Feedstocks for additive manufacturing, and methods of using the same

Abstract

Some variations provide a method of making an additively manufactured metal component, comprising: providing a feedstock that includes a high-vapor-pressure metal; exposing a first amount of the feedstock to an energy source for melting; and solidifying the melt layer, thereby generating a solid layer of an additively manufactured metal component. The metal-containing feedstock is enriched with a higher concentration of the high-vapor-pressure metal compared to its concentration in the additively manufactured metal component. The high-vapor-pressure metal may be selected from Mg, Zn, Li, Al, Cd, Hg, K, Na, Rb, Cs, Mn, Be, Ca, Sr, or Ba, for example. Additively manufactured metal components are provided. Metal-containing feedstocks for additive manufacturing are also disclosed, wherein concentration of at least one high-vapor-pressure metal in the feedstock is selected based on a desired concentration of the high-vapor-pressure metal in an additively manufactured metal component derived from the metal-containing feedstock. Various feedstock compositions are disclosed.

Description

PRIORITY DATA[0001]This patent application is a non-provisional application with priority to U.S. Provisional Patent App. No. 62 / 540,615, filed on Aug. 3, 2017, which is hereby incorporated by reference herein.FIELD OF THE INVENTION[0002]The present invention generally relates to processes for additive manufacturing using optimized metal-containing precursors (e.g., powders).BACKGROUND OF THE INVENTION[0003]Metal-based additive manufacturing, or three-dimensional (3D) printing, has applications in many industries, including the aerospace and automotive industries. Building up metal components layer-by-layer increases design freedom and manufacturing flexibility, thereby enabling complex geometries while eliminating traditional economy-of-scale constraints. In metal-based additive manufacturing, application of a direct energy source, such as a laser or electron beam, to melt alloy powders locally results in solidification rates between 0.1 m / s and 5 m / s, an order of magnitude increas...

AI Technical Summary

Benefits of technology

The present invention provides a method for making an additively manufactured metal component by exposing a metal-containing feedstock to an energy source and solidifying the feedstock to generate a first solid layer. The metal-containing feedstock contains a high-vapor-pressure metal and at least one other metal species different from the high-vapor-pressure metal. The concentration of the high-vapor-pressure metal in the feedstock is higher than the concentration in the first solid layer. The method can use various techniques such as selective laser melting, electron beam melting, laser engineered net shaping, selective laser sintering, direct metal laser sintering, integrated laser melting with machining, laser powder injection, laser consolidation, direct metal deposition, wire-directed energy deposition, plasma arc-based fabrication, ultrasonic consolidation, and combinations thereof. The method can also involve repeating steps to generate multiple solid layers with smaller grain sizes. The invention provides a metal component with improved mechanical properties and a microstructure with equiaxed grains.

Problems solved by technology

The vast majority of the more than 5,500 alloys in use today cannot be additively manufactured because the melting and solidification dynamics during the printing process lead to intolerable microstructures with large columnar grains and cracks.

3D-printable metal alloys are limited to those known to be easily weldable.

The limitations of the currently printable alloys, especially with respect to specific strength, fatigue life, and fracture toughness, have hindered metal-based additive manufacturing.

In contrast, most aluminum alloys used in automotive, aerospace, and consumer applications are wrought alloys of the 2000, 5000, 6000, or 7000 series, which can exhibit strengths exceeding 400 MPa and ductility of more than 10% but cannot currently be additively manufactured.

These same elements promote large solidification ranges, leading to hot tearing (cracking) during solidification—a problem that has been difficult to surmount for more than 100 years since the first age-hardenable alloy, duralumin, was developed.

This mechanism results in solute enrichment in the liquid near the solidifying interface, locally changing the equilibrium liquidus temperature and producing an unstable, undercooled condition.

As a result, there is a breakdown of the solid-liquid interface leading to cellular or dendritic grain growth with long channels of interdendritic liquid trapped between solidified regions.

As temperature and liquid volume fraction decrease, volumetric solidification shrinkage and thermal contraction in these channels produces cavities and hot tearing cracks which may span the entire length of the columnar grain and can propagate through additional intergranular regions.

Note that aluminum alloys Al 7075 and Al 6061 are highly susceptible to the formation of such cracks, due to a lack of processing paths to produce fine equiaxed grains.

Another problem associated with additive manufacturing of metals is that producing equiaxed structures typically requires large amounts of undercooling, which has thus far proven difficult in additive processes where high thermal gradients arise from rastering of a direct energy source in an arbitrary geometric pattern.

Yet another problem associated with additive manufacturing of metals arises from the vapor pressures of some metals themselves.

Most engineering alloys contain multiple alloying elements that vaporize rapidly at high temperatures and can be selectively lost during additive manufacturing or welding.

In particular, at high temperatures encountered during additive manufacturing, significant vaporization of alloying elements can happen out of the melt pool.

Since some alloying elements are more volatile than others, selective vaporization of alloying elements often results in a significant change in the composition of the alloy.

For example, during laser welding of aluminum alloys, losses of magnesium and zinc result in pronounced changes to their concentrations.

The composition change can cause degradation of mechanical properties (e.g., tensile strength) and chemical properties (e.g., corrosion resistance) in the final structure.

However, it is not always possible to minimize temperature due to the presence of high-melting-point metals that need to be liquefied during additive manufacturing.

Likewise, depending on the specific additive manufacturing set-up or three-dimensional object to be printed, it is not always possible to reduce surface-to-volume ratio of the melt pool—or even if that can be done, it may not be sufficient to prevent significant vaporization of high-vapor-pressure metals.

A lack of teaching in the art with respect to processing of alloy systems that undergo vaporization makes it very difficult to select targeted alloy feedstock compositions.

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