FUNCTIONALLY GRADED COATINGS AND CLADDINGS

Abstract

A shear assisted extrusion process for producing cladded materials wherein a cladding material and a material to be cladded are placed in sequence with the cladded material positioned to contact a rotating scroll face first and the material to be cladded second. The two materials are fed through a shear assisted extrusion device at a preselected feed rate and impacted by a rotating scroll face to generate a cladded extrusion product. This process allows for increased through wall strength and decreases the brittleness in formed structures as compared to the prior art.

Description

BACKGROUND OF THE INVENTION

Several techniques are currently employed to clad materials. These include techniques such as extrusion, rolling, electroplating, weld forming, explosive bonding and the like. Among these techniques extrusion and rolling are sensitive to variable flow stresses of the two materials and require strenuous optimization of processing parameters. Typical defects or challenges to be addressed in using these techniques include non-uniform thicknesses, porous interfaces, lack of metallurgical bonding, etc. and it especially becomes challenging when working with anisotropic hexagonal close packed (HCP) material such as magnesium, titanium or zirconium. The other aforementioned techniques are slow batch processes and unable to control the graded interface or create the desired texture in the final component.

Magnesium is a desirable material for lightweight structures is limited by its corrosion resistance, but cladding it with aluminum or similar material will significantly improve the corrosion resistance and its ability to join with other material systems. Several technical challenges arise when forming such structure such as controlling the texture of the magnesium such that the asymmetry in mechanical properties under compression and tension is eliminated, the preferred grain size of the magnesium is less than 5 micron with an aluminum cladding bonded to the magnesium and the same time forms a graded interface to minimize the corrosion rate in the system. The present disclosure provides a methodology that allows for making structures with specified cladding as well as making structures that have desired shapes and microstructural and mechanical characteristics that existing methodologies struggle to provide.

To meet these needs a process has been developed wherein functionally graded claddings and coatings are produced in a single step with tailored physical properties (such as microstructure, mechanical, electrical, thermal, etc.) and at the same time provide high corrosion resistance. Typically clad materials are preferred material systems for engineering applications, as one metal/alloy often does not satisfy the required application conditions. The major advantage of cladding is the ability to tailor properties such that the surface has a different chemical composition and properties relative to the core. For example aluminum clad copper wires provide excellent conductivity with improved corrosion life. Clad materials also offer minimal use of expensive materials, such as high temperature materials, and at the same time retain the desired physical properties such as thermal conductivity.

Over the past several years researchers at the Pacific Northwest National Laboratory have developed a novel Shear Assisted Processing and Extrusion (ShAPE™) technique which uses a rotating ram as opposed to the axially fed ram used in the conventional extrusion process. As described in the previously cited and incorporated references, in some embodiments the ram face contains spiral scroll features which when brought into contact with a solid billet and a forging load is applied, significant heating occurs due to friction, thus softening the underlying billet material. The combined action of the forging load together with the rotating action of the ram face, force the underlying material to flow plastically. The scroll features on the ram face help in the material flow and help in controlling the texture.

We have successfully demonstrated the scalability of this process, and we were able to alter and control the texture, grain size and also uniformly disperse the secondary particles by changing a few process parameters and at loads/pressure several orders or magnitude lower than conventional extrusion. We have now expanded applications of this tool and process to generate cladded materials by extrusion and to control various features of structures formed by this technique.

This provides significant promise over several of the prior art techniques which are typically employed to clad materials such as extrusion, rolling, electroplating, weld forming, explosive bonding, etc. Extrusion and rolling are sensitive to variable flow stresses of the two materials and require strenuous optimization of processing parameters. Typical defects or challenges to address using these techniques are non-uniform thickness, porous interface, lack of metallurgical bond, etc. and it especially becomes challenging when working with anisotropic HCP material such as magnesium, titanium or zirconium. Typically the aforementioned techniques are performed in a slow batch processes and are unable to control the graded interface or create the desired texture in the final component. Conventional linear extrusions typically have virtually constant crystallographic texture across the wall thickness.

Developing a method for forming extrusions while simultaneously varying the texture across the wall thickness could lead to improved bulk material properties. Such improvement could include but are not limited to increased strength, reduced susceptibility to corrosion and brittleness, Mechanical property improvements through breakdown and dispersion deleterious second phase particles, corrosion resistance though elimination of galvanically unfavorable second phases and precipitates, and extrusion of brittle intermetallic materials not possible by conventional means among them.

