TECHNICAU FIEUD

The present disclosure broadly relates to processes for modifying a surface of a workpiece.

BACKGROUND

Methods of modifying a surface of a workpiece include, for example, methods of finishing the surface of the workpiece and methods of hardening the surface of the workpiece.

In the case of molded parts (e.g., especially cast metal parts), it is common practice to subject the workpiece to abrasive post-processing to remove burs, mold lines, and otherwise smooth the surface of the workpiece. Examples of such processes include vibrating and/or blasting with abrasive media propelled by high velocity gas (e.g., nut shells, ceramic particles, steel balls, or sand). In these processes, unwanted raised surface features are reduced overtime.

Shot peening is similar to sandblasting, except that it operates by the mechanism of plasticity rather than abrasion: each particle functions as a ball-peen hammer. In practice, this means that less material is removed by the process, and less dust created.

Shot peening (i.e., peening with shot particles, hereinafter "shot") is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals and composites. It entails impacting a metallic surface with shot (i.e., round particles typically made of, for example, metal, glass, or ceramic) with sufficient force sufficient to create plastic deformation. In machining, shot peening is used to strengthen and relieve stress in components like steel automobile crankshafts and connecting rods. In architecture it provides a muted finish to metal. Typically, in shot peening a stream of shot is directed toward a workpiece.

Both abrasive finishing and shot peening can be manual, time-consuming processes that can last for hours or days.

SUMMARY

There remains a need for faster and improved methods of modifying the surface of a workpiece that involve impact by particles such as abrasive blasting and shot peening. Advantageously, the present disclosure provides rapid abrasive blasting and shot peening methods that are energy efficient and easy to carry out (e.g., no shot recycling or manual directing of particle streams necessary).

Accordingly, in one aspect, the present disclosure provides a method of modifying a surface of a workpiece, the method comprising: providing a system comprising a sealed mixing vessel having an interior chamber containing the workpiece and working bodies; uniaxially vibrating the sealed mixing vessel at a frequency between 15 hertz and 1 kilohertz, and at a vibrational amplitude between about 0.2 cm and 3 cm such that the working bodies impact the surface of the workpiece.

Features and advantages of the present disclosure will be further understood upon consideration of the detailed description as well as the appended claims.

DETAILED DESCRIPTION

Methods according to the present disclosure may be carried out using a vibratory system that includes a sealed mixing vessel having an interior, processing, chamber. The system may further include an actuator (e.g., a mechanical actuator) capable of vibrating the sealed mixing vessel. Preferably, a control module controls the actuator such that the sealed mixing vessel vibrates under resonant or near resonant conditions (e.g., resonant acoustic conditions) throughout the surface modification process. Use of vibrationally resonant conditions ensures high efficiency use of the supplied energy.

Commercially available mixing devices capable of accomplishing the above are marketed by Resodyn Acoustic Mixers, Butte, Montana. Laboratory-scale devices include LabRAM I and LabRAM II controlled batch mixers. Large scale devices are marketed under the trade designations OmniRAM, RAM5, and RAM 55. These devices typically operate at resonant vibrational frequencies of from 20 to up to < 1 kHz, preferably 40 to 100 hertz, more preferably 40 to 80 hertz, and more preferably 55-65 hertz, although this is not a requirement. The vibrating mixers are also characterized by actuator displacements that are on the order of 0.5 inch (1.3 cm), that may be accompanied by an acceleration g- force, where g = 9.8 m/s , of at least 20-g, 30-g, 40-g, 50-g, or even at least 60-g, although this is not a requirement. Further details concerning suitable resonant acoustic mixers can be found, for example, in U. S. Pat. Nos. 7,188,993 (Howe et al.) and 9,808,778 (Farrar et ah).

In practice, the working bodies and the workpiece(s) are disposed within the interior chamber.

The workpiece may be loose within the interior chamber or fixed in a given position relative to the sealed mixing vessel (e.g., mounted to a wall of the sealed mixing vessel. The latter configuration may be desirable in instances where selective modification of a portion of the workpiece surface is desired. The latter configuration may also be desirable if the workpiece has a large mass and/or is delicate, so that collisions between the workpiece and the vessel walls are prevented.

