Aluminum alloys with grain refiners, and methods for making and using the same

Abstract

We have developed a scalable approach to directly incorporate grain-refining nanoparticles into conventional hot-tear-susceptible aluminum alloy powders. These aluminum alloy powders may be additively manufactured into high-strength, crack-free aluminum alloys with fine equiaxed microstructures by incorporating nanoparticle nucleants to control solidification during additive manufacturing. Some variations provide an additively manufactured aluminum alloy comprising aluminum, one or more strengthening elements, and at least one grain-refining element, wherein the additively manufactured aluminum alloy has a microstructure with equiaxed grains. Aluminum alloys with grain refiners are useful in many processes beyond additive manufacturing. Some variations provide an aluminum alloy comprising aluminum, copper, magnesium, at least one of zinc or silicon, and grain-refining nanoparticles selected from zirconium, tantalum, niobium, or titanium, wherein the aluminum alloy has a microstructure that is substantially crack-free with equiaxed grains.

Description

PRIORITY DATA[0001]This patent application is a non-provisional application with priority to U.S. Provisional Patent App. No. 62 / 452,989, filed on Feb. 1, 2017, which is hereby incorporated by reference herein.FIELD OF THE INVENTION[0002]The present invention generally relates to metal alloys with grain refiners, and methods of making and using the same.BACKGROUND OF THE INVENTION[0003]Aluminum and its alloys are characterized by a relatively low density, high electrical and thermal conductivities, and a resistance to corrosion in some common environments, including the ambient atmosphere. Recent attention has been given to alloys of aluminum as engineering materials for transportation to reduce fuel consumption due to high specific strength. The mechanical strength of aluminum may be enhanced by cold work and by alloying. Principal alloying elements include copper, magnesium, silicon, zinc, and manganese.[0004]Generally, aluminum alloys are classified as either cast or wrought. Som...

AI Technical Summary

Benefits of technology

The patent text claims that the microstructure of additively manufactured aluminum alloy is free of cracks and porous void defects. This results in a high quality product that can be used in various applications.

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.

Producing equiaxed structures 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.

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