Research On Spatial Coordinated Development In Harbin-changchun Urban Agglomeration

After decades of development,titanium alloy investment casting technology has made great progress and successfully prepared a variety of structurally complex aerospace components.However,with the increasingly harsh service environment,as well as the fact that titanium alloy castings tend to be large,complex,and thin-walled,there are higher requirements for the investment casting process.Therefore,focusing on the bottleneck problems of large aerospace complex thin-walled titanium alloy components of investment casting and then carrying out in-depth systematic research to further improve its technology level is of great significance.Given that the TC4alloy has excellent comprehensive mechanical properties and is one of the most commonly used in the aerospace industry.Meanwhile,based on the current situation of the national key types of active fighter aircraft,it is necessary to increase the range and combat radius,extend the retention time,increase the amount of ammunition（cargo）,and improve combat effectiveness.In the present study,TC4 alloy asymmetric airfoil refueling tube casting was used as the research object.Based on numerical simulation and experimental evaluation,the solidification characteristics of TC4 alloy ingots with variable cross section and asymmetric airfoil refueling tube castings were studied,and the formation mechanisms of casting defects such as shrinkage porosity,stress concentration and interfacial reaction were clarified.Using the optimized casting process,the target casting was successfully prepared,the casting quality was systematically characterized,and the effects of hot isostatic pressing on the microstructure and properties of the castings were revealed.Then,advanced vacuum welding technology was used to successfully obtain a high-quality refuelling tube weldment.The effect of temperature field control on the solidification characteristics of TC4 alloy ingots with variable cross section was investigated.On the one hand,removal of the riser shortens the distance that the molten metal covers before reaching the bottom of the mold and reduces the total mass of the ingot,factors conducive to shortening the bottom-reaching time and the mold filling completion time,both of which are crucial factors in the volume reduction and change in the location of the shrinkage porosity.On the other hand,reducing the mold filling distance has the effect of decreasing the initial speed at which the molten metal reaches the bottom,slowing down the flow speed of the molten metal and improving the mold filling stability.In addition,it is also found that the shrinkage porosity volume decreases significantly with increasing the mold taper,significantly increasing the ingot utilization rate.Combined with the solid phase fraction and macrostructure analysis,the formation of shrinkage porosity in TC4 alloy ingots in this study is mainly due to the formation of an isolated liquid phase zone.This zone results from the formation of a grain frame caused by the intersection of a large number of equiaxed crystals.The inability of this last crystallization part to compensate for volume shrinkage ultimately leads to the formation of shrinkage porosity.The riser-free design results in a significant improvement in stress concentration and solidification shrinkage in ingot II（effective stress:ingotⅠis 283.3 MPa,ingot II is 172.2 MPa）.This is mainly related to the structural changes that have led to the cooling rate of ingot II being significantly slower.Compared with ingot II,the maximum effective stress of ingots III and IV is higher（ingot III:181.3 MPa;ingot IV:292.0 MPa）,but there is no discernible difference in solidification shrinkage（ingot III:0.1154 cm;ingot IV:0.1132 cm）.The tiny rise in the effective stress for ingot III may thus be ascribed mostly to a slight increase in cooling rate,whereas the considerable increase for ingot IV can be primarily due to a change in the casting structure.The optimized process parameters for each section of the refueling tube were obtained,and the filling and solidification behavior of the optimized scheme was systematically investigated.Suitable casting parameters for the upper:casting temperature 1680°C,shell preheating temperature 400°C,casting rate 3 kg·s^-1,centrifugal speed 350 r·min^-1;suitable casting parameters for the middle:casting temperature 1680°C,shell preheating temperature 400°C,casting rate 5 kg·s^-1,centrifugal speed 400 r·min^-1;suitable casting parameters for the lower:casting temperature 1680°C,shell preheating temperature 400°C,casting rate 3 kg·s^-1,centrifugal speed 450 r·min^-1;suitable casting parameters for the internal tube:casting temperature 1750°C,shell preheating temperature 400°C,casting rate 3 kg·s^-1,centrifugal speed 300 r·min^-1.Based on the analysis of filling and solidification behavior,each section of the refueling tube meets the sequential solidification characteristics（casting→pouring system）,which can effectively inhibit the formation of shrinkage porosity.The formation of the isolated liquid phase can be considered the reason for shrinkage defects.In combination with macrostructure simulation,both the upper and milddle have a significant number of columnar grains and a few equiaxed grains,which exhibit the characteristics of narrow crystallization temperature range alloy;the lower consists of a large number of equiaxed crystals and a small number of columnar crystals,showing the characteristics of a wide crystallization temperature range alloy;the threaded end of the internal tube consists of a large number of columnar crystals and a small number of equiaxed crystals,showing the characteristics of a narrow crystallization temperature range alloy;the flange end consists of a large number of equiaxed crystals and a small number of columnar crystals,showing the characteristics of a wide crystallization temperature range alloy.It is noteworthy that the stress concentration and solidification shrinkage of the castings occur mainly at structural changes or large cooling rates.The actual casting process of the TC4 alloy asymmetric airfoil refueling tube was studied.A high-quality wax pattern was successfully prepared by combining the optimized core material（Na₂CO₃+Ca CO₃+fibres）and the wax injection method;through in-depth analysis of the thermal property parameters of Zr O₂ shell,good quality shells were obtained;water-cooled copper crucible vacuum melting process was used to successfully cast TC4 alloy asymmetric airfoil refueling tube.Casting surface quality is good,without obvious defects such as misrun,cracks,flying edges,burrs,and so on.Casting internal shrinkage porosity does not exist,and the size can better meet design requirements.The casting interface does not occur for greater element diffusion,the reaction degree is low.The microstructure and properties of the casting itself and the excised bar samples were analyzed in the hot isostatic state,and both of them showed typical Widmanst?tten characteristics.Compared with the cast-on specimens,the microstructure of the attached specimen is significantly finer;accordingly,the attached specimen has better strength and plasticity,and the property uniformity is significantly improved.Subsequently,integral components of the TC4alloy refueling tube were developed using a rational welding process.Finally,an effort was made to investigate the applicability of centrifugal casting for various raw materials.The results demonstrate that the composition and hardness of TC4 alloy ingots produced from various materials satisfy national or national military standards.