Advantageously, the working bodies ricochet off the sides and top of the sealed mixing vessel during vibration such that the workpiece is bombarded from all angles.

On a volume basis, the working bodies may collectively constitute up to 20, 30, 40, 50. 60, 70, or 80 percent of the volume of the interior chamber, for example. However, in typical use the working bodies may collectively constitute from 5 to 35 percent of the volume of the interior chamber, although lesser and greater amounts may also be used.

Useful working bodies may include abrasive bodies and peening bodies. The abrasive bodies are typically irregular so that sharp-edged particles can cut away brittle surface deposits; however, this is not a requirement. Abrasive bodies useful for the methods of the present disclosure may include any abrasive bodies that are useful for abrasive blasting (commonly termed "sandblasting") or vibratory tumbling. There are several variants of the process, using various media; some are highly abrasive, whereas others are milder. Exemplary materials for the abrasive bodies may include sand, copper slag, nickel slag, coal slag, glass beads, plastic abrasive, crushed glass, silica, steel spheres, steel grit, stainless steel spheres, cut steel wire, ground-up plastic stock, walnut shells, corncobs, aluminum oxide (which includes brown aluminum oxide, heat treated aluminum oxide, and white aluminum oxide), co-fused alumina-zirconia, ceramic aluminum oxide, green silicon carbide, black silicon carbide, chromia, zirconia, flint, cubic boron nitride, boron carbide, diamond, garnet, sintered alpha-alumina-based ceramic as described, for example, by U.S. Pat. No. 4,314,827 (Leitheiser et al.) and in U. S. Pat. Nos. 4,770,671 and 4,881,951 (both to Monroe et al.). Preferentially, the abrasive bodies are aggregates of the aforementioned abrasive particles, bound by polymers, ceramics or metals, for example.

Usually, the abrasive bodies range in diameter from 0.01 millimeter (mm) to as large as 5 mm, preferably from 0.1 mm to 5 mm; however, this is not a requirement. In some embodiments, the abrasive bodies may be sized according to an abrasives industry specified nominal grade.

Abrasive particles graded according to abrasive industry accepted grading standards specify the particle size distribution for each nominal grade within numerical limits. Such industry accepted grading standards (i.e., abrasives industry specified nominal grade) include those known as the American National Standards Institute, Inc. (ANSI) standards, Federation of European Producers of Abrasive Products (FEPA) standards, and Japanese Industrial Standard (JIS) standards.

ANSI grade designations (i.e., specified nominal grades) may include: ANSI 4, ANSI 6, ANSI 8, ANSI 16, ANSI 24, ANSI 36, ANSI 40, ANSI 50, ANSI 60, ANSI 80, ANSI 100, ANSI 120, ANSI 150, ANSI 180, ANSI 220, ANSI 240, ANSI 280, ANSI 320, ANSI 360, ANSI 400, and ANSI 600. FEPA grade designations include P8, P12, P16, P24, P36, P40, P50, P60, P80, P100, P120, P150, P180, P220, P320, P400, P500, P600, P800, P1000, and P1200. JIS grade designations include JIS8, JIS12, JIS16, JIS24, JIS36, JIS 46, JIS 54, JIS 60, JIS 80, JIS 100, JIS 150, JIS 180, JIS 220, JIS 240, JIS 280, JIS 320, JIS 360, JIS 400, JIS 600, JIS 800, JIS 1000, JIS 1500, JIS 2500, JIS 4000, JIS 6000, JIS8000, JIS 10000, JIS 20000, and JIS 30000.

