| Aluminum and its alloys have high specific strength,high heat resistance and good processing performance.Aluminum alloy is one of the most popular materials in the automobile industry because of its important high performance structural metal,which has become the preferred material for automobile weight reduction.However,due to low surface hardness and poor wear resistance of aluminum and its alloy,its application in engineering machinery is limited.AlN is an ideal hard coating material with high hardness and wear resistance,high temperature resistance and better chemical stability.Therefore,the composite surface layer containing ceramic reinforcing phase is prepared on the aluminum alloy substrate by effective technique,and it is the good method to improve the wear resistance of the surface on the basis of maintaining a series of excellent qualities of the matrix material.Making full use of the advantages of low density and light weight of aluminum alloy,and it can also improve the reliability and service life of parts,which has important theoretical significance and practical value for promoting the application of aluminum alloy in mechanical industry,especially in automobile industry.In this paper,AlN reinforced surface layers were prepared on the 1060 pure aluminum,6082 aluminum alloy and Al-Si alloy substrates by the direct current(DC)/ alternating current(AC)nitrogen arc nitriding technology.Under the condition of pure aluminum substrate predeposited by titanium powders,the ceramic reinforced surface layers with the composites of AlN and TiN were prepared by DC nitrogen arc nitriding technology.The microstructures and formation mechanism of the ceramic reinforced surface layer were investigated and the rules of influence nitrogen to argon ratios and arc current on the microstructure and performance are studied.The AlN enhanced surface layers were prepared by DC nitrogen arc on the surface of 1060 pure aluminum,6082 aluminum and Al-Si alloy.The ceramic reinforced surfaces is composed of AlN phase and ?-Al solid solution phase prepared on 1060 aluminum and 6082 aluminum alloy,and the microstructures were similar.According to the characteristics of microstructure,the ceramic enhanced surface layer can be divided into three parts,the layer structure region composed of AlN and ?-Al solution alternatively distributed in the upper part,the AlN dendrite region in the lower part,and the transition region composed of AlN small particles or columnar crystals in the bottom part.The ceramic enhanced surface layer prepaired on Al-Si alloy substrate is mainly composed of AlN,?-Al solution and Si phase.The microstructure of the ceramic enhanced surface layer is composed of the upper thick AlN dendrite region,the middle AlN and Al-Si eutectic alternately distributed layered structure region and the bottom fine AlN dendrite crystal.The ceramic enhanced surface layer prepared by DC nitrogen arc on aluminum substrate is mainly formed by the capillary action of the aluminum melt in the molten pool formed by the transition zone and AlN in the dendritic zone,and the nitrification reaction occurred with the N which diffusion into the molten pool.According to the different microstructure of the ceramic reinfored layers on the different substrates,the formation mechanism of the AlN reinforced surface layer prepared by DC nitrogen arc on the aluminum and aluminum alloy surfaces is proposed.The hardness and wear resistance of the aluminium and aluminium alloy can be increased obviously by DC arc nitrogen nitriding technique.The distribution of hardness of the ceramic reinforced surface layer prepred on 1060 aluminum and 6082 aluminum alloy are convex parabola shape.The low surface hardness is mainly due to the layer structure region composed of AlN and ?-Al solution alternatively distributed in the upper part and ?-Al solution is more then AlN.AlN dentrites distributed in the middle of the creamic reinforced layer lead to the increasing of hardness.The hardness decreases gradually because of the AlN crystal size decreased with the increase of depth.The highest hardness currented on the surface of ceramic reinforced layer prepared on Al-Si alloy,and decreased gradually with the increase of depth.The ratio of nitrogen to argon in arc atmosphere has obvious influence on microstructure and properties of ceramic reinforced surface.With the increase of nitrogen ratio in arc atmosphere,the average hardness of ceramic reinforced surface layer increases,and the wear resistance increases accordingly.When the ratio of nitrogen to argon is 3/1,the maximum hardness on the surface of 1060 pure aluminum ceramic reinforced layer was 690 HV,and the relative wear resistance was 7 times of the matrix.The maximum hardness of 6082 aluminum alloy ceramic reinforced layer was 400 HV,and the relative wear resistance was 2 times of the matrix.The surface hardness of Al-Si alloy ceramic reinfored layer was up to 665 HV,and the wear resistance was increased to 6 times of the matrix.With the arc current increasing from 70 A to 130 A,the content of AlN dendrite crystal in the Al-Si alloy ceramic reinforced layer increased first and then decreased.When the arc current is 90 A,the ceramic reinforced layer has the highest average hardness and the the best wear resistance because of the highest AlN dendrite content,and the the wear resistance was 8 times that of the matrix.Therefore,the arc current of 90 A is more appropriate.AlN reinforced surface layers were prepared on 1060 aluminum and 6082 aluminum alloy substrates by AC nitrogen arc nitriding technology.The surface layer is composed of AlN and ?