The Preparation And Characterization Of Gray Cast Iron Powders Based On The Fabrication Ideas Of Structure Transplantation

As one of the surface treatment processes,thermal spray technology has been increasingly concerned and valued due to its high efficiency and low cost of coating preparation.Simultaneously,the lower cost and the self-lubricating effect of of cast iron powders make plasma-sprayed cast iron coatings as one of the effective ways to improve the wear resistance of aluminum alloy engine cylinder block surface.Unfortunately,it is quite difficult to precipitate the graphite crystal in plasma-sprayed cast iron coatings owing to rapid cooling of molten droplets during deposition.It is encouraging that the difference between the melting points of graphite and gray cast iron matrix makes it possible for maintaining the solid(graphite)-liquid(matrix)two-phases state of the gray cast iron particles in the plasma jet.Based on this,through designing appropriate composition of gray cast iron powders and spray paramenters,it is entirely possible to reserve graphite in gray cast iron powders in the plasma sprayed gray cast iron coating.Therefore,preparing the gray cast iron powders containing graphite structure has become a key to obtain plasma-sprayed gray cast iron coating by the way of retaining graphite.However,gray cast iron powders are usually fabricated by gas or water atomization,which also results in rapid solidification of cast iron droplets.As a result,the carbons in cast iron droplets have to precipitate in the form of cementite rather than graphite.On the basis of above analyses,a method of powder preparation was developed based on the fabrication ideas of structure transplantation.Futhermore,in order to optimize experimental parameters,an orthogonal experiment method was used to study the effects of ball to powder mass ratio,rotational speed,and milling time on the powder properties.The phase composition of the milled powders was determined using an X-ray diffractometer.A semi-quantitative analysis was carried out according to the total area of the strongest three peaks in XRD patterns to estimate graphite content.The grain size and microstrain of gray cast iron powders after ball-milling were estimated based on the modified Williamson-Hall method.The surface morphology and cross-sectional structure of the powders were characterized by a field emission scanning electron microscope(FESEM)equipped with energy spectrum analysis(EDS)to determine the distribution of elements in the powders.The flowability of gray cast iron powders was measured by a Hall flowmeter,and the microhardness of the powders was measured by a Vickers microhardness tester.The results of range and variance analyses of orthogonal experiment show that the ball to powder mass ratio has a significant effect on the performance of the powders,and the rotational speed and milling time have no significant effects on it.The FESEM observations of precursor powders show that the powders were of irregular shape with a long flake shape.The surface and edge of the particles are rough with cracks and sharp shape.After ball-milling,the surface morphology of precursor powders changed from irregular shape to oval shape with smooth surface.Specially,a passivation effect of the sharp edges of particles emerged with a "seaming" mechanism.The results of EDS and XRD analyses show that both the precursor powders and the milled powders contain flake graphite.Moreover,with the increase of the ball to powder mass ratio,the average grain size of gray cast iron powders decreases,the microstrain increases,and the microhardness increases.All these are mainly resulted from the crystal defects,new grain boundaries and work hardening due to the severe plastic deformation of the powder particles during the milling process.The results of flowability of the powders show that the flowability of ball-milled powders has been remarkably improved in comparison with that of the precursor powders.However,with the increase of the ball to powder mass ratio,the flowability of the powders become overall worse with a complex variation.In view of this fact,the fractal dimension was used to quantitatively characterize the characteristics of the powders.The results indicate that with the increase of the ball to powder mass ratio,(1)the fractal dimension for particles size distribution decreases firstly and then increases gradually and finally tends to be stable,which shows that the homogeneity of particles size distribution gets worse firstly then becomes better;(2)the boundary fractal dimension based on perimeter-area method decreases,while the circularity of the powders increasesand the irregularity of profile of the powders decreases;(3)the surface fractal dimension decreases firstly and then increases,which indicates that the surface roughness of the powders decreases firstly and then increases.All these fully suggest that the powder morphology,surface roughness,particle size distribution and other factors will affect the flowability of the powders,and the change in the flowability of the powders is related to the change in the primary and secondary status of these factors under certain conditions.