Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Heat and pressure simultaneously to effect sintering
Reexamination Certificate
2000-02-28
2001-04-03
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Heat and pressure simultaneously to effect sintering
C264S604000
Reexamination Certificate
active
06210633
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to powder metallurgy, and more particularly to forming articles or parts of complex shape by subjecting metal powder material to Hot Isostatic Pressing (HIP).
DESCRIPTION OF BACKGROUND ART
There exist different methods of manufacturing shaped parts from powders using Hot Isostatic Pressing technique. The main feature of these techniques is that free shaping (volumetric shrinkage) of a piece is performed by isostatic gas pressure at high temperatures and therefore there is no any rigid tool conventional for traditional processes of hot metal forming. All these methods use metallic cans or capsules as a plastically deformed tool to give initial shape to powder and to transfer external HIP pressure on it.
Usually capsules with constant wall thickness of 2-3 mm are used for HIP. Along with advantages of this method such as the possibility of manufacturing large-size parts with isotropic structure of material and 100% density it has some serious disadvantages while complex shape parts are considered:
during HIP in capsules with constant wall thickness substantial distortions caused by radial shrinkage occur and lead to poor dimensional precision of the powder parts and low material yield;
irregularity of powder tap density in different sections and channels of the capsule and local deviations in capsule and powder material properties lead to difficulties in controlling the shrinkage and final shape of complex shape parts such as turbine and compressor disks with blades and as a result-cause shape distortions after HIP;
if powders with tap density less than 65-70% are used for HIP, it leads to strong distortions during shrinkage and makes it impossible to manufacture shaped parts using the above method;
During conventional HIP of powders with 65-70% initial density in capsules with constant wall thickness the values of radial and axial shrinkage constitute 12-13% and 14-16% correspondingly.
For large size parts such as turbine disks of 500-600 mm (20-24″) diameter these values of radial shrinkage can reach 60-70 mm (2.5-3″). Such large radial deformations inevitably lead to geometrical distortions of parts during HIP. Therefore it is necessary to reduce the absolute values of radial deformations during HIP. Besides, while manufacturing parts by conventional HIP method in capsules with constant wall thickness only powders with high tap density (more than 55-60%) can be used, otherwise initial internal pressure inside the capsule is so low that the capsule looses its shape under high external pressure during radial shrinkage. This does not enable to manufacture by HIP parts with desired geometry from many perspective powder materials such as powders of refractory alloys with initial density less than 30%. In order to have active control of the shrinkage process and of the final geometry the following manufacturing method based on shrinkage regulation by a new design of capsules with variable wall thickness is proposed.
SUMMARY OF THE INVENTION
The present invention offers a novel method of manufacturing articles of a complex shape by subjecting powder material to Hot Isostatic Pressing (HIP). The method involves manufacturing a capsule with at least one insert. The capsule is filled with outgassed powder. Thereafter, the powder in the capsule is subjected to hot isostalic pressing. The capsule is removed to produce a finished article, such as a bladed disk.
In accordance with the present invention, the thickness of capsule walls is made variable so as to provide substantially unidirectional axial deformation of the powder during the hot isostatic pressing.
In a preferred embodiment of the present invention, the capsule may have upper and lower butt elements and lateral cylindrical elements with the thickness of butt elements exceeding the thickness of lateral cylindrical elements. Masses or volumes of metal of upper and lower butt elements may be made equal.
Dimensions of the upper and lower butt elements may be determined as a function of initial tap density of the powder, and target dimensions of the article. For example, the thickness of the lower butt element may be less than that of the upper butt element provided they have equal masses or volumes.
Preferably, the capsule and inserts are provided with additional cavities to be filled with powder. These cavities provide suppressing capsule distortion during HIP. Lateral surfaces of the insert may be made concave.
The object of the present invention is to develop the method of manufacturing complex “net” and “near net” shape parts (including those with non-machined surfaces) from powder materials (including those with low tap and loose density) by HIP as well as to develop the design of the capsules in order to suppress the above described distortions.
The object is attained according to the invention by making capsules with variable controlled wall thickness providing substantially uniformal axial deformation during HIP. For predicting dimensions of a HIPed part there exist various computer based models of the process which use as an input theological properties of densified powder and capsule materials at elevated temperatures. These data originate from some model experiments.
However these model experiments as well as their results and process model based on them cannot account all the peculiarities of deformation and consolidation during HIP and therefore the accuracy of dimensional prediction based on existing models is about 1-2% of corresponding linear displacements during HIP.
Therefore, it is necessary to minimize radial shrinkage during HIP and it can be performed by changing the construction of the said capsule by changing the ratio of thickness between different capsule elements.
For example, if the thickness of the upper and lower butt elements (responsible for radial stiffness) is increased and that of cylindrical elements (responsible for axial shrinkage)—reduced it is possible to re-distribute considerably the values of radial and axial deformations during HIP.
If the ratio of thickness for the butt and cylindrical elements increases to 5:1 the corresponding values of axial and radial shrinkage for the same capsule described above change from 14-16% to 35-40% and from 12-13% to 3-4%. It means that the value of the radial shrinkage becomes 3-4 times less than while using conventional capsules, and the capsule with powder is subjected to substantially unidirectional deformation. It leads to much better dimensional precision of HIPed parts (their accuracy also increases 3-4 times), reduces 5-10 times distortions caused by radial deformations and what is very important—provides higher reproducibility of dimensions in large manufacturing lots.
For favorable distribution of shrinkage the ratio of plastic stiffness of butt and cylindrical elements (which is proportional to the volume of corresponding capsule elements multiplied by the yield stress value of the capsule material) should be kept in the range of 5-10.
However the change of the capsule wall thickness can lead to the changes of volumes (masses) of upper and lower capsule butt elements which determine their radial stiffness. As a result of this non-equilibrity of capsule volumes and their different stiffness capsule can warp or twist during shrinkage under HIP. Therefore it becomes necessary to keep the balance of the plastic stiffness between the upper and lower parts of the capsule. If this principle is not accounted large distortions and bending of the capsule during HIP will occur.
Besides during manufacturing by HIP of bladed disks with powder blades formed during HIP there usually happen some distortions of the edges of blades due to the non-uniform plastic stiffness of different capsule elements. When a solid insert (for example a ring with slots for shaping blades) is placed inside a capsule it can also lead to additional local distortions during shrinkage as the local stiffness of the construction can change. Also “barrel effect” on the walls of the blade channel is observed due to non-uni
Haykin Roman
Kratt Eugene
Samarov Victor
Seliverstov Dmitry
Jenkins Daniel J.
Laboratory of New Technologies
McDermott & Will & Emery
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