Microstructures And Formation Mechanisms Of Ceramic Reinforced Layer Prepared On Aluminum Substrates By Hot Nitrogen Plasma

Because of their high specific strength and strong corrosion resistance,aluminum and its alloys have attracted more and more attention.However,the shortcomings of aluminum and its alloys like low hardness and poor wear resistance limit their application.For improving the surface hardness and the wear resistance of the aluminum substrate,AlN ceramic reinforced layer was in-situ prepared on the surface of the aluminum substrate by hot nitrogen plasma,which was ionized from nitrogen gas by gas tungsten arc welding equipment.The structures,the performances and the formation mechanisms of the strengthening layer were also analyzed.Firstly,a strengthening layer reinforced by AlN ceramic was prepared by hot nitrogen plasma on the surface of cast aluminum alloy ZAl Si12 substrate.The preparing time of the strengthening layer is very short and the influence of the oxygen partial pressure is quite small.The strengthening layer is mainly composed of Al phase,AlN phase and a small amount of Si phase.When the nitriding current is in DC polarity,the AlN phase in the strengthening layer is densely distributed,and the proportion of AlN phase is 96.4%.The strengthening layer can be divided into two regions from bottom to top: the transition region and the dendrite region.Si element plays a catalytic role in the formation of AlN by forming the intermediate product Si3N4.During the formation of the strengthening layer,Al melt transports upward from the substrate through the gaps between the AlN structures to the top of the strengthening layer,and reacts with the hot nitrogen plasma to form the AlN structure.It is found that with the increase of the nitriding current,the weight loss of the abrasive wear test without lubricant increases first and then drops,and reaches the lowest at 130 A,which is about 1/13 of the substrate,when the average hardness of the outmost region is 8.1 times that of the substrate.The volume loss of the frictional wear test with lubricant decreases first and then increases with the increase of the current,and reaches the lowest at 90 A,which is about 1/38 of the substrate.When the nitriding current is in AC polarity,the AlN structure in the strengthening layer is less dense,and the proportion of AlN phase is only 71.8%.Secondly,a strengthening layer was prepared by hot nitrogen plasma with pre-coated Ti powder on pure aluminum 1060 substrate.This layer is mainly composed of AlN phase,Al3Ti phase,Al phase and a small amount of Ti N phase.AlN lamellae are formed on the top region of the layer,and Ti element is concentrated in the middle and bottom region forming brittle needle-like Al3 Ti structures.During the formation process of the strengthening layer,Al melt transports upwards,part of the Al melt transports to the top surface of the strengthening layer reacting with the nitrogen plasma to form AlN structure,and part of the Al melt is mixed with Ti melt to form the Al-Ti mixed melt.When the temperature decreases,small Ti N grains and needle-like Al3 Ti structures are precipitated from the Al-Ti mixed melt.When the strengthening layer is prepared at different gas ratios,the average hardness of the outmost region of the strengthening layer is significantly higher than the substrate hardness,and the weight loss of the abrasive wear test is significantly smaller than that of the substrate.Nevertheless,a large number of brittle needle-like Al3 Ti structures are distributed in the strengthening layer,which can cause the stress concentration.Furthermore,many defects are formed at the layer adjacent to the substrate,including the unmelted Ti powder at the bottom of the strengthening layer,and voids in the interface of the strengthening layer and the substrate.To improve these defects,an AlN/Ti-Al-N composite strengthening layer was prepared on the surface of ZAl Si12 substrate by preparing an AlN layer in advance and then cladding Ti powder for nitridation.The Al3 Ti structure in the AlN/Ti-Al-N composite strengthening layer on ZAlSi12 substrate is in the granular or bulk shape,which significantly reduces the crack tendency of the layer.The morphology of the boundary between the Ti cladding layer and the AlN layer is also significantly improved.It is found that Si element improves the structure of the Al3 Ti phase and reduces the crack tendency of the strengthening layer.Among the strengthening layers prepared with pre-coated Ti powder,the AlN/Ti-Al-N composite strengthening layer prepared on ZAl Si12 substrate at a mixed atmosphere of nitrogen and argon can get the best structure: the structure of brittle Al3 Ti phase is greatly improved,no cracks are found,few loose tissues are formed,and the content of Al melt in the strengthening layer is quite little.Finally,the microstructures of the ceramic reinforced layers on aluminum 1060 substrate prepared by the direct nitridation method and the Si assisted nitriding method were compared.The strengthening layer prepared by the direct nitridation method on aluminum 1060 substrate is mainly composed of laminar and dendritic AlN phase and Al melt phase.AlN phase is sparsely distributed in the strengthening layer,and the proportion of AlN phase is only 58.0%.The hardness and the wear resistance of the strengthening layer are both significantly lower than those of the strengthening layer on ZAl Si12 substrate.Prepared by both the pre-applied Si powder nitriding method and the wire filling nitridation method,the AlN structures are denser than that of the direct nitridation layer,and the proportions of AlN phase in the strengthening layers are 82.4% and 91.5% for both methods.For the pre-applied Si powder nitriding method,the pre-applied powder has different melting rates at different locations.The middle part of the strengthening layer sags downward into the substrate,and Si powder on both sides of the layer is only partly melted forming an unmelted particle layer.The morphology of the strengthening layer is not good.In addition,the lateral transport of the Al melt is fast,making the nitriding reaction insufficient,and the AlN structure is not as dense as the strengthening layer on ZAl Si12 substrate.For the wire filling nitridation method,the AlN phase in the strengthening layer is very dense and the proportion of AlN phase is close to that of the strengthening layer on ZAl Si12 substrate.Meanwhile,the welding wire and the substrate are well fused,with fewer defects observed,and the morphology of the strengthening layer is excellent.