3D printing for metals: indirect and selective laser sintering

Abstract

Much has been published recently for 3D printing. For polymers, it is already a reality, considering various processes as rapid prototyping or even for mass production. For metals, however, there is still some uncertainty related to technical and economic issues. Equipment producers for additive manufacturing has presented solutions, and one of the most promising is called Selective Laser Melting, or Selective Laser Sintering. Although some manufacturers call it as a sintering process, which actually occurs is melting layer-by-layer, from a region selected by a digital model that "moves" a laser beam of high intensity, in this case. This process, direct sintering layer by layer, has two disadvantages: the powder should possess physical characteristics so specific that makes the cost very high; and the fact that it involves melting causes a final heterogeneous microstructure, with intense segregation. Because of this, alternatives such as "indirect laser sintering", object of this project, seem to be an interesting alternative. In this case, the metal powder is mixed with a fraction of polymeric material, and the mixture feeds, layer by layer, a chamber that receive the action of low intensity laser which "sintering" the organic part, forming the product into desired shape. Next, the product passes through a "debinding" step followed by sintering, similar to injection molding, or MIM (Metal Injection Molding).In this context, two companies, BRATS (www.brats.com.br) and ALKIMAT (www.alkimat.com.br) joined forces to testing the process for at least one product, sintered stainless steel filter, produced conventionally by BRATS. Two expertises, one in laser additive manufacturing (ALKIMAT) and other related to Powder metallurgy has made companies asked directly for Phase II. The ALKIMAT machine has a "pilot" laser machine, which was tested in the manufacture of sintered filters, where "debinding" and sintering has been conducted by BRATS. The objective of this project is therefore to improve the production capacity of porous media in stainless steel, with greater flexibility in shape and size (compared to the conventional process, uniaxial compaction and sintering) and reaching "structural" components (densities exceeding 95 % relative) also in stainless steel. For this, it will be carried out a "scale up" for current laser sintering machine. A major effort will be in the conditioning of raw material. Special attention will be paid to the types of powders and polymeric materials, and particularly to a good mixture of these components ensuring a good supply (flowability), layer-by-layer, and a proper "debinding" without distortion and an effective sintering to produce a porous component. Dense components will be also considered for applications for which the mechanical properties and corrosion resistance are necessary. The methodology to be employed in this project will involves: process consolidation, using the laser pilot equipment in the production of stainless steel porous media (filters); improvements of the feedstock (mixture of metal powder with organic material); design and construction of flexible semi-industrial laser equipment ; and validation of this equipment for applications presented here (porous and dense component). The investigation of adequacy of the developed process considering other products will be considered, and it will depend on the demands coming from various. The project team, strong point of this proposal will be composed of highly qualified professionals from both companies, and with extensive experience in laser manufacturing and powder metallurgy. (AU)