Patent Application
20160158843
1. Field of the Invention
The present invention relates to binder-jet printing and, more particularly, to the use of binder-jet printing and hot isostatic press (HIP) processing in the preparation of printed metallic articles.
2. Description of the Background Art
Additive manufacturing has been demonstrated to be highly effective for rapid prototyping and small lot production for plastic articles. More recently, the application of additive manufacturing to metallic materials has been growing, first in medical, dental and specialty consumer products, and now developing in aerospace applications.
The benefits of direct metal additive manufacturing are best realized in applications involving: high material loss during processing involving high value materials, complex geometries requiring high cost material removal processes or assembly from many fabricated sub-parts, or high tooling expense amortized over short production runs. In these situations, the reduction in expensive material buy-to-fly ratio, specialty machining costs, part number reduction, or tooling overhead cost make direct metal additive manufacturing an attractive alternative.
Direct metal additive approaches for digital metallic part formation include a variety of methods based either on wire form or powder form feed stock. Wire form is generally applied to larger articles by direct fusion of the wire to build up material much like welding. Powder may be used for smaller articles bye-beam or laser direct fusion or binder-jet printed articles that are subsequently sintered to full density in a secondary high temperature operation. For many alloys and applications these processes are effective.
Some specialty alloys such as titanium or superalloys experience undesirable microstructural changes when processed using e-beam or laser direct melt fusion or the high temperature sintering near the melt temperature used to densify binder-jet printed articles. High temperatures close to the melt temperature are needed for sintering most metals including titanium and superalloys to near (>90%) full density.
The above methods have restrictions on powder size that significantly impact cost. The direct melt fusion methods perform better with smaller particle sizes to achieve optimal densification, but the Binder-jet print/high temperature sintering method requires very fine powder size to achieve full density. Since spherical powder is the most desirable for these processes and spherical powder cost is greatest for fine particle sizes, the need for fine powder size in the binder-jet process adds significantly to cost.
Direct melt fusion methods, whether based on wire or powder, create a high temperature differential in the part as it is being built. That temperature difference causes thermal expansion residual stresses to accumulate in the article. This limits the amount of material that can be deposited and the size of the article that can be built without experiencing residual stress cracking. Depending on the article size and thickness, intermediate stress relief operations have to be performed to relieve the stresses before more material can be deposited. This adds cost and time to the process.
Binder-jet printing is an ambient temperature process that mimics ink jet printing. A powder bed of the desired alloy powder is ‘printed’ with a binder pattern layer-by-layer to form a powder assembly held together by binder. After printing, the powder bed is baked at moderate temperatures to cure the binder and give the articles sufficient strength to be handled. After curing, the article(s) can be removed from the powder bed for further processing. Binder-jet printing resolves the residual stress buildup encountered with fusion melt processes since it is an ambient temperature process. This enables complete builds of large article(s) without distortion or cracking.
After creation of the green binder-jet printed form, further processing is performed to remove the binder and sinter the article(s) to near full density at high temperature. This process can create a sintered article, however, the sintering temperatures are typically close to the melt temperature, above the solids but below the liquids, to achieve near full density creating undesirable microstructural modifications and in some cases requiring extremely long sintering times.
Combining binder-jet printed and cured article(s) with near net shape hot isostatic press (HIP) processing solves both problems encountered with fusion melt processes as well as the baseline binderjet process of high temperature sintering. In this improved process, a binder-jet printed and cured article is packed in a HIP container with shape stabilizing powder media. This media can be non deforming such as Zircon sand or deforming media such as steel or glass powder depending on the desired compaction dynamics. All thermal operations are conducted at temperatures below which detrimental microstructural modification will occur. Typically this is below the beta transus for titanium and titanium alloys, which is significantly below the melt point. These thermal operations may include: binder removal, oxide reduction for iron, nickel, and cobalt base superalloys, light sintering for shape stability during transport to the HIP location and the HIP processing conditions. In the preferred example, a vacuum is maintained in the HIP container from before binder removal to HIP thus maintaining a compressive force on the powder assembly to assure shape retention of the printed article(s). In this way, full density titanium and superalloy articles can be processed while avoiding the two primary production problems. This improved process is also effective independent of powder size, thus significantly reducing product cost by enabling the use of lower cost powder.
Referring to
The first operation is to produce a metallic 3-D printed article using a binder-jet printing method. The result is a near net shaped article with a solid metal fraction between 40 and 80 percent held together with a removable polymeric binder. Printing machines of this type are primarily produced by ExOne and Voxeljet.
This operation establishes a hot isostatic press (HIP) container with the green as-printed article(s) and surrounding stabilizing powder fill.
Container—The container can be cylindrical, rectangular, or other convenient shape, sized and shaped to hold one or more articles. The container material is selected so that it is elastic at HIP conditions, tough enough to withstand HIP compaction and exhibits minimal reaction with the stabilizing fill powder at HIP conditions. Examples of HIP container materials include mild steel, stainless steel, and titanium.