The purpose of the foregoing abstract is to enable the United States Patent and Trademark Office and the public generally, especially the scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.

Various advantages and novel features of the present invention are described herein and will become further readily apparent to those skilled in this art from the following detailed description. In the preceding and following descriptions I have shown and described only the preferred embodiment of the invention, by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the drawings and description of the preferred embodiment set forth hereafter are to be regarded as illustrative in nature, and not as restrictive.

SUMMARY

The advantages of the present disclosure lie in the application of a shear assisted extrusion process for producing cladded materials wherein a cladding material and a material to be cladded are placed in sequence with the cladded material positioned to contact a rotating scroll face first and the material to be cladded second. The two materials are fed through a shear assisted extrusion device at a preselected feed rate and impacted by a rotating scroll face to generate a cladded extrusion product.

In one example the cladding material is aluminum, and the material to be clad is magnesium or a magnesium alloy. In some instances the preselected feed rate is 0.05-1.0 inches per minute, in others the rotating scroll face rotates at a rate of 10-1000 rotations per minute. The rotating scroll face can have at least 2 starts. In some embodiments the axial extrusion force is less than 50 MPa and the temperature of the billet (aluminum, magnesium or both) is less than 100° C. In various applications feed rates are varied to include a rate of less than 0.2 inches (0.51 cm) per minute and the rotational shearing force is generated from spinning the die or the billet at a rate between 100 rpm to 500 rpm.

In another embodiment, a process for creating an aluminum cladded magnesium product comprising the steps of placing a thin sheet of aluminum having a hole defined therein on center of a magnesium billet in a shear assisted extrusion device, impacting the billet with a rotating scroll face rotating at rate of (10-1000 RPM) and a feed rate of 0.05-1.0 inches per minute to extrude an aluminum cladded magnesium material. This extrusion process can include the steps of: simultaneously applying a rotational shearing force and an axial extrusion force to a billet while contacting one end of the billet with a scroll face configured to engage and move plasticized billet material toward an orifice whereby the plastically deformed billet material flows substantially perpendicularly from an outer edge of the billet through the orifice forming an extrusion product with microstructure grains about one-half the size of the grains in the billet prior to extrusion. In some variations the extrusion of the plasticized billet material is performed at a temperature less than 100° C. In other applications the axial extrusion force is at or below 100 MPa.

The resulting materials developed by such a process provide materials with mechanical property improvements through breakdown and dispersion deleterious second phase particles enabled by such a process. This includes corrosion resistance though elimination of galvanically unfavorable second phases and precipitates. The extrusion of brittle intermetallic materials not possible by conventional means and other advantages not available in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, shows the placement of a billet of aluminum with hole on center in front of a magnesium billet also having a hole in the center in an arrangement that creates an aluminum cladded magnesium extrusion product.

FIG. 2 shows the cross section of a magnesium alloy wire/rod where the outer surface is clad with a high fraction of aluminum with highly refined grain size.

FIGS. 3(a)-3(c) show illustrative examples of different scroll geometries on the face on various extrusion dies.

FIG. 4 shows an example of the (0001) basal texture at two cross section locations for the 60 mil thick tube made with a 4 start scroll.

FIGS. 5(a) and 5(b) summarize the data for grain size and texture orientation for 60 mil thick tube walls made with 2, 4 and 16 start scrolls.

FIGS. 6(a) and 6(b) show for a 120 mil thickness tube made with a 4 start scroll.

FIGS. 7(a)-7(b) show the microstructure of AZS312 in the as-cast materials

FIGS. 7(c)-7(d) show the microstructure of AZS213 in the extruded materials formed by the claimed process.

DETAILED DESCRIPTION OF THE INVENTION

The following description including the attached pages provide various examples of the present invention. It will be clear from this description of the invention that the invention is not limited to these illustrated embodiments but that the invention also includes a variety of modifications and embodiments thereto. Therefore the present description should be seen as illustrative and not limiting. While the invention is susceptible to various modifications and alternative constructions, it should be understood, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims.

Various methods and techniques are described wherein application and manipulation and modification of the ShAPE™ technique and device as shown for example in demonstrated the ability to control microstructure such as crystallographic texture through the cross sectional thickness, while also providing the ability to perform various other tasks such as cladding aluminum to magnesium.

