Study On Microstructure Characterizations And Properties Of Laser Cladding And Remelting On Surface Of AZ91D Magnesium Alloy

Material surface processing is one of the most rapid developments of material modification fields. Many of them start and go on and complete on the their surface and interface of materials because no matter what are the corrosion, friction, abrasion, fatigue, or sound, light, electricity, magnetic, heat, pression, displacement, angle, acceration, chemical change, biochemical process, et al and their signal conversions, as a result, many product performances mainly depend on their surface performances and states. Reducing the raw material consumption, saving enengy resources, incresing serviced properties and securty of products, prolonging operational life span of products, enhancing the compatibility of environment and materials and relieving the environmental pression, are not only the demands of technology but also the results of economic development and environmental pression of increasing overload operation, then it is meaning of the surface engineering from this point.Increasing attention has been paid to magnesium and its alloys in application of industries, especially in automotive, communication apparatus and aerospace applications, because of its low density, high specific strength and stiffness, high heat and electrical conductivity, good damping and shock absorption, excellent electromagnetic shielding capability and easy processing, as well as a good using of recovery and recycling capability. However, as an engineering structure material, the range of application for magnesium alloys in industry has been greatly limited due to their undesirable properties, such as low strength, poor wear and corrosion resistance. It has hardly met the needs of the rapid development of science and technology and large-scal industry application for light materials only by alloying so far. What is more, most of components of magnesium alloys are used under the die pressure or as-casting, although employing the advanced casting technology can produce all kinds of large volume and complex parts, however, it is often uneconomic and very difficult for these components to employ overall surface processing. Local surface processing has both evidently economic performance and far-reaching social effect. Therefore, how to improve the comprehensive properties of magnesium alloys such as strength, hardness, wear, heat and corrosion resistance et al, and find the suitable way to strengthen the surface, has been become an important subject of magnesium alloy research. As a consequence, one of the best routes is the modification on surface magnesium alloy to solve the problems of magnesium and its alloys on the basis of the internal attribute of material system. Laser surface modification technique has shown its potential preponderance among many surface modification techniques due to its short time-consuming, flexibility of operation and orther advantages. Therefore, our researches employed laser remelting and cladding lower melted point aluminium based coating alloys with surface modification technique to improve wear and corrosion resistance of magnesium alloys, the major research efforts of the present study are as follows: (1) Microstructure characterisation and performance by laser surface remelted AZ91D alloyStudies on the AZ91D microstructure characterisation and performance by laser surface remelted have been carried out, and different laser processing parameters of Nd:YAG and CO2 emitters were compared with variation of remelted layer performances. The result of DTA test showed that the Mg17Al12 phase can be formed in uniform grain (9wt. %Al) corresponding to AZ91D composition and its reaction is endothermic, which means Mg17Al12 phase melted at this temperature. Moreover, except for magnesium, aluminium and Mg17Al12 phase, no other magnesium and aluminium compounds were found formation in the test; the analysis showed that the microstructure of remelted layer by Nd:YAG laser processing short columnar crystals, granular crystals and hexagonalαphase crystals from the substrate to the remelted layer sequentially, and the average grain diameter is 1-2μm; the microstructure of remelted layer by CO2 laser processing isβphase which showed the discontinuous granular distributed in or between the dendritic stripαphase crystals and the average grain diameter is 3-5μm; the grain size of the microstructure of remelted layer by Nd:YAG laser processing is evidently finer and smaller than that of the microstructure of remelted layer by CO2 laser processing; the result of performances for remelted layer showed that compared with original magnesium alloy the microhardness of layer by Nd:YAG laser processing enhanced 100-110%, and wear resistance increased 93%; the microhardness of layer by CO2 enhanced 79-84%, and wear resistance increased 89%; however, the corrosion resistance did not obviously improve whether Nd:YAG or CO2; the reinforcement mechanics of magnesium alloy remelted by laser beam are grain refinement, solid solution and aging, Orowan deposition strengthening and dislocation strengthening produced by geometric constraint force.(2) Microstructure characterisation and performance by laser surface cladding AZ91D alloyIt was the first time that Al+Si+Al2O3 and Al+Al2O3+Mg-Y cladding layers were successfully fabricated on the surface of AZ91D magnesium alloy by laser cladding processing. The coating has a well bonded metallurgic interface with substrate due to the preferable compatibility between Al-Si or Al and magnesium alloy substrate. However, Al2O3 has a poor wettability with the magnesium and aluminium, in order to solve the problem, a small amount of Mg-Y intermediate alloy powder has been added into the powder of alumina ceramic. As a result, the addition of Mg-Y intermediate alloy reduced the pollution of interface and improved the wettability between alumina and aluminium, and also strengthened the cladding layer of Al+Al2O3+Mg-Y. Yttrium and yttrium oxide can not only improve the wettability of alumina and aluminium, they can but also enhance the mechanical properties and effectively improve the corrosion resistance; the addition of nano-alumina in the cladding powder, which allows a small amount of nano-alumina