Alternatively, abrasive bodies can be graded to a nominal screened grade using U.S.A. Standard Test Sieves conforming to ASTM E-l 1 "Standard Specification for Wire Cloth and Sieves for Testing Purposes." ASTM E-l 1 proscribes the requirements for the design and construction of testing sieves using a medium of woven wire cloth mounted in a frame for the classification of materials according to a designated particle size. A typical designation may be represented as -18+20 meaning that the abrasive particles through a test sieve meeting ASTM E-ll specifications for the number 18 sieve and are retained on a test sieve meeting ASTM E-ll specifications for the number 20 sieve. In one embodiment, the abrasive bodies have a particle size such that most of the particles pass through an 18 mesh test sieve and can be retained on a 20, 25, 30, 35, 40, 45, or 50 mesh test sieve. In various embodiments of the disclosure, the abrasive bodies can have a nominal screened grade comprising: -18+20, -20+25, -25+30, -30+35, -35+40, -40+45, -45+50, -50+60, -60+70, -70+80, -80+100, -100+120, -120+140, -140+170, -170+200, -200+230, -230+270, -270+325, -325+400, -400+450, -450+500, or -500+635.

Optionally the sealed mixing vessel may contain a fluid such as, for example, water. The fluid may contain optional additives such as, for example, surfactant, defoamer, or in the case of abrasive bodies an etchant (e.g., an alkali metal hydroxide).

Useful peening bodies may include any bodies known for use in shot peening. Examples include: spherical metal shot (e.g., cast steel, iron steel, stainless steel, tungsten, molybdenum, tungsten, titanium, tantalum, cobalt-chrome, or cobalt), spherical ceramic/cermet beads (e.g., zirconia, alumina, silicon carbide, or tungsten carbide/cobalt), spherical glass beads, and conditioned (rounded) cut wire (e.g., conditioned cut steel wire). Conditioned cut wire shot may be preferred in some applications, because maintains its roundness as it is degraded, unlike cast shot which tends to break up into sharp pieces that can damage the workpiece. Conditioned cut wire shot can last five times longer than cast shot. Mixtures of two or more working body compositions, shapes, and/or sizes may be used. Usually, the peening bodies range in diameter from 0.1 millimeter (mm) to as large as 3.2 mm, preferably from 0.7 to 1.2 mm; however, this is not a requirement.

When peening a surface finished workpiece (e.g., deburred and/or smoothed), any peening bodies useful for shot peening may be used in practice of the present disclosure. Exemplary useful peening media includes rounded metallic (e.g., cast steel, stainless steel, molybdenum, tungsten, titanium, tantalum, cobalt-chrome, or cobalt) particles and conditioned cut wire versions thereof, glass (e.g., glass beads), ceramic particles (e.g., tungsten carbide, silicon carbide, titanium carbide, corundum, and Zirshot ceramic media (60-70% ZrC>2, 28-33% S1O2, <10% AI2O3 marketed by SEPR Saint-Gobain ZirPro, Le Pontet Cedex, France), and combinations thereof.

Peening may be beneficially practiced on metallic (e.g., including aluminum, steel, steel forgings and machine parts) workpieces. The effect of peening is a surface phenomenon that typically does not exceed several hundred microns in depth, so it is typically only necessary that the surface of the workpiece be metallic in order to achieve a benefit. However, in many instances the entire workpiece may be metallic.

Methods according to the present disclosure may be especially beneficial for workpieces, fabricated by powder jet or laser sintering additive manufacturing (3D printing) methods, since the working bodies and the workpiece may be free to move within the sealed chamber, the working bodies (if sufficiently small) can penetrate into interior passages that are accessible from the surface of the workpiece, and which may not be easily accessible using other methods. EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight.

The system used for all examples described below was a LabRAM Resonant Acoustic mixer from Resodyn Corporation, Butte, Montana. The machine, which was equipped with a sealed mixing vessel, was run at 100% intensity in the auto frequency mode. Roughness measurements: Ra, were measured using a MarSurf PS 10 stylus profilometer and Sa roughness measurements were recorded using a MikroCAD surface metrology system.

EXAMPLE 1

This example demonstrates abrading aluminum alloy with loose abrasive grain.

The workpiece was a machined aluminum alloy (Grade BS EN 755 6082-T6), 16 mm x 3 mm x 50 mm cuboid. The surface was scratched by hand with P36 coated abrasive to produce an initial surface roughness R _a of 6.2 microns. The workpiece part was placed in a polypropylene straight-sided sealed cylindrical container with 102 mm internal height and 52 mm internal diameter. P80 semi-friable fused aluminum oxide BRFPL (175 g, Imerys, Paris, France) was placed in the container along with the workpiece. The LabRAM was run at 100% intensity in the auto frequency mode for 30 mins. Afterward, the surface roughness R _a of the workpiece was 4.1 microns. The mass loss of the workpiece during this time period of processing was 0.036 g.