In one embodiment, such as the arrangement shown in FIG. 1, where in the typical mandrel is not present, a billet of aluminum with hole on center is placed in front of a magnesium billet also having a hole in the center, and processed within an extrusion die using a novel Shear Assisted Processing and Extrusion (ShAPE™) technique which uses a rotating ram. When brought into contact with the billet and a forging load is applied, significant heating at a high shear region near the contact between the die and the billet, thus softening the underlying billet material. The combined action of the forging load together with the rotating action of the ram face, force the underlying material to flow plastically. The scroll features on the extrusion die help in the material flow and help in controlling the texture.

It has been shown that the geometry of the scroll face, using 2, 4 and 16-start tools shown in FIG. 3(a)-3(b), and accompanying process parameters directly affect texture through the tube wall. With these scrolled patterns, the billet is rotated in the counterclockwise direction to gather material into the extrusion orifice as the billet is pressed against the die face.

In this arrangement where the aluminum billet and the magnesium billets are together within the structure the impact of the aluminum billet with the scroll face at relatively slow rate (150 RPM) and slow feed rate 0.3-0.15 inches per minute at room temperature, created an aluminum cladded magnesium extrusion product in tubular, wire, or rod form. In addition to this arrangement other arrangements have been shown to produce cladded materials such as rods and wire. In one particular instance this process was used to form actual extrusion components including ZK60 tubing having an outer diameter of 2.0″ and wall thickness of 60 and 120 mils.

FIG. 2 shows the cross section of a magnesium alloy wire/rod where the outer surface is clad with a high fraction of aluminum with highly refined grain size. Not only does the aluminum cladding provide a corrosion barrier for the magnesium, the highly refined microstructure within the aluminum clad is also know to reduce corrosion rate. This advantages are particularly seen with claddings that are otherwise difficult to form, examples include but are not limited to applications such as rivets and fastener applications, wires for electrical applications (inner aluminum and outer copper or vice versa or steel based), nuclear fuel, piping/conduits.

In other applications, methodologies using the ShAPE™ design have also been developed to allow for through-wall texture control, extrusion of additional materials that were previously considered not possible for extrusion by conventional extrusion processes (such as AZS 312/317 magnesium alloys), increasing the strength, ductility, corrosion resistance and energy absorption in various materials, significant grain refinement and basal texture alignment, creation of extruded materials with reduced potential for microgalvanic corrosion, breakdown of Mg₂Si intermetallics to nanoscale, elimination of Al—Zn precipitates by dissolution into solid solution, elimination of Mg₁₇Al₁₂ (β phase), uniform dispersion of second phases, improved ductility, and increased compressive yield stress. The application of this process is not limited to these alloys but unconventional systems like high entropy alloys have also been processed to create a single phase alloy and eliminated the dire homogenization step.

As will be described below in more detail, in addition to modifying various parameters such as feed rate, heat, pressure and spin rates of the process, various mechanical elements of the tool assist to achieve various desired results. For example, scroll patterns on the face of extrusion dies (in the ShAPE™ process) can be used to affect/control crystallographic texture through the wall thickness of extruded tubing. This can be used to advantageously alter bulk materials properties such as ductility and strength. These properties can in turn be tailored for specific engineering applications such crush, pressure or bending.

The die design and process parameters can offer unprecedented control over the microstructure of materials. An illustrative example is the use of different scroll geometry as shown in FIGS. 3(a)-3(c) for ZK60 magnesium tubing. In one set of experiments, the 2-start scroll gave a constant texture through the wall thickness, and then varying process parameters led to changes in texture. This system also enhanced the microstructure and eventually mechanical properties of the system. The basal texture of the material was not parallel to the extrusion axis, which is typical of traditional extrusion processes. Utilizing differing scroll patterns (4 starts and 16 starts) has been shown to vary texture and grain size across the thickness of the tube wall with process parameters held constant. This is yet another example of the ShAPE™ process enabling material properties that are not possible with conventional linear extrusion.

In a first set of examples, the process parameters were as follows: material: Magnesium alloy ZK60, rotational speed 250 revolutions per minute (range can be 10-1000 rpm), extrusion rate: 0.15 inches per minute (range can be 0.05 to 1.0 ipm), die face temperature: 450 degrees Celsius (range can be 200 to 500 degrees Celsius). Under these conditions tubes with 60 mil wall thickness were extruded using 2, 4 and 16 start scrolls. One tube with 120 mil wall thickness was extruded using a 4 start scroll. This makes for a total of four tubes for which data has been collected supporting this invention.

