Improvement of Thermal Fatigue Resistance of a Wrought Nickel-Base Superalloy by Laserglaze
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IMPROVEMENT OF THERMAL FATIGUE RESISTANCE OF A WROUGHT NICKEL-BASE SUPERALLOY BY LASERGLAZE
Zhao Qi, Ge Yunlong, Hu Zhuangqi, Jiang Ming and Shih Changshu, Metal Research, Academia Sinica, Shenyang, Liaoning, China
Institute of
ABSTRACT Laserglaze with appropriate post heat treatment has improved the thermal fatigue resistance of a wrought nickel-base superalloy. It was found that laserglaze was able to eliminate the blocky MC phase, refine grains and form a very interesting microstructure of serrated grain boundaries. Careful selection of post heat treatment markedly increased the strength in the laser irradiated region. The initiation and propagation of thermal fatigue cracks were suppressed by this novel microstructure. INTRODUCTION The use of laserglaze for the controlled modification of surface sensitive properties has been a rapidly growing research field in the last decade. It is a very useful metallurgical tool for studying basic mechanisms in material research areas such as rapid solidification, corrosion and oxidation, tribology, etc., as well as a powerful means of beneficial modification of the mechanical and chemical properties of materials for formation of amorphous or microcrystalline metals and alloys. For this reason, laserglazing has attracted vast attention in the world. Some of the advantages have already been reported, for example, refinement of grains, dendrites and phases, elimination of harmful phases, extension of solute solubility and reduction of segregation [1-7]. As a result of the above mentioned surface modification, mechanical, corrosion-resistant and wearresistant
properties
can be improved.
In
this
paper we summarize
of laserglaze, with appropriate post heat treatment, wrought nickel-base superalloy.
on thermal
the effect
fatigue of a
EXPERIMENTAL PROCEDURE The chemical composition of the wrought nickel-base superalloy is listed in Table I. The main strengthening phase is y', about 42wt%. There are also MC, M6 C, M33 C6 and M3 B2 phases present. The alloy was produced by vacuum induction an% vacuum arc duplex melting. The standard heat treatment 0 was: 1220 C, 4hr, AC (air cool) + 10500C, 4 hr, AC + 950 0 C, 2hr, AC. A 2kW CW CO laser was used to scan over 25xl5x5 mm specimens, notched with a depth of 1.5 mm and a root curvature radius of 0.1 mm. Parameters were chosen as follows: output power 1.3 kW, beam diameter 2.5 mm, scanning speed 15 mm/sec and melt depth about 0.5 mm. Black spray paint was used as an absorbing coating. We rapidly raised the specimen temperature to 9000C (or 950'C, 1000'C) within 0.5 min., exposed at the upper temperature limit for 2 minutes and 0 water-cooled to 200 ± 5 C. This thermal cycle was repeated many times. RESULTS AND DISCUSSION The key factor controlling the high temperature mechanical properties of this wrought nickel-base superalloy is the y' phase, including its amount, size morphology and distribution. Fig. 1 shows there are two kinds of y' phase, large cuboids and very small globular ones. After laserglaze of this alloy, the mic
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