Analysis on the dynamic properties of metal powder sintering


The geometrical shape of welding pool is controlled by the technological parameters (such as laser power, scanning speed, powder layer thickness and so on), material properties and other factors. However, this kind of welding pool is composed by an area rather than a thread, which will solidify firstly. Except for the partial melting region in the welding pool, the rest part of regions all belong to the melting zone which will begin to solidify only when the laser beam removed from the welding pool for a period of time. Once the temperature in this region is lower than the liquidus surface, it will automatically nucleate and solidify. Obviously, this is a kind of heteroepitaxial growth nucleation mode. Regarding the melting zone, of which the solidification nucleation including two modes: powder particles that are partially melted will exist in this region, which may become the heterogeneous nucleation core; powder particles are completely melted, which may become the homogeneous nucleation in this region.

Solidification characteristics of melting zone

In the nucleation and growth process of solidified crystal, the crystal will compete for growth mutually due to the fast and dynamic fusing characteristics of welding pool. It can be seen that the growth rate of one kind of crystal orientation can be expressed as vhkl=vb (cos/cos), in which vb refers to the scanning speed of laser beam and vnkl refers to the solidification rate of dendrite. Therefore, the growth rate of crystal orientation will be comprehensively influenced by the angle between laser movement direction and normal direction of solid-liquid interface in the molten pool boundary and the angle between the normal direction of solid-liquid interface in the molten pool boundary and the orientation of crystal. Parameters that influence the crystal orientation can be changeable due to the dynamic sintering process. As a result, the growth direction of crystal will also change constantly.

Regarding the melting zone, due to the different powder particle size (powder particle size distribution is in the range of 15-130m), small size of powder particles may be completely melted and big size of powder particles will be melted partially when the laser power is not very large, thus the particles that are partially melted will exist in the melting zone and may become the heterogeneous core; when the laser power is large, powder in the melting zone can be completely melted and may become homogeneous nucleation. When the laser power is very small, the powder particles are easy to form sphere which will has bad effect on sintering and forming.

Therefore, large power density is generally used in the metal powder sintering and forming process, in which the homogeneous nucleation is the main status of its melting zone that can help to form the isometric crystal. The growth orientation of the melting zone crystal will be disturbed and the crystal orientation will become disordered due to the strong effect of melt convection. Analysis on multilayer and multipass laser sintering samples shows that microstructure heterogeneity is existed in the microstructure of sintered body.

Research has shown that a certain amount of rare earth elements can be added into the laser melting zone so as to eliminate the formation of dendrite in its bottom part. The solidification mechanisms of metal powder laser sintering mainly include two types: a. melt in the partially melting zone will solidify in the form of heteroepitaxy; b. melt in the melting part will solidify in the form of homogeneous nucleation. As the solidified crystal orientation is influenced by the molten pool shape, melt flow and other technological parameters, the dendrite orientation in the partially melting zone will be same with the normal direction of solid-liquid interface, while the crystal orientation in the melting zone is rather disordered. The structure of sintered body is formed by alternately superposing dendrite and isometric crystal, while the structure that forms the whole product will reappear periodically.