Effects of Sand Blasting on Surface Integrity and High Cycle Fatigue Properties of DD6 Single Crystal Superalloy
LI Jiarong(), DONG Jianmin, HAN Mei, LIU Shizhong
Science and Technology on Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095, China
Cite this article:
LI Jiarong, DONG Jianmin, HAN Mei, LIU Shizhong. Effects of Sand Blasting on Surface Integrity and High Cycle Fatigue Properties of DD6 Single Crystal Superalloy. Acta Metall Sin, 2023, 59(9): 1201-1208.
DD6 is a second generation single crystal superalloy independently developed in China, which offers advantages such as high-temperature strength, stable structure, and satisfactory casting process performance. Currently, it is being widely used in development of aviation engine turbine blades. Sand blasting can be performed for surface cleaning and adjusting surface roughness of single crystal turbine blades and is an essential process in manufacturing of single crystal blades. Although sand blasting has been widely used in the manufacturing process of DD6 alloy turbine blades, only few reports studied the impact of sand blasting on the surface integrity and fatigue performance of DD6 alloy. Therefore, research is required to provide a theoretical basis for the safe service of DD6 alloy turbine blades. In this work, several specimens after a standard heat treatment are blasted with white corundum sand with 150, 124, and 100 μm diameters at 0.5 MPa to study the effect of sand blasting on the surface integrity of DD6 alloy; the rotary bending high cycle fatigue properties of the specimens without and with sand blasting (blasting with white corundum sand with 150 μm diameter) were tested at 760 and 980oC, respectively, to study the effect of sand blasting on the alloy's fatigue property. The results show that sand blasting destroys the surface integrity of the single crystal superalloy, resulting in irregular pits on the surface caused by the cutting by sand particles while changing the surface morphology as well; the surface roughness and microhardness increase with sand particle size increase; after sand blasting, many dislocations slip in the γ phase, and the dislocation density near the surface is high. Additionally, many dislocations shear the γ′ phase, forming antiphase domain boundaries and stacking faults; sand blasting results in deformation strengthening and residual stress; blasting with 150 μm diameter sand at 0.5 MPa has a small effect on the rotary bending high cycle fatigue properties of DD6 alloy at 760oC, but it considerably reduces the alloy's fatigue properties at 980oC in the low stress amplitude region, decreasing the fatigue strength of the alloy by 7.3%. The combined action of the notch effect, oxidation damage, deformation strengthening, and residual compressive stress leads to the changes in fatigue life without and with sand blasting.
Fig.1 Schematic of shape and size of specimen for high cycle fatigue test (unit: mm)
Fig.2 3D (a, c, e) and 2D (b, d, f) morphologies of DD6 superalloy specimens after sand blasting with granularities of 150 μm (a, b), 124 μm (c, d), and 100 μm (e, f)
Fig.3 Variations of specimen hardness with distance from surface after sand blasting with different granularities
Fig.4 Dislocation morphologies of DD6 superalloy specimen after sand blasting with 150 μm and 0.5 MPa sand (APB—antiphase domain boundary) (a) near surface (b) slightly away from surface
Fig.5 Stress-fatigue life curves of DD6 superalloy specimens at 760oC (a) and 980oC (b) before and after sand blasting with 150 μm granularity (S—stress, Nf—fatigue life)
Fig.6 High cycle fatigue fracture (a-d) and source zone (e-h) morphologies of DD6 superalloy specimens before (a, e, c, g) and after (b, f, d, h) sand blasting with 150 μm granularity under rotational bending at 760oC (a, e, b, f) and 980oC (c, g, d, h) (a, e) S = 460 MPa, Nf = 1.10 × 106 cyc (b, f) S = 500 MPa, Nf = 1.97 × 105 cyc (c, g) S = 440 MPa, Nf = 7.46 × 106 cyc (d, h) S = 500 MPa, Nf = 1.44 × 105 cyc
Fig.7 Cross sectional microstructures of DD6 superalloy specimens after sand blasting with 150 μm granularity under rotational bending at 760oC (a) and 980oC (b)
Fig.8 Dislocation morphologies near high cycle fatigue fracture of DD6 superalloy specimens under rotational bending at different temperatures after sand blasting with 150 μm granularity (a) 760oC, S = 430 MPa, Nf = 7.54 × 106 cyc (b) 760oC, S = 600 MPa, Nf = 8.07 × 104 cyc (c) 980oC, S = 370 MPa, Nf = 8.71 × 106 cyc (d) 980oC, S = 700 MPa, Nf = 2.78 × 104 cyc
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