United Technologies Corporation

Richard W Salkeld

Anthony F Giamei

Neal P Anderson

Thomas T Field

Nicholas E Ulion

Otis Y Chen

Earle A Ault

Arthur R Geary

Unwanted recrystallization during heat treatment of a directionally solidified nickel alloy single crystal casting is prevented by chemical milling the part before heat treatment. Removal of a layer of as little as 0.013-0.050 mm thick, less than two percent of the part thickness, has been found effective.

Preparation of single crystal superalloys for post-casting heat treatment

During conventional directional solidification of a single crystal article having a large overhanging flange lying along its length, crystal nucleation defects and shrinkage occur in the flange. To form articles without such defects, the part is cast so that the solidification front moves transverse to the length of the article, and essentially parallel to the flange surfaces where the defects tend to occur. The solidification front is initiated at a seed crystal oriented to produce the desired crystal structure in the cast article. Dendritic effects produce somewhat different properties in articles cast in accord with the invention, compared to those cast with prior art techniques.

Transverse directional solidification of metal single crystal articles

Concepts relating to the heat treatment of single crystal superalloy articles are disclosed. A first concept is the use of a heat treatment sequence which includes incipient melting of the article being heat treated followed by one or more steps which essentially heal the incipient melting damage. A second concept relates to the treatment of previously heat treated single crystal articles which have incipient melting. Methods are disclosed for healing this damage and for recovering the mechanical properties of such articles. A third concept disclosed is a particular cycle which permits effective heat treatment of a specific single crystal alloy composition.

Heat treatment of single crystals

A method of epitaxially casting single crystal metal articles in molds, which comprises causing the solidification interface velocity vector to rotate through at least 90 degrees, preferably 180-540 degrees, about the axis of overall directional solidification growth. In addition, a mold section has a small passageway configured to prevent line of sight communication between the portion where the seed is contained and the article portion of the mold. Preferably, the passageway is shaped like a helix of 0.8-1.5 turns. A second seed can be formed in the mold of an article made from a first seed, and used to make a second casting. The invention reduces the normal increase in crystallographic variability which would otherwise be associated with such practice.

Casting a metal single crystal article using a seed crystal and a helix

In the manufacture of a single crystal article, by the directional solidification of a metal in a ceramic mold the melt-back of a seed crystal is controlled to provide high casting yield. The melt-back, measured as a fraction of the original length of the seed, is preferably between 25-75%, more preferably between 33-67%. Diametrical clearance between the seed and mold is also closely controlled, between 0.06-0.25 mm, to eliminate extraneous grain nucleation.

Control of seed melt-back during directional solidification of metals

An improved casting apparatus for directionally solidifying molten metal is disclosed. The apparatus typically includes a furnace for heating a mold containing molten metal, a liquid metal coolant bath positioned below the bottom of the furnace and means for gradually withdrawing the heated mold from the furnace into the coolant bath to effect directional solidification of the molten metal. The apparatus is improved by positioning a heat insulating baffle between the bottom of the furnace and coolant bath, the baffle being constructed to float on the bath surface during the solidification process and minimize heat loss from the mold until it is immersed in the bath. The provision of the floating baffle between the furnace bottom and coolant bath provides several important advantages including increased thermal gradients, shorter casting cycles and improved cast microstructures. In addition, the floating baffle reduces vaporization of the liquid metal coolant and maintains a smooth, ripple-free coolant surface.

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South Windsor, CT, United States of America

Richard W Salkeld