-Si eutectic compositions.By OM, SEM, TEM micro structure observation of the hot-extruded composite tube in longitudinal section, and combined EDS analysis of the interface components, it was found that recrystallization occurred in3003and4343alloy, and rod-like (MnFe)Al6and Al-Si eutectic composition distributed evenly, respectively. There was no significant elements spreading near the interface, but with the high temperature, the Si occurred short range diffusion in local area, and generated some α-Al(MnFe)Si phases. Plastic deformation occurred between the real contact surfaces of the two metals during extrusion process, the oxide film ruptured and the metallurgical bonding was formed, but a small amount of unbound clearance still existed.By tensile tests and hardness tests of the samples of extrusion, cold rolling and cold drawing semi-finished tubes, it was found that the tensile strength and yield strength of the composite tubes and the hardness of the substrate increased, the elongation after fracture decreased with the increasing amount of cold deformation. By OM and SEM observations, we can see that the morphology and distribution of second phases didn’t change; the equiaxed recrystallization grains changed into fibrous microstructure. Because of the decrease in the deformation capacity, the interface was difficult to slide separately and to achieve full bonding.The influence of annealing temperature on the microstructure and the mechanical properties of composite tubes were studied. Tensile test and hardness test results showed that: when annealing at250℃, the mechanical properties of composite tube slightly decreased; between250℃～400℃, the tensile strength and the yield strength range of composite tube and hardness of the substrate decreased rapidly, the elongation after fracture increased; between400℃～500℃, the mechanical properties kept stable.EDS was employed to analyze the elements distribution of the interface area of the400℃annealing sample. It showed that no significant element diffusion occurred, indicating that the temperature has litter impact on element diffusions of the two aluminum alloys. By OM and SEM observation, it was found that in the samples annealed between250℃～400℃, recrystallization occurred in4343alloy, but3003alloy was still composed of fibrous microstructure. The difference of plastic deformation between two alloys made interfacial bonding poor.3003alloy annealed above400℃recrystallized, so the deformation capacity of the two alloys became similar causing a better bonding. Increasing the annealing temperature, recrystallized grains didn’t grow significantly and the small a-Al(MnFe)Si phase precipitated in grains.The composite tubes annealed at400℃possess good microstructure and mechanical properties. The samples annealled at400℃in different time from0.5to3hours, it was found that the tensile test and the hardness test results showed that the mechanical properties of composite tube kept stable; by OM, SEM observation, it was found that both the alloys recrystallized after0.5hour annealing at400℃; with prolonging annealing time, the grains grew up in4343alloy, while in3003alloy they didn’t, so the interface bonding became worse. The annealing experimental results showed that the production efficiency can be improved by increasing the annealing temperature, and get products with good microstructure and mechanical properties at the same time. |
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