Influence Of Cryogenic Treatment On Microstructure And Mechanical Properties Of AlMnFe Alloy

Cryogenic technology has been widely applied in industrial production, the main purpose of this study is to improve AlMnFe alloy by cryogenic process and mechanical properties of alloy's organization, to expand their application fields and prolong the service life. In this experiment with liquid nitrogen as refrigerant, aluminium foils used as heat exchanger's wings were cryogenic treated by gas or liquid method. Through tensile experiment, microhardness test, XRD, TEM and SEM, the influences of different cryogenic temperature and different cryogenic time on the aluminium foil sample's mechanical properties and microstructures were studied, to ascertain the optimization of AlMnFe alloy's cryogenic process parameters. The annealing organizational change and the tensile property after cryogenic pretreatment were also studied. And the variation of low temperature properties and the anisotropy were discussed. According to the experimental results, obtained the following conclusions:(1) The influence of cryogenic temperature on the microstructures and mechanical properties of AlMnFe alloys are mainly as follows:when the cryogenic temperature is above-150℃, mechanical properties of aluminum foils change little, when the cryogenic temperature reach-196℃, the tensile strength and elongation of AlMnFe alloyH18, H26 aluminum samples are certain to improve relatively to cryogenic temperature at -50℃and room temperature's samples. So in this experiment, the optimal cryogenic temperature should be -196℃. But the study finds that the mechanical properties of aluminum AlMnFe alloy O state behave a relatively stable performance after cryogenic treatment. This can provide certain support of the experiment for the follow-up studies of cryogenic processing time influence on the performance of AlMnFe alloy. On the other hand, after-196℃cryogenic treatment, the dislocations of the sample at H26 state distribute disordered in the crystal, and compared with untreated sample there is not obvious bulk of dislocations. And finite black grain shape second phase precipitates comparatively centralize distribution.(2) In the paper, the microstructures and the properties of AlMnFe alloy H18 aluminum foils which are cryogenic treated at different times were investigated. The results show that, after 5 minutes cryogenic treatment, the tensile strength is improved from 233MPa to 263MPa and the micro-hardness is obviously enhanced, but there is no obvious change in elongation. With the increase of cryogenic treatment time, the strength and the hardness remain stable, however, the elongation is improved slightly and it is 1.5 times that of originality after 24 h cryogenic treatment. TEM and XRD analysis show that second phase precipitates out in the alloy with 5 minutes cryogenic treatment, and further to increase the cryogenic treatment time, the precipitated quantity is finite. With 5 h cryogenic treatment, the grains begin to break into much smaller ones. After 24 h cryogenic treatment, the grains are fully broken into close grain with the grain size of 0.3～2.5μm. The tensile strength of 0.1mm H26 state aluminum foils after short cryogenic time treatment is improved with about 7% increase amplitude. With the increase of cryogenic treatment time, the strength improves stably; elongation changes little after cryogenic treatment, but when cryogenic treatment 1h, it increases a lot with about 24% amplitude. The fibrous grains of H26 state aluminum foils after long time cryogenic treatment as H18 state fully break into smaller grains with the size of 100～200 nm, and the shape of quadrangle and polygon.(3) The effects of cryogenic pretreated on the microstructures and mechanical properties of AlMnFe alloy are investigated. The aluminum foils are cryogenic pretreated for different times and then annealed at 330℃for 7 h and 22 h. The results show that cryogenic pretreatments inhibit anomalistic coarse grains which are produced in the annealing process, and prolong recovery periods. The inhibitation effect becomes more and more obvious as the cryogenic pretreatment time increase. Then different time cryogenic treatment affects aluminum foils, tensile strength, elongation and microhardness can be changed in a certain extent. XRD analysis shows that grain orientations of annealed aluminum foil change due to the cryogenic pretreatments, and change trend intensified with the increase of cryogenic time.(4) Low temperature performance of AlMnFe alloy is mainly as follows:with the decrease of experiment temperature, as a whole, the trend of tensile strength and elongation of AlMnFe alloy foil is improving, but at -50～-100℃, the elongation low point appears, then rises up and is higher than the room temperature's value. In addition, the tensile fracture morphology is investigated. It is found that the quantity of nest microstructure increases after low temperature tensile experiment. Thirdly, the primary theoretical analysis about low temperature performance advantages of aluminum foil has been discussed:the temperature is lower, the dislocations are more difficult to slip in the aluminum crystal, the deformation strain hardenability augments, and the fibrous grain broken into smaller ones is one of the important reasons.