All four tubes were cross sectioned through at least two locations along the length of the tube to ensure that potential variations in texture along the tube length were also captured. FIG. 4 shows an example of the (0001) basal texture at two cross section locations for the 60 mil thick tube made with a 4 start scroll. A full 60 degree change in texture is observed between the inner and outer surface of the tube wall thickness. The same data was also acquired for 60 mil thick tubes formed with 2 and 16 start scrolls and a 120 mil thick tube made using a 4 start scroll.

FIGS. 5(a) and 5(b) summarize the data for grain size and texture orientation for 60 mil thick tube walls made with 2, 4 and 16 start scrolls. The horizontal blue bar for 2 start scrolls indicates that the grain size and texture are essentially constant across the wall thickness. Grain size does not appear to change as a function of the scroll geometries and process conditions explored. However, texture is seen to vary substantially based on the scroll geometry. With a 2 start scroll, texture was not seen to vary across the wall thickness, but texture was seen to vary dramatically with the 4 and 16 start scrolls.

FIGS. 6(a) and 6(b) show for a 120 mil thickness tube made with a 4 start scroll. Again the grain size is relatively constant through the wall thick but the texture again varies dramatically through the wall thickness changing by a full 90 degrees. The ability to control and tailor texture through the thickness of a thin-walled tube is a novel discovery enabled by the ShAPE™ process. From detailed microstructural investigations we have determined the texture is developed as the material is gathered toward the extrusion orifice and obtains its final orientation as it enters the orifice. The combination of scroll geometry and process conditions are used to tailor the basal texture orientation as it enters the extrusion orifice, including across the wall thickness, which in turns sets the texture for the entire length of the extrusion.

In addition to being able to providing a process for cladding, and obtaining a desired through wall thickness. The ShAPE™ technology platform can be used to obtain structures from various materials that have not been demonstrated in other prior art configurations.

For example, Mg alloys containing Si are attractive for automotive, aerospace and high temperature applications. The maximum solubility of Si in Mg is less than 0.003 at % and the Si atoms react to form Mg₂Si precipitates, which results in forming an alloy that has high melting point, low density, low coefficient of thermal expansion and increases the elastic modulus. It is also known that the Mg₂Si precipitates have the same galvanic potential as that of the mg alloy matrix which results in minimization or elimination of microgalvanic corrosion making for a more corrosion resistant alloy. However, casting these alloys results in very low ductility and strength due to the formation of large Mg₂Si precipitates and Chinese script brittle eutectic phase and thus cannot be easily extruded. In order to overcome this challenge several others have tried hot extrusion, rapid solidification and extrusion and mechanical alloying. All these techniques help refine the microstructure and the precipitate morphology but involve additional steps which increases the cost of the processing and does not entirely solve the issues associated with extruding such a brittle material. Even with these approaches, the extruded products also have brittle properties.

Wire and rod of brittle magnesium alloys AZS312 and AZS317 has been extruded with 2.5 mm and 5.0 mm diameters using the ShAPE™ process. Process parameters range from 0.05 to 1.0 for feed rate, 10 to 1000 for rpm rotational speed with extrusion ratios demonstrated up to 160:1 and anticipated as going as high as 200:1. Process parameters will vary depending on the material and desired extrudate dimension and the parameter values mentioned are indicative of the material/geometry investigated and are not restrictive to the process of extruding brittle materials in general. Table 1 shows mechanical test data for AZS312 and AZS317 extruded by ShAPE™ into 5.0 mm rod. The table also shows data for conventionally extruded AZ31 as a benchmark for comparison.

	TABLE 1
	Tensile	Compression
		Ultimate			Com-
	Yield	Tensile		Yield	pressive
	Strength	Strength	Elongation	Strength	Strength	CYS/TYS
Alloy	(MPa)	(MPa)	(%)	(MPa)	(MPa)	Ratio
AZ31	200	255	12	97	NA	0.48
AZ312	170	252	17	160	403	0.94
AZS317	145	200	7	155	281	1.06

The AZS alloys compare similarly with AZ31 in terms of ultimate strength but show a marked improvement compressive yield strength form 97 MPa for AZ31 to 160 MPa and 155 MPa for AZS312 and AZS317 respectively. The higher compressive strength for the AZS alloys also leads to a dramatic improvement in the ratio of compressive yield strength to tensile yield strength (CYS/TYS) with 0.48 for AZ31 and 0.94 and 1.06 for AZS3112 and AZS317 respectively. This is important because the optimum value for CYS/TYS is 1.0 for energy absorption applications. In addition, the elongation at failure improves from 12% for AZ31 to 17% for AZS312. In the case of AZS alloys, ShAPE™ not only enables the extrusion of brittle materials directly from castings, but the unique shearing conditions intrinsic to ShAPE™ also enable novel microstructures which lead to the improved properties shown in Table 1.

