The Mechanism For Powder Refinement In Plasma Milling And Preparation Of Flaky Iron Powder For Wave Absorbing

High-energy ball mil ing is the most commonly used method to achieve powder refinement and mechanical al oying.However,only mechanical energy is input in conventional ball mil ing,which brings some problems such as low efficiency,high energy consumption,and powder contamination.Plasma mil ing has unique advantages to solve the above problems.In this dissertation,the applications and basic principle of high-energy ball mil ing and plasma mil ing technology were reviewed at first.Then,by controlling the process of plasma mil ing,the flaky iron powders were prepared for wave absorbing,the effects of discharge parameters and ball mil ing parameters on the morphology of powder were studied.Finally,the mechanism for the formation of flaky iron powder in plasma milling was discussed.It was found that the electron temperature of the plasma was 0.6-3.2 e V and the electron density was 10¹⁹-10²¹/m³ under the conventional discharge parameters.The energy of discharge could make Sn,Zn,Al,Fe and other metal powders melt or sinter in different degree.The depth of the surface melting layer was about tens of nanometers,and a certain temperature gradient was formed from the surface to the core of powder,resulting in thermal stress.After discharging for 1 h,the temperature range of aluminium powder was 130-166?,and the maximum thermal stress was about 82 MPa.The temperature range of iron powder was 41-95?,and the maximum thermal stress was about 140 MPa.SEM and XRD results showed that the flaky reduced iron powders with 4-10?m in diameter and 18 nm in average grain size could be obtained by plasma mil ing for 6 h,and the flaky carbonyl iron powder with 9.5?m in average diameter,0.6?m in average thickness and9 nm in average grain size could be obtained by plasma mil ing for 1 h.Compared with conventional mil ing,the flaky carbonyl iron powder prepared by plasma mil ing had larger diameter thickness ratio and stronger shape anisotropy,which led to the improved microwave absorbing properties.The maximum reflection loss measured by simulated thickness method was-25 d B for the coaxial sample,the reflection loss was less than-10 d B in the frequency range of 13.6-16.7 GHz,and the bandwidth was about 3 GHz.Numerical simulation method was used to characterize and calculate the strength of plasma mil ing.The impact force of mil ing was 20-200 N,the contact stress of mil ing was900-8000 MPa,the collision frequency was 10-100 s^-1,and the collision duration was about 10?s.The duration of discharge was about 100 ns,and a single discharge cycle was about 60?s.The"heating and micro-forging"model could be used to describe the formation of flaky carbonyl iron powder in the process of plasma mil ing.The impact force of mil ing made the powder forge slightly,and the surface layer of iron powder melted after discharge heating,the overall macro temperature was 330-470?.The yield stress of iron powder was reduced,the degree of work hardening decreased,and thus,the spherical powder was easier to be extended into flaky powder with larger diameter and thinner thickness.