Study On Microstructure And Properties Of High-silicon Aluminum Alloys Prepared By Spray Conform

Hypereutectic Al-Si alloys are very interesting materials for automotive,aerospace and electronics applications due to attractive properties such as low density,high hardness,good wear resistance and low coefficient of thermal expansion.These properties can be improved by further increasing silicon content.However,high silicon content leads to coarse microstructures,inherent brittleness,and poor workability when conventional methods are applied to produce such alloys.Spray Conform(SC)is a new approach for the manufacture of advanced materials with enhanced properties and performances.Variety forms of products including tubes,solids and complex profiles can be continuously produced by SC in one step.This technique can be expected to be a new way for the high efficiency and low cost preparation of high silicon aluminum alloy.In this dissertation,seven kinds of high-silicon aluminum alloys have been prepared by SC.The microstructures,mechanical and wear properties of these alloys have been investigated.When the conventional atomizer is used in SC,the wide spray plume will cause that a majority of droplets deposit out of the range of the wheel groove,resulting in a great waste of material.In order to control the profile of the deposit,spray pattern controller was used to confine and control the shape of the spray plume.The effects of melt temperature and gas pressure on the shape and microstructure of spray formed Al-20Si alloy have been investigated.The optimization parameters in the spray deposition stage were obtained by combining the artificial neural network model.The effect of Fe content on the lattice parameter of a-Al phase in SC Al-20Si-xFe alloy has been investigated by X-ray diffraction analysis.The higher the Fe content,the more the lattice distortion and the larger lattice parameter.The formation mechanism of Fe-containing intermetallic compound in SC Al-20Si-xFe alloy was discussed.The droplets impacting the deposit surface at a high velocityin spray depositionstage andthe intensive shear behavior in continuous extrusion stage are the main reasons for the formation of fine iron-bearing intermetallic phase.The effects of extrusion ratio on microstructure,mechanical property and wear behavior of Al-20Si alloy prepared by SC were studied.The results show that the primary silicon phase can be refined effectively by SC,and the size and shape of Si particle become more fine and regular with the increasing extrusion ratio.With the increasing extrusion ratio,the hardness,ultimate strength and wear resistance gradually increase.When the extrusion ratio reaches 20,fully dense material with uniform distribution of Si particlescan be obtained.Al-xSi(x = 20,25,30 wt.%)alloys fabricated by SC exhibit fine and regular Si particle as against coarse and polygonal Si phase observed in the as-cast microstructures.The average equivalent diameter of Si phase in the former is only about 1/20 of that in the latter.Higher Si content can lead to higher strength for the alloy fabricated by SC,which can be ascribed to the large volume fraction of fine Si phase.Compared with as-cast alloys,ultimate strength,plasticity and wear resistance of SC alloys are all significantly improved.It is worth noting that the higher the silicon content,the greater the improvement of mechanical property.The second phases of SCAl-20Si-xFe(x = 2,3,4,5 wt.%)alloysare mainly composed of fine silicon particles,short rod 8 phase and short needle β phase.Compared to casting materials,the size of Si particle is smaller by more than an order of magnitude,and the lengths of β phase and δ phase are shorter by almost two orders of magnitude in SC alloys.The tensile strength of SC Al-20Si-xFe alloy increases with the increase Fe content,which can be increased to 367MPawhen the Fe content is 5%.This should be ascribed to the refinement in the second phase constituent.The higher iron content leads to greater amounts of the second phase,resulting in a more significant enhancement of the strength.In contrast with as-cast Al-20Si-xFe alloy,ultimate strength,plasticity and wear resistance of the SC alloy are all significantly improved.When test temperature increases from RT to 200 ℃,the strength of SC Al-20Si-xFe alloy decreases,while the plasticity can be greatly improved.The elevated temperature strength increases with the increasingFe content,which can be increased to 192MPawhen Si content is 5%.SC Al-20Si-xFe alloys have excellent wear resistance at RT and 200℃.When Si content is 4%,the elevated wear rate is only 3.8 X 10-10g/(N· mm).