Sintering Support
An object to be sintered is supported on a bed comprising of rolling or flowable particles made from a material that does nor reacts with, and does not adhere to, the object or the surface it is sliding on or the tray holding the particles. For a metallic object ceramic balls are used, rolling on a second metallic surface made of a refractory metal.
The invention relates to the art of sintering, in particular sintering metal and ceramic parts from powders.
BACKGROUND OF THE INVENTIONIt is well known that when metal or ceramic powders, preferably compressed, are heated to a temperature close to their melting point, the individual powder particles fuse together into a solid object. Metals have to be heated in an inert or reducing atmosphere. This process is accompanied by shrinkage, since the voids between the powder particles have to disappear. The shrinkage is typically between 15 to 20 percent. At the high temperatures used in sintering, the sintered object tends to stick to the plate supporting it inside the sintering furnace, The object being sintered is quite weak at this high temperature, it will often develop cracks or tears during sintering unless it is completely free to move and shrink. This is shown in
An object to be sintered is supported on a bed comprising of rolling or flowable particles made from a material that does nor reacts with, and does not adhere to, the object or the surface of the tray holding the particles. For a metallic object ceramic balls are used, rolling on a second metallic surface made of a refractory metal.
Referring to
A secondary problem during sintering is that dust and small particles 6 from the sintered objects accumulate on tray 4. These particles sinter to each other and sometimes to the tray 4, making the granules 3 less mobile. For sintering metals, it is best to use a tray material that is covered by an oxide layer, such as Kanthal. Such an oxide layer prevents particles 6 from bonding to the tray.
The term “shelf” in this disclosure should be understood to mean any horizontal surface used to support the sintered object. The shelf can comprise of several layers, such as a metal tray on top of a ceramic plate. The word “tray” in this disclosure should be interpreted broadly as any surface supporting the granular matter used to support the object being sintered.
Example #1The ceramic and metal balls used in all the examples are available from suppliers of blasting and milling media such as www.precisionfinishing.com
An object to be sintered made of type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a single layer of Zirconia (YSZ type) balls having a diameter of 2 mm. The weight of the object was about 1 Kg. The balls were free to roll on a 1 mm thick Molybdenum sheet. The object was sintered in hydrogen at 1350 degrees C. with very low distortion and no cracks.
Example #2An object to be sintered was made type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a single layer of alumina balls having a diameter of 3 mm. The weight of the object was about 1 Kg. The balls were free to roll on a shelf made of HBN. The object was sintered in hydrogen at 1350 degrees C. with very low distortion and no cracks.
Example #3An object to be sintered was made type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a bed of Hexagonal Boron Nitride (HBN) powder with average particle size of 50 um. The powder was supported on a tray made of Kanthal. The object was sintered in hydrogen at 1350 degrees C. with very low distortion and no cracks.
Example #4An object to be sintered was made type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a bed of zirconia balls having a diameter of 2 mm. The weight of the object was about 1 Kg. The balls were free to roll on a 3 mm thick tray made of Kanthal APMT. The object was sintered in hydrogen at 1350 degrees C. with very low distortion and no cracks.
Example #5An object to be sintered was made of commercial porcelain paste (available from pottery supplies). It was supported on 2 mm diameter Tungsten balls. The weight of the object was about 0.3 Kg. The balls were free to roll on an alumina shelf. The object was sintered in air at 1200 degrees C. with very low distortion and no cracks.
Comparative ExampleAn object to be sintered made of type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a single layer of Zirconia (YSZ type) balls having a diameter of 2 mm. The weight of the object was about 1 Kg. The balls were free to roll on a zirconia shelf. The object was sintered in hydrogen at 1350 degrees C. It had severe distortion and several cracks. After sintering it was found out that the object did not move freely as the ceramic balls partially adhered to the ceramic shelf.
Claims
1. A method of supporting a metallic object being sintered, the method comprising of:
- supporting the object on a bed of flowable ceramic granules, and
- supporting said granules on a tray made of a refractory metal.
2. A method of supporting an object during sintering, the method comprising of placing the object on a horizontal bed of flowable granules made of a material that is different than the material being sintered.
3. A method of supporting a ceramic object being sintered, the method comprising of:
- supporting the object on a bed of metallic granules, and
- supporting said granules on a metallic tray.
4. A method as in claim 1 wherein the bed comprises of a single layer of spherical granules.
5. A method as in claim 1 wherein the bed comprises of multiple layers of spherical granules.
6. A method as in claim 1 wherein the tray is made of Kanthal APMT.
7. A method as in claim 1 wherein the granules are zirconia spheres.
8. A method as in claim 1 wherein the granules are alumina spheres.
9. A method as in claim 1 wherein the granules are Hexagonal Boron Nitride powder.
10. A method as in claim 2 wherein the granules are Hexagonal Boron Nitride powder.
11. A method as in claim 2 wherein the granules are supported by a ceramic tray.
12. A method as in claim 2 wherein the granules are supported by a tray made of Hexagonal Boron Nitride.
13. A method as in claim 3 wherein the granules are Tungsten spheres.
14. A method as in claim 1 wherein the tray material does not form a bond with the metal the object is made off.
Type: Application
Filed: Sep 4, 2019
Publication Date: Jun 4, 2020
Inventor: Daniel Gelbart (Vancouver)
Application Number: 16/560,848