Bin Xiao

Laser sintering of metal powders on top of sintered layers under multiple-line laser scanning

A three-dimensional numerical model for multiple-line sintering of loose powders on top of multiple sintered layers under the irradiation of a moving Gaussian laser beam is carried out. The overlaps between vertically deposited layers and adjacent lines which strengthen bonding are taken into account. The energy equation is formulated using the temperature transforming model and solved by the finite volume method. The effects of the number of the existing sintered layers, porosity and initial temperature coupled with the optimal combination laser intensity and scanning velocity are presented. The results show that the liquid pool moves slightly towards the negative scanning direction and the shape of the liquid pool becomes shallower with higher scanning velocity. A higher laser intensity is needed to achieve the required overlaps when the number of the existing sintered layers increases. Increasing porosity or initial temperature enhances the sintering process and thus less intensity is needed for the overlap requirement.

Marangoni and Buoyancy Effects on Direct Metal Laser Sintering with a Moving Laser Beam

A three-dimensional model describing melting and resolidification of a direct metal laser sintering process under the irradiation of a moving Gaussian laser beam is developed. Effects of shrinkage and natural convection driven by the surface tension and buoyancy force are taken into account. The energy equation is formulated using a temperature-transforming model and solved by the finite-volume method. The temperature distribution and velocity field are investigated. The results show that increasing the initial porosity of the powder bed enlarges the depth of the melt–solid interface and that the laser intensity has great influence on both the depth and width of the liquid pool. The whole molten pool shifts toward the direction opposite the laser scanning as the scanning velocity increases.

Partial melting and resolidification of metal powder in selective laser sintering

Partial melting and resolidification of single-component metal powders bed with a Gaussian laser beam is investigated numerically. Because laser processing of metal powder is a very rapid process, the temperature of the liquid layer and solid core of a partially molten particle may not at thermal equilibrium, that is, the temperature of the liquid part is higher while the temperature of the solid core is lower than the melting point. To use an equilibrium model to describe melting of single-component metal powder, the local temperature of regions with partially molten particles can be assumed to be within a range of temperature adjacent to the melting point, instead of at the melting point. In addition, the whole powder bed shrinks as the gas is driven out during the melting process. A temperature transforming model is employed to simulate the melting and resolidification process over a temperature range with the consideration of shrinkage. The convection driven by capillary and gravity forces in the melting liquid pool is formulated by using Darcys law. Effects of laser beam intensity and scanning velocity on the shape and size of the heat affected zone and molten pool are analyzed.

Analysis of Partial Melting in Metal Powder Bed with Constant Heat Flux

Rapid melting of a subcooled single-component metal powder bed in Selective Laser Sintering (SLS) is analyzed in this paper. Under irradiation of a pulse laser beam, the surface of the powder particle is molten first while the core of the particle remains solid. The temperature of the liquid layer is higher than the melting point, while the temperature of the solid core is below the melting point. Therefore, the mean temperature of the partially molten particle is within a range of temperature adjacent to the melting point. In addition, the powder bed experiences a significant density change during melting because the interstitial gas initially in the pore space is driven out as melting progresses. Melting in SLS of single-component metal powder can therefore be modeled as that occurring in a range of temperature with significant density change. The temperature distributions in the solid, liquid, and mushy zones and locations of the various interfaces are obtained by using an integral approximate method. The effects of initial porosity, dimensionless initial temperature, and dimensionless thermal conductivity of the interstitial gas on the surface temperature and locations of the interfaces are investigated.

Numerical simulation of pulsatile turbulent flow in tapering stenosed arteries

Purpose – The purpose of this paper is to investigate the geometric effects and pulsatile characteristics during the stenotic flows in tapering arteries.Design/methodology/approach – The low Reynolds number k − ω turbulence model is applied to describe the stenotic flows in the tapering arteries in this paper. The results are divided into two sections. The first section characterizes the geometric effects on the turbulent flow under steady condition. The second section illustrates the key physiological parameters including the pressure drop and wall stress during the periodic cycle of the pulsatile flow in the arteries.Findings – The tapering and stenoses severity intensify the turbulent flow and stretch the recirculation zones in the turbulent arterial flow. The wall shear stress, pressure drop and velocity vary most intensively at the peak phase during the periodic cycle of the pulsatile turbulent flow.Originality/value – This paper provides a comprehensive understanding of the spatial‐temporal fluid dy...

Analysis of Melting in a Single-Component Metal Powder Bed Subject to Constant Heat Flux Heating

To model Selective Laser Sintering (SLS) of single-component metal powders, melting of a subcooled powder bed with single-component metal powder is investigated analytically. Since laser processing of metal powder is a very rapid process, the liquid and solid phases of a partially molten powder particle may have different temperatures: the temperature in the liquid phase is higher than the melting point, and the temperature in the solid phase is below the melting point. Therefore, the local temperature of regions with partial molten particles is within a range of temperature adjacent to the melting point, instead of at melting point. In addition, the powder bed experiences a significant density change during melting. Therefore, melting of a metal powder bed can be modeled as a melting that occurs in a range of temperature with significant density change. The temperature distributions and locations of the various interfaces were obtained by solving the governing equations for solid, liquid and mushy zones in a one-dimensional system using an integral approximate method. The effects of porosity, sub-cooling, dimensionless thermal conductivity of gas, and dimensionless heat flux on the surface temperature and locations of the interfaces were investigated.Copyright

Numerical Simulation of Direct Metal Laser Sintering of Single-Component Powder on Top of Sintered Layers

A three dimensional model describing melting and resolidification of direct metal laser sintering of loose powders on top of sintered layers with a moving Gaussian laser beam is developed. Natural convection in the liquid pool driven by buoyancy and Marangoni effects is taken into account. A temperature transforming model is employed to model melting and resolidification in the laser sintering process. The continuity, momentum, and energy equations are solved using a finite volume method. Effects of dominant processing parameters including number of the existing sintered layers underneath, laser scanning velocity and initial porosity on the sintering process are investigated.© 2006 ASME

Partial Melting and Resolidification of Single-Component Metal Powder With a Moving Laser Beam

Partial melting and resolidification of single-component metal powders with a moving laser beam is investigated numerically. Since laser processing of metal powder is a very rapid process, the liquid layer and solid core of a partially molten powder particle may not at thermal equilibrium and have different temperatures: the temperature of the liquid part is higher than the melting point, and the temperature of the solid core is below the melting point. Therefore, the local temperature of regions with partial molten particles is within a range of temperature adjacent to the melting point, instead of at the melting point. The partial melting of the metal powder is also accompanied by shrinkage that drives out the gas in the powder bed and the powder structure is supported by the solid core of the partially melted powder particles. Melting with shrinkage and resolidification are described using a temperature transforming model. The convection driven by capillary and gravity forces in the melting liquid pool is formulated by using Darcy’s law. The effects of laser beam intensity and scanning velocity on the shape and size of the heat affected zone and molten pool are investigated.Copyright