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Study Of Laser Modification And Alloying Of Ni-P Deposited Aluminum And Laser Cladding Of Aluminum-based Powders On Magnesium Alloys

Posted on:2007-03-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y YangFull Text:PDF
GTID:1101360182997127Subject:Materials science
Abstract/Summary:PDF Full Text Request
Aluminum alloys and magnesium alloys are a group of the most widely usedmaterials in the fields of automotive and aerospace industries. They possess manyadvantages: aluminum alloys have low specific weight, good corrosion resistanceand high thermal conductivity, and magnesium alloys have low densities (from1.75 to 1.85 g/cm3) and high specific strength. However, these alloys possess poortribological properties and load bearing capacity due to their low melting points,soft surface and weak interatomic bonding nature which limits their applicationfields.One of the most effective ways to improve the corrosion and wear resistance ofaluminum alloys is to coat protective layer. Electroless Ni-P coating has achievedwidespread acceptance for its excellent properties, such as high hardness and betterwear and corrosion resistance. Electroless Ni-P coating are widely used forcorrosion protection application in a variety of environments. In recent years,various thermal treatments to the electroless Ni-P layer have received muchattention as they could provide considerable improvement in the desiredcharacteristics of the coatings. It is shown that lasers are powerful tool for thesurface modification of materials. Based on the consideration, Ni-P layer (P content6.74%) was deposited on aluminum alloys, and then laser treatment was conductedwith different laser power densities. The process of laser beam interacting with thedeposited layer was divided into two kinds of process by adjusting the powerdensity: recrystalline process of deposited Ni-P layer and laser alloying process ofNi-Al.One of the solutions to the problems is to coat the surface of magnesium alloys,and hence laser surface processing has attracted considerable interest as a methodof improving the mechanical and chemical resistance of these alloys. Laser surfacecladding (LSC) is one such laser surface processing techniques. It has been provedthat LSC is an appropriate technique to improve the wear resistance and corrosionresistances of magnesium alloys. It has been reported that laser cladding ofmagnesium alloys with Al-Al2O3 aiming in improving the corrosion and wearresistance and has found a significant improvement in these properties. However,there are only limited reports on the laser cladding of magnesium alloys withAl-Ti-C powders. In this regard, it may be mentioned that TiC particles introducedinto a molten magnesium matrix may give rise to a poor interfacial strengthbetween the ceramic particles and the magnesium substrate. On the other hand, Alas an alloying element is known to enhance corrosion resistance of magnesium andits alloys significantly. Hence, laser cladding of mixed powders Al-Ti-C on themagnesium alloy may, perhaps, simultaneously improve the corrosion resistanceand wear resistance of Mg substrate. Based on the consideration, laser cladding twokinds of powders on magnesium alloys was conducted: Al-Al2O3 and Al-Ti-Cpowders.By adjusting laser parameters, the laser treated zone was observed and theproperties of laser treated specimens were examined. We have made severalconclusions through theoretical and experimental investigation:(1) The surface temperature distribution profiles under the different laser powerdensity were obtained by the analysis of laser beam with deposited layer interaction,at the same time, the temperature distribution profiles with depths under same laserpower density were also obtained. Based on the analysis, we found that: when theemployed power density was 0.284×109W/m2 , 0.515×109W/m2 and0.68×109W/m2, respectively, the surface temperature could reach 573K,996K and1122K which is far lower than the melting point of Ni;when the employed powerdensity was 3.18×109W/m2,3.66×109W/m2,4.31×109W/m2,5.20×109W/m2,5036×109W/m2 and 6.46×109W/m2, respectively, the surface temperature couldreach 1921K,2086K,2281K,2513K,2691K and 2991K which is all higherthan the melting point of Ni1726K. Hence, the process of laser beaminteracting with the deposited layer was divided into two kinds of process:recrystalline process of deposited Ni-P layer and laser alloying process of Ni-Al.(2) When the recrystalline process of deposited Ni-P layer was carried out, thesurface roughness, the adhesion of deposited layer with substrate and the corrosionresistance of the layer were improved. The amorphous Ni-P microstructure wasobserved when the employed laser power density was 5.36×109 W/m2. Heattreatment was conducted to the laser alloyed samples, the equilibrium phases ofNi+Ni3P was obtained by X-ray diffraction, the grain size in the alloyed layer wascalculated by the Scherra equation which was in good agreement with theexperimental results.(3) The laser parameters have strong affection to the quality of laser alloyed layer.The main defects in the alloyed layer were cracks, pole and void. The surfacequality of laser alloyed layer could be improved by adjusting laser parameters.Then, the stacking in the layer was discussed: the stacking density was differentwith the depths in the molten pool, the stacking density was higher at the interfaceof molten pool and substrate, the stacking density was lower at the middle part ofthe molten pool and near surface. The twinning deformation was also observed atthe interface of molten pool with the substrate.(4) The microstructure in the alloyed layer was observed, the alloyed layer wasdivided into several parts: featureless part, cellular dendrite and dendrite. The effectof laser parameters on the microhardness was also studied, the maximum hardnessalong the depths of the molten pool increased with the increasing of power density,on the other hand, the hardness varied with the scanning speed.(5) The solidified microstructure in the layer, dendrite growth and eutectic structureof Al+Al3Ni were also discussed, the microstructure in the molten pool wastheorically explained.(6) The powder Al-Ti-C and Al-Al2O3 were successfully clad on magnesium alloyunder appropriate laser parameters. For the laser cladding of Al-Ti-C powders, theappropriate laser parameters: power density 1.8-2.2×109W/m2, scanning speed1.0-1.5mm/s;for laser cladding of Al+Al2O3, power density 0.8-1.0×109W/m2,scanning speed 1.0-1.5mm/s.(7) For the laser cladding of Al-Ti-C powders, the main phases in the clad layerwere TiC, TiAl3 and Mg12Al17;when the Al content was 40%, the main phaseswere TiC particles and little amount of Mg12Al17. For the laser cladding ofAl-Al2O3, the products in the clad layer were Al2O3 particles, intermetalliccompounds of Mg-Al Mg12Al17 and cellular α-Mg, the quantities of Mg12Al17increased with the increasing of amounts of Al content.(8) The microhardness and wear resistance of laser clad layer were far better thanthe magnesium substrate. When Al content was 40% in the mixed powder, the cladlayer showed the best wear resistance;when Al and Al2O3 was 3:1, the clad layershowed the best wear resistance.
Keywords/Search Tags:pulsed laser, laser cladding, recrystalline, amorphous microstructure, wear resistance
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