Any joints included in the container during fabrication should be leak proof under HIP conditions by a joining method such as tungsten inert gas (TIG) welding. The HIP container is first fabricated with an open top to enable placement of the article(s) and stabilizing powder packing around the article(s).
Stabilizing Powder—The stabilizing powder fill acts to hold the powder that makes up the article(s) in position after the binder is removed until the compaction operation has been completed. It also acts as the force transmission medium during compaction to translate the HIP forces to achieve compaction of the article(s). The powder might be non-deforming at HIP conditions such as Zircon sand, or deforming at HIP conditions such as steel or glass powder. Non-deforming stabilization powder might be used when the article(s) are simple shapes in which the non-deforming stabilization powder would not inhibit the desired compaction motion of the article(s). Deforming stabilization powder might be used when the geometry of the article(s) is more complex with pockets or shapes that require the stabilization powder to compact along with the article(s) to achieve the desired final shape. It is necessary that the stabilizing powder be removable by some mechanism after compaction without damaging the article(s) themselves. It is desirable that the stabilizing powder have minimal reaction with the article(s) so as to not create an unwanted surface reaction layer that can't be removed.
Packing Article(s)—The article(s) are placed in the container and packed in stabilizing powder such that the entire container is filled with no gaps. A vibrating action may be applied to enhance the packing of the stabilizing powder around the article(s). In some cases the article(s) may first be coated with a high temperature fine ceramic slurry such as alumina, zirconia or yttria. This slurry would have the effect of improving the surface quality of the printed part after HIP consolidation and, if a deforming glass powder is used it would prevent infiltration of the glass into the article surface.
A top or closure is placed on the container and sealed to it by means that would create a leak proof joint, for example TIG welding. The closed container includes one or more gas flow tubes that enable the application of vacuum to the inside of the container and the introduction of beneficial gases. In some cases it may be desirable to initially process the packed container without a lid in a vacuum furnace.
A vacuum is applied to the container either as an enclosed container with gas tubes or in a vacuum furnace with an open top. The application of vacuum removes atmospheric gases that could interfere with subsequent operations such as binder removal or sintering. The introduction of an inert sweep gas such as argon may be useful to enhance the kinetics of atmospheric gas removal and subsequent binder by product removal. The binder used to bond powder particles together in the green as-printed article(s) is removed by heating the container to decompose the binder and drawing off the decomposition by-products with the vacuum system.
After binder removal, in the case of iron, nickel and cobalt alloys, the sweep gas may be mixed with or replaced by hydrogen gas to reduce any oxide on the surface to the powder making up the article(s). This can be achieved by further increasing the container temperature above that of binder removal. This operation is useful for reducing oxides in superalloy articles based on water atomized powder. Water atomized superalloy powder typically has an oxide rich surface on the powder particles.
Further stabilization of the powder making up the article(s) is achieved by partially sintering the article(s) to reduce the risk of movement during handling and shipping the HIP containers to the HIP unit. This is achieved by further raising the temperature above that of binder removal and oxide reduction and holding at that temperature for a sufficient time. The goal is to limit the sintering temperature such that the microstructure of the powder metal making up the article(s) is not be modified in an undesirable way.
If the container has been processed without a lid, a lid would be applied at this time as described in Operation 4. One or more gas tubes are part of the assembly either on the lid or the container body to enable application of vacuum to the interior of the container.
A vacuum is applied to the container by means of the gas tube(s) to remove all process gases and establish a clean vacuum inside the container. This can be performed while the container is hot and the container is allowed to cool under vacuum. Once cooled, the gas tube(s) are crimp sealed and cut to establish a hermetically sealed container with vacuum pressure inside. The cut tube(s) may be further TIG sealed to assure stability during the HIP compaction.
The HIP container is compacted under heat and pressure to remove all porosity in the articles. The HIP conditions are selected based on the alloy of the article(s), and are limited to those conditions that would not modify the microstructure in an undesirable way.
After the HIP compaction, the article(s) is removed from the container by first cutting the container open and subsequent removal of the stabilizing medium. In the case of a non-compacting material such as Zircon sand, the sand can be removed by light mechanical work. In the case of compaction media such as glass powder, the media can be removed by grit blasting. In the case of compaction media such as steel powder, the media can be removed by acid milling.
After removal of the stabilizing media, the article(s) is finished to final form. This could include minor machining, polishing or other surface finishing operations.
The method of
This operation may be useful if the as-printed article is particularly low in solid metal fraction, or if a higher precision is required on consolidation. The cold isostatic pressing operation consists of encapsulating the as-printed articles(s) in a flexible molding material such as silicon rubber and subjecting them to a non-heated compression cycle in a high pressure gas chamber. Pressures up to 30,000 psi may be used. This operation will debulk the as-printed green form to assure particle-to particle contact throughout the article(s) that may not have been achieved in the printing operation.
In the method of
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
This application claims the priority of Provisional Patent Application No. 62/088,009 filed on Dec. 5, 2014 and entitled ACHIEVING FULL DENSITY BINDER JET PRINTED METALLIC ARTICLES.
| Number | Date | Country | |
|---|---|---|---|
| 62088009 | Dec 2014 | US |