For example FIG. 7 shows the microstructure of AZS312 in the as-cast materials and after extrusion. Comparing microstructure before and after extrusion, the data in FIG. 7 shows grain refinement from ˜1 mm to ˜4 microns, basal texture alignment from random to 45 degrees to the extrusion axis, break down of Mg₂Si second phase particles from mm to nm scale, uniform dispersion of Mg₂Si second phase, and dissolution of Al into the matrix which result in the elimination of the Al—Zn impurity. In addition, the brittle Mg₁₇Al₁₂ intermetallic present in AZ31 is not present in the AZS castings. From a corrosion standpoint, AZS312/317 alloys also offer improved corrosion resistance compared to AZ31 as shown in Table 2 where the galvanic corrosion potentials are listed for the constituents within each material.

TABLE 2
AZ31	AZS312
	Corrosion		Corrosion
Phase	Potential	Phase	Potential
Mg(matrix)	−1.65	Mg(matrix)	−1.65
Mg2Si	−1.65	Mg2Si	−1.65 (broken down into
			nanoscale particles)
Al6Mn	−1.52	Al6Mn	−1.52
Al4Mn	−1.45	Al4Mn	−1.45
Al—Zn	−1.42	Al—Zn	Does not exist in
	(approx.)		extrusion, Zn absorbed
			into particles
Mg17Al12 (β)	−1.20	Mg17Al12 (β)	Does not exist in
			casting- Mg combines
			with Si instead

First, the brittle Mg₁₇Al₁₂ intermetallic present in AZ31 is not present in the AZS castings because Mg combines favorably with Si instead of Al during the casting process. As such, second phase with the lowest corrosion potential is eliminated which reduces corrosion rate. Second, Al from the Al—Zn impurity dissolves into the Mg matrix during ShAPE™ processing which further reduces the overall corrosion potential. Third, the fracturing of mm scale Mg₂Si particles to the nm scale is known to reduce microgalvanic corrosion.

While various preferred embodiments of the invention are shown and described, it is to be distinctly understood that this invention is not limited thereto but may be variously embodied to practice within the scope of the following claims. From the foregoing description, it will be apparent that various changes may be made without departing from the spirit and scope of the invention as defined by the following claims.

Claims

1. A shear assisted extrusion process for producing cladded materials wherein a cladding material and a material to be cladded are placed in sequence with the cladded material positioned to contact a rotating scroll face first and the material to be cladded second the two materials are fed through a shear assisted extrusion device at a preselected feed rate and impacted by a rotating scroll face to generate a cladded extrusion product.

2. The process of claim 1 wherein the cladding material is aluminum.

3. The process of claim 1 wherein the preselected feed rate is 0.05-1.0 inches per minute.

4. The process of claim 1 wherein the rotating scroll face rotates at a rate of 10-1000 rotations per minute.

5. The process of claim 1 where in the rotating scroll face has at least 2 starts.

6. The process of claim 1 wherein the axial extrusion force is less than 50 MPa and the temperature of the billet is less than 100° C.

7. The process of claim 3, wherein the feed rate is less than 0.2 inches (0.51 cm) per minute and the rotational shearing force is generated from spinning the die or the billet at a rate between 100 rpm to 500 rpm.

8. The process of claim 5, wherein the material is to be clad contains a magnesium alloy.

9. A process for creating an aluminum cladded magnesium product comprising the steps of placing a thin sheet of aluminum having a hole defined therein on center of a magnesium billet in a shear assisted extrusion device, impacting the billet with a rotating scroll face rotating at rate of (10-1000 RPM) and a feed rate of 0.05-1.0 inches per minute to extrude an aluminum cladded magnesium material.

10. An extrusion process, comprising the steps of: simultaneously applying a rotational shearing force and an axial extrusion force to a billet while contacting one end of the billet with a scroll face configured to engage and move plasticized billet material toward an orifice whereby the plastically deformed billet material flows substantially perpendicularly from an outer edge of the billet through the orifice forming an extrusion product with microstructure grains about one-half the size of the grains in the billet prior to extrusion.

11. The process of claim 10, wherein extrusion of the plasticized billet material is performed at a temperature less than 100° C.

12. The process of claim 11, wherein the axial extrusion force is at or below 100 MPa.