EXAMPLE 2

This example demonstrates abrading aluminum alloy with loose abrasive grain and a chemical etchant.

The workpiece was a machined aluminum alloy (Grade BS EN 755 6082-T6), 16 mm x 3 mm x 50 mm cuboid. The workpiece was scratched by hand with P36 coated abrasive to produce an initial surface roughness R _a of 7.1 microns. The workpiece was placed in a polypropylene straight-sided sealed cylindrical container with 102 mm internal height and 52 mm internal diameter. P80 semi-friable fused aluminum oxide BRFPL (175 g, Imerys) was placed in the container along with the part. 1M potassium hydroxide solution (75 ml) was added to the container. The LabRAM was run at 100% intensity in the auto frequency mode for 30 mins. Afterward, the roughness R _a of the workpiece after 30 mins of processing was 5.1 microns. The mass loss of the workpiece during this time period of processing was 0.094 g.

EXAMPLE 3

This example demonstrates abrading aluminum alloy with abrasive agglomerates.

The workpiece was a machined aluminum alloy (Grade BS EN 755 6082-T6), 16 mm x 3 mm x 50 mm cuboid. The workpiece was scratched by hand with P36 coated abrasive to produce an initial surface roughness R _a of 5.1 microns. The workpiece was placed in a polypropylene sealed cylindrical container with 55 mm internal height and 80 mm internal diameter. Premium Ceramic Fast Cutting Triangles (75 g, 2 mm x 2 mm, Kramer Industries, Piscataway, New Jersey) were placed in the container along with the workpiece and 50 g of water. The LabRAM was run at 100% intensity in the auto frequency mode for 30 mins. Afterward, the surface roughness R _a of the workpiece was 2.6 microns.

The mass loss of the workpiece during this time period of processing was 0.024 g.

EXAMPLE 4

This example demonstrates abrading additively manufactured aluminum alloy with abrasive agglomerates.

The workpiece was an additively manufactured aluminum alloy (AlSilOMg) 20 mm diameter tube with 2 mm walls. The part was printed by Direct Metal Laser Sintering (DMLS). The initial roughness R _a was 4.7 microns. The workpiece was placed in a polypropylene straight-sided thick-walled sealed cylindrical container with 84 mm internal height and 60 mm internal diameter. Abrasive agglomerates (200 g, 720-micron cubes comprising P600 aluminum oxide FRPL grit and a vitrified binder from 3M Company, Maplewood, Minnesota) was placed in the container along with the workpiece and 100 g of water. The LabRAM was run at 100% intensity in the auto frequency mode for 30 mins. Afterward, the roughness R _a of the workpiece on the inside surface the tube was 2.9 microns and outside surface of the tube was 2.4 microns. The mass loss of the workpiece was 0.010 g.

EXAMPLE 5

This example demonstrates abrading additively manufactured polymer with loose abrasive grain.

The workpiece was an additively manufactured FormLabs Clear Resin (methacrylic acid esters with a photoinitiator) 20-mm diameter tube with 2 mm walls. The workpiece was printed by stereolithography (initial roughness S _a = 51 microns). The workpiece was placed in a polypropylene straight-sided thick-walled sealed cylindrical container with 84 mm internal height and 60 mm internal diameter. PI 20 semi-friable fused aluminum oxide BRFPL (100 g, Imerys) was placed in the container along with the workpiece. The LabRAM was run at 100% intensity in the auto frequency mode for 30 mins. Afterward, the roughness S _a of the workpiece on the surface of the tube was 2 microns (98% improvement). The mass loss of the workpiece was 0.18 g (8% of the total initial mass).

EXAMPLE 6

This example demonstrates peening of additively manufactured aluminum alloy (AlSilOMg).

