Research On Pore Structure And Mechanical Properties Of Directionally Solidified Porous Aluminum Alloys Prepared By Mold Casting Technique

Many studies have been published about various directionally solidified pure metals and some alloys,though it was noticed that very few research results are available about Al-based alloys.It is well-known that aluminum is remarkable for the metal’s low density and for its ability to resist corrosion due to the phenomenon of oxidation.Structural components made of aluminum and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials.Considering the benefits of aluminum alloys,the experimental aspect of this work is focused on the investigation of pure aluminum,Al-Mg alloys,Al-Si alloys and Al-Cu alloys of different chemical compositions,produced by mold casting technique;their directionally solidified porous structure and mechanical properties.Metal-gas eutectic method or gasar process is based on the simultaneous cooperative growth of two phases:the crystals of metal and gas pores.The experiments were carried out in a tailor-made apparatus that ensures directional solidification.Hydrogen was driven out of the melt,and then the quasi-cylindrical pores normal to the solidification front were usually formed.Pure aluminum,Al-3%Mg,Al-6%Mg and Al-33%Mg alloys with oriented pores have been successfully produced under processing conditions of varying gas pressure.The relationship between the final pore structure and the solidification pressure,as well as the influence of Mg quantity on the pore size,porosity and mechanical properties of Al-Mg alloys were investigated.The results showed that the higher pressure of solidification tends to yield smaller pores in aluminum and its alloys,while the higher saturation pressure is the higher porosity of the specimen we get.In case of Al-Mg alloys,it was proved that with the increasing of Mg amount,the mechanical properties of the alloys sharply deteriorate.However,since Al-3%Mg and Al-6%Mg alloys are ductile metals,their porous samples have compressive strength similar to the bulk samples due to the existence of pores.It gives the opportunity to use them in industry at the same conditions as bulk alloys with savings in weight and material consumption.The germination of gas bubbles during the gasar operation and their expansion in the molten metal have also been studied.Models were developed to describe the germination of bubbles within molten aluminum alloys and the growth of bubble engendered by temperature and pressure gradients.It was found that the ability of hydrogen to saturate in liquid aluminum is significantly affected by the alloying elements.Hydrogen solubility increases in liquid Al-Mg alloy with an increase of Mg content.The pore shape also undergoes changes.The experimental tests and numerical simulation of gasar process showed similar results.In this work directionally solidified porous Al-Si alloys with different Si content(4%,8%,12%and 14%)were also prepared under solidification pressure-0.075MPa and different values of hydrogen saturation pressure.The influence of Si content on the porosity,pore shape and size,pore circularity of the specimen were studied.Appropriate parameters to get the relatively uniform porous structure were defined.Thus,Al-4%Si and Al-8%Si alloys were marked out as the ones possessing the good combination of porous structure and compress tests results.It was proved that with the increasing of Si content level pores acquire more spherical shape and porosity decreases.Pore diameter grows with the increasing of the Si content in the melt.Some interesting results were described to prove the fact that specimens produced by different casting techniques have differences in porosity,shape of pores and the tendency to increasing/decreasing porosity with changirng of Si content in the alloys.Compression tests were also carried out to reflect the advantages of porous specimens compared to the bulk ones.The advantages were proved for specimens with the Si content of 4%and 8%,revealing anisotropic properties.Al-Cu alloys are very common in aerospace and automotive industry,due to their excellent heat resistance and processing properties.The most widely used Al-Cu alloys contain 4-6%of Cu.Even though Al-Cu alloys with different Cu content do not have a wide range of applications,it is still interesting to investigate processes of porous structure formation from the scientific point of view.Thus,Al-Cu alloys of such chemical compositions as 5%,10%,15%,20%and eutectic alloy 33%were chosen to be alloys of interest for this study.The influence of the Cu content on the pore structure and porosity of these alloys was investigated.The results showed that both the porosity and the average pore diameter of the specimens increase with the increasing of the Cu content.Meanwhile,it was noticed that the shape of pores gradually transforms from elongate to nearly spherical one.It was revealed that pores are almost absent in the alloys with Cu content 33%which is the eutectic composition of Al-Cu alloy.It was proved that primary-precipitated phase and the width of the mushy zone have significant effect on the pore formation.