Properties And Interface Characteristics Of Silica Sol Shell Reinforced With Sepiolite Fibers

Silica sol shell is the most widely used type of shell in investment casting.However,the main problem of silica sol shell is low green strength,making it prone to damage during transportation.At present,to acheieve higher strength for silica sol shell,it is often achieved by increasing the thickness of the shell back layer.Although this solves part of the problem to a certain extent,with the increase of the wall thickness of the mold shell,the permeability of the mold shell is poor and the mold filling is difficult,resulting in poor mechanical and organizational properties of the final casting,and increasing the complexity of the shell making process and cycle time.In order to obtain the shell with higher strength and thinner shell wall,this paper uses sepiolite fiber as the reinforcement phase.Due to the presence of impurities such as calcium carbonate and dolomite in the protosepiolite fiber,the specific surface area of the fiber is affected.Therefore,hydrochloric acid is used to activate the sepiolite fiber.The influence of sepiolite fiber coating amount after different hydrochloric acid concentration and activation time on the green strength,fired strength,retained strength,high temperature strength,high temperature self-loaded deformation and permeability was studied.In order to make the fiber evenly dispersed into the shell back coating,the fiber is added to the shell back coating in the way of fluidization spread technology.The fracture morphology of shell specimens was observed by SEM,and the fiber,shell phase composition and thermal stability were analyzed by XRD and TG-DSC.It was found that compared with the original sepiolite fiber,the sepiolite fiber activated by hydrochloric acid enhanced the bending strength of the shell samples more significantly.When 4 mol/L hydrochloric acid was used to activate sepiolite fiber for 7 h,the fiber reinforcement effect on shell samples reached the best.When the fiber coating amount was 2.79%,the green strength,fired strength,retained strength and high-temperature strength of shell reached the maximum,which were 3.73 MPa,8.62 MPa,9.48 MPa and 21.60 MPa.The strength of samples without fiber shell was increased by78.47%,79.58%,40.24%and 110.73%.TG-DSC test on the thermal stability of sepiolite fiber shows that the fiber is not burned after high temperature roasting,so it can be seen that the fiber still plays a strengthening role in silica sol shell at high temperature.SEM was used to observe the fracture morphology of fiber-reinforced composite shell samples.It was found that the sepiolite fibers without hydrochloric acid activation were in the shape of fiber bundles,and the combination with the matrix was poor,and the strength enhancement amplitude of the shell samples was low.After the activation of sepiolite fiber by hydrochloric acid,the impurities in the fiber are fully removed,the fiber changes from bunched state to dispersed state,and the effective contact area between fiber and matrix increases.In addition,there is a good combination between fiber and matrix.When the shell sample is subjected to bending load,the fiber fails in the form of debonding,pulling out and breaking,which consumes part of the energy and increases the strength of the shell.XRD analysis shows that the phase composition of the sepiolite fiber composite shell does not change compared with the shell without fiber.After roasting,the characteristic peak d_（200）of Zr Si O₄did not deviate,but the peak strength increased obviously.The thermal stability test of silica-sol shell before and after fiber addition shows that the thermodynamic characteristics of silica-sol shell are basically the same,and it has good thermal stability.The addition of sepiolite fiber has a certain resistance to high temperature self-loaded deformation of shell samples,and the high temperature self-loaded deformation of fiber reinforced composite shells is less than 1%,which meets the requirements of investment precision casting shell.When the amount of fiber coating is fixed,the concentration of hydrochloric acid is too large or the activation time is too long.Due to the enhancement effect of sepiolite fiber on the permeability sample of the shaped shell,the temperature of microcracks in the permeability sample is delayed,and the inflection point temperature of the permeability of the fiber reinforced composite shell is increased.