By: Shen Xianfeng
Selective Laser Sintering (SLS) of metal powders, namely Direct Metal Laser Sintering (DMLS) is an emerging Rapid Prototyping and Manufacturing (RPM) technology which can be used to produce three-dimensional metal parts directly from a CAD model by the selective laser sintering (SLS) of successive layers of metallic or pre-alloyed powders. SLS of metal powders is becoming a significant trend of Direct Manufacture (DM) for the unique advantage in direct manufacturing of metal parts and eliminating the expensively time-consuming pre- and post-processing steps, compared to indirect laser sintering or other conventional processes. In the laser sintering process of metal powder, the substrate plays an important role in DMLS. As a matter of fact, the substrates are commonly used for avoiding the balling effect in powder melting and dislocation of sintered part. So the issue of the substrate role in the process is becoming one of the research focuses.
The change of material thermal conductivity in powder-to-solid transition cannot be ignored for the heat transference by the loose powder is much lower than by the nearly full-density solid body in SLS of metal powders, for example, thermal conductivity of Cu is about 100 times bigger than that of Cu powder. Modeling of nonlinear and transient powder-to-solid transition of material thermal conductivity during DMLS process is proposed. The thermal conductivity model between solidus and liquidus temperatures for powder-to-solid transition is established according to liquid phase sintering (LPS) theory. In addition, the thermal conductivity model of the whole temperature scope is developed in sections. Thermal conductivity model including powder-to-solid transition effect is shown in Fig.1.
Fig.1: Thermal conductivity model including powder-to-solid transition effect.
Experiments and simulations have been carried out using multi-component Cu-based metal powder. Element-dependent material properties in SLS are determined according to the history maximum temperature. The thermal conductivity and heat capacity of the compounds is calculated using the weighted average approach. Some conclusions related to the simulation results are reached: When using substrate, the maximum temperature in powder bed becomes lower and sintered zone is reduced, which is mainly caused by the higher consistent thermal conductivity of substrate. Measures of employing moderate thermal conductivity substrate and preheating substrate should be taken lest input-heat mainly transfers to substrate.
The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here