to enter the microporesities of micron scale Al2O3 particles and increases the densities of Al+Si+Al2O3 and Al+Al2O3+Mg-Y laser cladding layers, and decreases the surface roughness of cladding layers, moreover, when the mass content less than 5%, there were well cladding processing properties and cladding layer properties; the short columnar crystals were found in the bond zone at the bottom of two kinds of laser cladding layers adjacent to the substrate, and the growth direction of them was perpendicular to the bond line and pointed to cladding layer, and what is more, the short columnar crystals formed metallurgical bonding with the substrate. It is due to these short columnar crystals that enhancing the contact area between the cladding layer and the substrate and that increasing the bond strength of the cladding layer and the substrate; when the laser energy and scanning speed and preheating temperature are 500-700mJ, 60-80mm/min and 150-260℃, respectively, it is beneficial for avoiding crack, keeping uniform distribution of elements and improving of microhardness and wear resistance of the cladding layer; there were two kinds of microstructrue characterisations in the cladding layer, that is, equiaxial and cellular crystals, the short columnar crystals existed in equiaxial crystals of the cladding layer and their morphologies were somelike that of Al+Si+Al2O3 cladding layer; however, the cellular crystals in bond zone of cladding layer had not the strong direction like sawtooth crystals, but it was infiltration in the crystals as a substitution and its dierection pointed to the substrate. It is beneficial to increase bond strength between the cladding layer and substrate, because of enhancing the contact area and forming metallurgical bond with substrate; after having modified powder, the laser cladding layer has more homogeneous distribution of elements and reunion of Al2O3 was decreased, and its microhardness and wear resistance evidently surpassed over the unmodified cladding layer; alumina particles exhibited gradient distribution in cladding layer, and the size of particles gradually increased from the bottom to the near surface of cladding layer, the result from nucleation rate and supercooling degree in the solidified process.(3) Interface microstructure of cladding layer and solidification behavior of laser poolThe result of HRTEM analysis showed that there was no interface reaction in the interface of alumina and aluminium and silicon. There were two kinds of interfaces in the cladding layer of Al+Si+Al2O3 and Al+Al2O3+Mg-Y, namely, direct bonding and amorphous transition form, and an amorphous transition layer with a width of 3~5nm was found at the interface between alumina particulate and aluminium substrate in the Al+Al2O3+Mg-Y laser cladding layer. It has been found that the crystallographic orientation relationship between the alumina particulate and aluminium substrate is as follow:Although interfacial reaction products were not found in the converge of Al2O3 and Al, however, HRTEM analysis evidently revealed that the vestige of dislocation tangle was found around the Al2O3 particulate, therefore, the presence of dislocation tangle strengthening mechanism was possible in the laser cladding layer.It was discussed that the formative conditions of solidified microstrcture and kinetics on tip of dendritic growth and orther aspects. The formative mechanism of sawtooth crystals between the cladding layer and substrate was revealed by applying fundamentals of metal solidification, and it was proposaled that the diffence from boundary tension and solidified driving force was the reason why interface of solid-liquid could not put forward at stable speed in planar growth, but only solidification formed in the dendritic growth, therefore, the short columnar paralleled sawtooth crystals came into being in the bond zone.(4) Frictional wear behavior of AZ91D substrate and laser cladding layer Under the condition of dry sliding frictional wear, the friction coefficient of cladding layer was smaller than that of AZ91D substrate. The wear resistance decreased with increasing of applied load, and enhanced with increasing of particulate volume fraction. What is more, when the volume fraction of Al2O3 particulates in the the cladding layer is less than 35wt. %, it is beneficial for increasing of wear resistance for the cladding layer; at the low applied load, the wear mechanisms of AZ91D alloy were oxide, adhensive and abrasive wear, with increasing of load and sliding rate(more than 1m/s), the wear developed to the melted wear which was signal of severe wear; at the lower applied load, the wear mechanisms of cladding layer were oxide and plow effect, at the higher applied load, the wear mechanism of cladding layer were delamination, oxide and plow effect wear due to repeat rolling compaction made the cladding layer fragile and appeared to'disturbed plate'in the cladding layer. Therefore, the wear resistance of laser cladding layer is much higher than that of AZ91D substrate due to bearing loads, strengthening substrate and resisting particulates intruding into substrate.(5) Corrosion behavior of AZ91D substrate and laser cladding layerThe test of salt fog corrosion, AC impedance spectrum and polarization curve showed that the corrosion resistance of laser cladding layer with Al+x%Al2O3 evidently surpassed over the AZ91D alloy. Moreover, corrosion current density of 1.7588E-5A/cm2 for laser cladding layer was less by one magnitude than that of 3.7406E-4A/cm2 for AZ91D alloy. The corrosion of AZ91D was due to its Al element unhomogeneoues distribution in cladding layer, and it started from boundary betweenαandβphase and closed toαphase, and extended to the innerαphase, the corrosion mechanisms of AZ91D were mainly pitting corrosion; however, the corrosion mechanisms of cladding layer were mainly uniform and small partial pitting corrosion.In summary, the research of the present study lies not only in the further investigation on the present laser cladding technique, but also it will enhance more application of magnesium alloys in industry and broaden the horizon of our knowledges as well as the correlation theories. At the same time, it provides the referencable ideas and routes for the further studies of magnesium alloys.