Direct selective laser sintering of metals

Details Direct selective laser sintering of metals Direct selective laser sintering of metals

2001-06-01

2004-01-13

Jenkins, Daniel J. (Department: 1742)

Powder metallurgy processes

Powder metallurgy processes with heating or sintering

Making composite or hollow article

Reexamination Certificate

active

06676892

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method for using a directed beam of energy to selectively sinter metal powder to produce a part. Specifically, the invention relates to the use of selective laser sintering (SLS) in order to produce a full density metal part.
BACKGROUND OF THE INVENTION
Solid Freeform Fabrication (SFF) is a group of emerging technologies that have revolutionized product development and manufacturing. The common feature shared by these technologies is the ability to produce freeform, complex geometry components directly from a computer generated model. SFF processes rely on the concept of layerwise material addition in selected regions. A computer generated model serves as the basis for making a replica. The model is mathematically sliced and each slice is recreated in the material of choice to build a complete object. A typical SFF machine is a “miniature manufacturing plant” representing the convergence of mechanical, chemical, electrical, materials and computer engineering sciences.
The first application of SFF technologies was in the area of Rapid Prototyping (RP). Rapid Prototyping enables design and manufacturing engineers to quickly fabricate prototypes in a fraction of the times and at typically less than half the costs in comparison with conventional prototyping methods. The tremendous economy of RP is facilitated by its high degree of automation, both in the design and fabrication cycles. On the design side, the advantages are at least fourfold. First, the use of Computer Aided Design (CAD) solid modeling software allows the design of a component to be stored digitally, obviating the need for detailed technical drawings and the extensive manual labor associated therewith. Second, the advent of so-called “parametric” CAD modeling software allows facile incorporation of design changes into an existing CAD design quickly. Third, the same CAD solid model can be fed to a variety of dynamic, mechanical and thermal analyses software, resulting in a high degree of design integration. Finally, each CAD model can be electronically “tagged” for incorporation into databases that store information on parts assemblies, design variants and manufacturing methods. Lately, a move towards standardizing such information integration is taking place in the specification of the Standard for Exchange of Product Data (STEP). On the manufacturing side, computer driven RP machines accept the CAD solid model as input to automatically create a physical realization of the desired component. The major advantage realized here is the substantial elimination of process planning, operator expertise, additional tooling and set-up. The overall advantage of this powerful combination is the ability to rapidly iterate through several design and prototyping cycles before “freezing” the design for final production at significantly lowered costs and shorter “time to market”.
Most RP technologies were initially developed for polymeric materials. These technologies allowed designers to rapidly create solid representations of their designs in a surrogate material for design visualization and verification. Further demand for functional prototypes led to the development of materials and processes that enabled production of prototype patterns and parts that could be subjected to limited testing for form and fit. Major developments have taken place next in the area of SFF known as Rapid Tooling. The focus of this area has been to develop SFF technologies to enable rapid production of prototype tooling for a variety of manufacturing techniques including injection molding, electro-discharge machining and die casting. The growth in this area has been spurred by the economical advantages of making limited run prototype tooling via SFF as compared to conventional techniques.
Over the past ten years, there has been an explosion in the development and growth of SFF technologies. These technologies can be broadly categorized into three classes, namely transfer, indirect and direct SFF methods. Transfer methods are those methods that use a pattern or sacrificial intermediary to generate the desired component whereas “indirect” methods are those SFF methods that directly produce intermediate density parts that undergo post-processing such as conventional sintering and infiltration to attain full density. Direct methods are methods that directly produce fully dense or near fully dense complex shaped parts in the desired composition (e.g. metal, ceramic or cermet) by applying a geometry and property transformation to the material with minimal post-processing requirements. In the context of making metal components by SFF, a number of “transfer” and “indirect” methods are available.
Selective laser sintering (SLS) is a SFF technique that creates three-dimensional freeform objects directly from their CAD models. An object is created by selectively fusing thin layers of a powder with a scanning laser beam. Each scanned layer represents a cross section of the object's mathematically sliced CAD model.
Direct Selective Laser Sintering (Direct SLS), the relevant field of this invention, is a direct SFF technique. Direct SLS is a laser based rapid manufacturing technology that enables production of functional, fully dense, metal and cermet components via the direct, layerwise consolidation of constituent powders. In Direct SLS, a high energy laser beam directly consolidates a metal or cermet powder to a high density (>80%), preferably with minimal or no post-processing requirements. The main advantages associated with this technique are elimination of expensive and time-consuming pre-processing and post-processing steps. In comparison with “indirect SLS”, direct SLS is a binderless process. Direct SLS also does not involve furnace de-binding and infiltration post-processing steps as in “indirect SLS”. Compared to conventional bulk metal forming processes (e.g. casting or forging), direct SLS does not require the use of patterns, tools or dies. The metal powder being processed by direct SLS directly undergoes a shape and property transformation into final product that may require minimal post-processing such as finish machining.
Several processing requirements differentiate direct SLS of metals from SLS of polymers or polymer coated powders. Perhaps the most important distinguishing characteristic is the regime of high temperatures involved in direct SLS of metals. At the temperatures necessary for processing metals of interest (generally >1000° C.), the reactivity of the melt poses serious process control issues. Control of the processing atmosphere takes on paramount importance since it not only enables successful layerwise buildup but also addresses safety concerns. In one particular application of SLS known as SLS/HIP, the goal of in-situ containerization of a part fabricated during SLS processing requires that it take place under vacuum to ensure full densification of the canned shape during HIP post-processing.
Until recently, no work was reported on direct SLS of high performance materials, such as Nickel and Cobalt base superalloys, superalloy cermets, Titanium base alloys and monolithic high temperature metals such as Molybdenum. These materials are used for high performance components that typically experience high operating temperatures, high stresses and severe oxidizing or corrosive environments. Direct SLS, with its ability to produce components in such materials is especially useful for functional prototype, low volume or “one of a kind” production runs. To manufacture a typical prototype lot of 100 superalloy cermet abrasive turbine blade tips, direct SLS can achieve acceptable microstructure and properties with 80% cost savings over the traditional methods. Aerospace industries face typical lead times of several dozen weeks for functional, metallurgical quality prototypes. Direct SLS can lower cost and drastically reduce lead times by eliminating pre-processing and post-processing steps, and by eliminating the need for specialized tooling.
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