-Al solution phases.The microstructure of ceramic reinforced surface layer is evener,which is composed mainly of fine AlN columnar crystals and small AlN granular crystals regularly repeat distribution in the layer structure region.With the increase of nitrogen to argon ration,the average hardness and wear resistance of the ceramic reinforced layer prepared on 1060 pure aluminium substrate by AC nitrogen arc were increased.The relative wear resistance is about 1.7 times that of the substrate with the ratio of nitrogen to argon is 7/3.When the ratios of nitrogen to argon were 1/1,the ceramic reinforced layer has the highest average hardness and the best wear resistance,and the relative wear resistance was 2 times that of the substrate.Compared with the ceramic reinforced layer prepared by DC nitrogen arc,the ceramic reinforced layer prepared by AC nitrogen arc has the smaller thickness,and the average hardness and wear resistance are less improved.However,the surface quality of the ceramic reinforced layer prepared by AC nitrogen arc is well formed,and the microstructure of the surface layer is uniformly fine.The AC nitrogen arc nitriding technique has practical value,which can improve the wear resistance of aluminum and aluminum alloy matrix surface as well as the ceramic reinforced surface layer with good surface forming quality.The ceramic reinforced layer prepared on pure aluminum substrate which was predeposited titanium powders by DC arc technique.The surface layers were composed of Al-Ti compound and aluminum solid solution under the condition of pure aluminum substrate precoated titanium powders and melted by argon arc.The average hardness of the surface layer is 232 HV and the relative wear resistance is as high as 4.56,which is more than four times higher than that of pure aluminum substrate.The nitrided zone consisting of AlN and aluminum solid solution was formed at the top of the surface layer by changing the nitrogen ratio in DC arc atmosphere.Beneath the nitrided zone,Al-Ti intermetallic compounds with thin strips were formed and coexisted with some aluminum solid solutions to form a mixed microstructure zone by these phases.However,the relative wear resistance is only 1.61 when the nitrogen ratio is 3/12.The relative wear resistance of the surface layer increases to 2.60 when the nitrogen ratio reaches 6/12.The surface layers were composed of lamellar nitrided zone,mixed microstructure zone and transition zone between the preceding two zones under the condition of pure aluminum substrate pre-deposited by titanium or its alloy powders and then prepared by twin arc melting and nitriding system.The lamellar nitrided zone consisted of AlN,(Al,Ti)N and aluminum solid solution phases;The mixed microstructure zone consisted of Al-Ti intermetallic compound and aluminum solid solution;But the transition zone consisted of Al-Ti compound,(Al,Ti)N,AlN and aluminum solid solution phases.Their crystallization temperature ranges are different because of Al-Ti compound,(Al,Ti)N and AlN phases with high melting point,while pure aluminum solid solution with low melting point.Thus a periodic distribution by these phases was obtained in the transition zone.In pure argon cladding,the hardening phase in the surface layer was mainly Al-Ti compound with high hardness,which leads to the highest microhardness and the best wear resistance.When a small amount of nitrogen is added into the arc atmosphere,the microhardness and relative wear resistance of the surface layers decreased.With the ratio of nitrogen increasing continuously,they rose again.When the aluminum substrate is pre-deposited by titanium alloy powders and nitrogen ratio is increased to 7/12,the relative wear resistance of the surface layer was as high as 5.16.With the increase of nitrogen ratios in the arc,the microstructures and properties of the upper part in the surface layer changed as follows: The Al-Ti intermetallic compound was the main phase without nitrogen in the arc,which has high hardness and good wear resistance,but is prone to cracks and other defects;When a small amount of nitrogen is added,the upper nitrided zone was mainly composed of AlN phase.And it was thickened because more aluminum melt effused upwards to the zone from the molten pool in the aluminum substrate at the same time,which resulted in low microhardness and poor wear resistance.The upper nitrided zone was composed of AlN and more(Al,Ti)N compound nitride and less proportion of aluminum solid solution,which lead to picking up on hardness and wear resistance of the surface layer with higher ratios in the arc.Therefore,the microhardness and wear resistance of the surface layers strengthened with ceramic phases decreased first and then increased.The wear resistance of the surface layers prepared by twin arc melting and nitriding system is higher than that of those prepared by DC single arc. |
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