| The current technique for investment casting,as a near-net shape forming process,is one of the key technologies for high-end equipment and components with complex structures employed in the field of aerospace.However,this conventional process,for its low service performance of shells,cannot meet special requirement of various metals and structures for shells.Besides,there is an increasingly severe environmental problem of huge volume pollutants from waste and / or used shells.Therefore,it’s urgent for this technology to develop further an innovative design and making process of shell with higher strength,thinner-wall,and low solid waste emissions.It is in want of a new and refreshing approach to the problem since there is a strong negative correlation between high strength and thinwall.Short fibers as reinforcement in composite material have been employed successfully in many fields and these practices are more valuable for thin-wall shells.However,it should be noticed that the performances of shells vary with large variations in temperature during service process,different from common composite materials.For this reason,it is necessarily controlled so that shells can achieve a high green-,appropriate high-temperature-,low residual-strength,and excellent permeability for service requirements.Hence,the short fibers with different thermal damage,fluidized by a reactor are employed to spread and build a multi-overlay network structure in shell and the performance of shells are investigated in this work.It is expected that a support in theory should be provided for improvements or developments of current technology of shell making for investment casting.In order to improve these properties of the silica sol shell,the two categories of aluminum silicate short fibers(φ3-5 μm×4-6 mm)and cattail short fibers(φ9-11 μm×4-9 mm)were selected as reinforcements and induced into silica sol shell through fluidization spread method during the fabrication process of shells.The amount of fibers in the shells was determined by the air flow covering time and the differences between initial and residual amount of fibers in the reactor.In the test,aluminum silicate fibers were preferentially coated,and the coating time was controlled in 1s,3s,5s,7s,and 9s,respectively;and cattail fibers coating time controlled in 2s,4s,6s,8s,and 10 s,separately.The initial total amount of the two fibers in the reactor was 1000 mg.The influence of different amount of two kinds of fibers on the green flexural strength,fired flexural strength,residual strength,permeability and self-loaded deformation at elevated temperature of reinforced shell were investigated.The results show that with the increase of the amount of cattail fibers added,the strength curve shows a trend of rising first and then falling,when alumina silicate fibers coating amount is constant.The maximum green flexural strength of shell specimens reinforced with aluminum silicate fibers of 18.3 mg reaches 3.87 MPa,about 52.96% higher than that of unreinforced shell specimens,meanwhile the flexural strength of fired shell specimens reaches 5.58 MPa,ca.35.44% higher than that of the shell specimens without fibers.At the same time the residual strength of fired shell specimens is 6.38 MPa,16.05% lower than that of the unreinforced shell specimens.Furthermore,while the strength of the shell specimens shows better performance,the permeability of the shell specimens has also improved significantly and the self-loaded deformation at elevated temperature has decreased,too,which can meet the requirements of investment casting.To sum up,the silica sol shell performance can be improved by changing the fiber coating amount.While the coating amount of aluminum silicate fibers is 18.3mg and the amount of cattail fibers is 3.2mg,the silica sol shell has excellent strength performance,permeability and low self-loaded deformation at elevated temperature. |
PDF Full Text Request