Apparatus and method of producing a fine grained metal sheet for forming net-shape components

Abstract

A method and apparatus for producing ultra-fine grained magnesium metal alloy material sheets. The apparatus molds and rapidly solidifies a metal alloy material to form a fine grain precursor. The precursor is then subjected to deformation strains that alter the grain structure of the precursor so as to form a ultra fine grained structure in sheet form. The sheet form may then be subjected to superplastic forming to form a net shaped article.

Description

BACKGROUND [0001] 1. Field of the Invention [0002] The present invention relates to producing net shaped components of increased strength. More particularly, the invention relates to producing magnesium alloy sheet components, having micrometer sized grain structures, that can be subsequently used in the production of net shaped sheet components of increased strength. [0003] 2. Related Technology [0004] Over the last several decades, magnesium (Mg) alloy development has been inhibited by certain barriers. While wrought magnesium has the potential for making thinner structures, anisotropy in mechanical properties limits the applications of Mg alloys and their wrought products. The strength of Mg alloys is rather low in certain directions in comparison to most widely used structural materials, such as steels and precipitation-hardened aluminum (Al) alloys. For example, in-plane compression, yield strength can be only 85 MPa in basal textured Mg alloy; AZ31 B Mg alloy sheet in the H24 ...

AI Technical Summary

Benefits of technology

[0163] Potential markets for products manufactured by the present invention are envisioned in the automotive and aerospace fields, among others, where weight savings can be gained by replacing steel and aluminum with magnesium. Complex 3-D net-shapes can be SPF to greatly reduce the number of sub-assemblies and the costs of multiple fabrication and assembly. High tensile strength and high toughness will be attained by sub-micron grain sizes, second phase nanocrystals and by the selection of ductile alloys. The unique microstructure so attained will greatly reduce texture and its usual barrier to formability.

[0166] It is envisioned that the deformation strain processing can be accomplished by integrating injection molding machines for metal with conventional pressing and rolling equipment and should be feasible on process equipment already used in the aerospace and automotive industries. Deep drawing also can be practiced on conventional presses.

[0167] As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementations of the principles of this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.

Problems solved by technology

While wrought magnesium has the potential for making thinner structures, anisotropy in mechanical properties limits the applications of Mg alloys and their wrought products.

A high value of normal anisotropy in sheet is helpful for deep drawing, but may not be suitable for other applications, particularly if in-plane strength is also anisotropic.

In fact, this base element has not been a friendly host for extensive alloy strengthening.

The alloying elements that improve corrosion resistance and castability, such as Al, unfortunately introduce eutectic intermetallic phases.

Furthermore, it is difficult to attain efficient age hardening by fine precipitates within the grains, as exemplified by the case of inefficient Al additions.

Elements that promote age hardening, such as rare earth metals, are costly, detrimental to castability and ineffective in resisting corrosion.

At room temperature, “basal a” slip {0001} is predominant, while “prism a” and slip are difficult because of their significantly high critical resolved shear stresses (CRSS), which are reported in regions of high stress concentration such as grain boundaries and twin interfaces.

A single twinning mode cannot fully accommodate plastic deformation.

When basal slip is inhibited at ambient temperatures, twinning deformation can be localized, which leads to low ductility in Mg.

Two major drawbacks restrict the application of wrought Mg alloys.

First the symmetry of hexagonal close packed crystal structures has the effect of limiting the number of independent slip systems, thus providing alloys with poor formability and ductility near room temperature.

), although helpful to overcome the restriction of slip, makes oxidation problems more severe.

However, for HCP metals, such as Mg alloys, grain refinement may also cause texture variation, and inadequate strengthening in certain directions.

These processes are costly, time consuming and have not enjoyed commercial success.

Several other schemes for severe plastic deformation (SPD) have proven to be unpractical for manufacturing larger quantities of ultra-fine grain metals.

When these processes are used in a repeated manner, the overlap of shear zones within the bulk material from individual steps causes extensive grain subdivision and formation of fine grain structure.

Deformation by several deformation passes through equal channels (so called ECAP) has been practiced in the lab on Mg bars, but is not practical for Mg sheets.

To date, Mg alloys have not enjoyed this advantageous processing in commerce.

First, Mg alloy castings do not have the prerequisite grain boundary crystal structure and, secondly, wrought Mg sheets have been too coarse grained and / or too textured for superplastic forming.

However, construction and assembly of such a microstructure for bulk structural parts, ab-initio from nano-powders, is a very costly and laborious.

Also, there are safety and health concerns for handling such fine particles in the workplace.

The grain boundary structure in conventionally prepared Mg alloy is not favorable to complex deformation without premature fracture, unless an elevated forming temperature is used.

The pressing and deep drawing of 3-D shapes is limited by the texture and the inherent non-uniform deformation that results from twinning, such as for example, “earring”, where shapes resembling ears are formed in portions of the grain microstructure.

Although twinning in some directions of the sheet causes increased elongation during tensile testing, twinning is an impediment to the formation of complex parts due to the anisotropy it produces in coarse grain Mg alloy, resulting in anomalies in work hardening and non-uniform deformation.

Further, the modeling of forming processes and performance in the dies is not reliable with such non-uniformity in structure.

Also, the coarse surface finish of present coarse grain Mg alloys poses a challenge to their acceptance as automotive sheet parts.

The TRC product is typically too thin to refine the grain size below 7 μm by such hot processing.

The TRC structure also suffers from centerline porosity.

Continuous cast Mg alloy may have promise, but currently this technology is not fully developed and many individual pieces of technologies are required for its full implementation, the scope of which is incompatible with small business operations and may not have the flexibility offered by the process of the present invention.

The many stages involved in breaking down large-grained conventional sheet precursors to produce the sheet form cause current wrought Mg alloys to be expensive, on the order of $5.00 to $10.00 / lb.

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