The workpiece was an additively manufactured aluminum alloy (AlSilOMg) 20 mm diameter tube with 2 mm thick walls. The part was printed by the Direct Metal Laser Sintering (DMLS) method (initial roughness R _a = 4.7 microns). The workpiece was placed in a polypropylene straight-sided thick- walled sealed cylindrical container with 84 mm internal height and 60 mm internal diameter. Spherical zirconia milling media (250 g, 3 mm diameter, Retsch, Haan, Germany) was placed in the container along with the workpiece and 50 g of water. The LabRAM was run at 100% intensity in the auto frequency mode for 15 mins. Afterward, the roughness R _a of the workpiece (both inside and outside surfaces of the tube) was 1.2 microns.

EXAMPLE 7

This example demonstrates peening of additively-manufactured stainless steel workpiece.

The workpiece was an additively manufactured 17-4 PH stainless steel bracket printed by DMLS within 3M. After printing, the workpiece was left unfinished, with an initial roughness R _a of 11.7 microns. The workpiece was then placed in a polypropylene sealed cylindrical container with 55 mm internal height and 80 mm internal diameter. Stainless steel round shot (100 g, 2 mm diameter, CousinsUK) was placed in the container along with the workpiece and 50 g of water. The LabRAM was run at 100% intensity in the auto frequency mode for 60 mins in total. The roughness R _a of the workpiece after 15 mins of processing was 2.9 microns. After 60 mins, the R _a was 1.0 microns.

EXAMPLE 8

This example demonstrates peening of an additively-manufactured cobalt chromium alloy workpiece.

The workpiece was an additively manufactured cobalt chromium alloy (Co-Crl30) 20mm diameter tube with 2 mm walls printed by DMLS. The roughness R _a after printing was 11.6 microns.

The workpiece was placed in a polypropylene sealed cylindrical container with 55 mm internal height and 80 mm internal diameter. Tungsten carbide spheres (100 g, 1 mm diameter, Bearing Warehouse Ltd., Sheffield, United Kingdom) were placed in the container with the workpiece along with 50 g of water.

The LabRAM was run at 100% intensity in the auto frequency mode for 15 mins. Afterward, the R _a was 3.0 microns.

EXAMPLE 9

This example demonstrates peening of an additively-manufactured titanium workpiece.

The workpiece was an additively manufactured titanium alloy (Ti6A14V) rectangular tab (10 x 30 x 1mm) printed by DMLS. The initial roughness after printing, R _a was 6.9 microns. The workpiece was placed in a polypropylene sealed cylindrical container with 55 mm internal height and 80 mm internal diameter. Tungsten carbide spheres (120 g, 3 mm diameter, Bearing Warehouse Ltd.) were placed in the container with the workpiece along with 50 g of water. The LabRAM was run at 100% intensity in the auto frequency mode for 60 mins total. After 15 mins, the R _a was 4.4 microns, and after 60 mins, the R _a was 2.2 microns. EXAMPLE 10

This example demonstrates peening of a machined aluminum alloy (Grade: BS EN 755 6082-T6) workpiece.

The workpiece was a machined aluminum alloy (Grade: BS EN 755 6082-T6) cuboid 15.9 mm x 3.2 mm x 50 mm. The workpiece was scratched by hand with a P36 grade coated abrasive to a roughness

R _a of 7.5 microns. The workpiece was placed in a polypropylene straight sided thick walled container (from United States Plastic Corp., Lima, Ohio) with 84 mm internal height and 60 mm internal diameter. Spherical ceramic tumbling media (250 g of 3 mm K-Polish Premium Ceramic Tumbling Media, Kramer Industries ) was placed in the container along with the workpiece. The LabRAM was run at 100% intensity in the auto frequency mode for 30 min. The roughness R _a of the workpiece after 30 mins of processing was 1.8 microns. The maximum compressive residual stress in the surface of the material was -100 MPa before the process. After the 30 mins of processing, the maximum compressive residual stress was -250 MPa. The depth of the compressive stress in the surface increased by 100 microns. Results are reported in Table 1, below.

TABLE 1

All cited references, patents, and patent applications in this application are incorporated by reference in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in this application shall control. The preceding description, given in order to enable one of ordinary skill in the art to practice the claimed disclosure, is not to be construed as limiting the scope of the disclosure, which is defined by the claims and all equivalents thereto.