Additive manufacturing of metallic components – Process, structure and properties

Additive manufacturing of fatigue resistant materials: Challenges and opportunities

Effect of surface nanostructure on tensile behavior of superalloy IN718

Fatigue crack initiation in the direct aged version of the nickel-based superalloy Inconel 718 has been investigated at room temperature in the low stress/very high cycle regime via ultrasonic fatigue testing. Three different microstructures have been examined at the same strain amplitude in order to understand the influence of non-metallic inclusions (NMIs), i.e. carbides, carbonitrides and nitrides, and Σ3 twin boundary density on lifetime and failure mode. A slight refinement in grain structure and a higher Σ3 twin boundary density is associated with substantial reductions in lifetime. Decreasing Σ3 twin boundary density for fine grain microstructures results in a change in crack initiation mechanism from strain localization within grains at the high end of the grain size distribution to cracking of NMIs. To study the early stages of crack initiation and growth, specimens with pre-cracked NMIs were also tested in order to examine the role of the surrounding grain structure. Pre-cracked NMIs mainly result in macroscopic failure initiation at NMIs independent of the wrought microstructure. However, pre-loading specimens within the plastic domain highlighted the competition in crack initiation mode between cracked-NMIs and favorably oriented twin boundaries. Crack arrest from most of the pre-cracked NMIs demonstrates that surrounding grain structure (grain orientation, local plasticity and roughness in the vicinity of crack tip due to pre-straining) play a key role in the fatigue life of components stressed in the nominal elastic regime.

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Crack initiation sensitivity of wrought direct aged alloy 718 in the very high cycle fatigue regime: the role of non-metallic inclusions

Recent advances in the field of cold spray have put forward the potential of this deposition technique to be used as a non-thermal additive manufacturing process with significantly high deposition rates. In this study, we use the additive manufacturing potential of cold spray for fabrication of freestanding three-dimensional Inconel 718 samples, which is a challenging material for cold spray due to its high hardness and limited deformability. Additionally, we fabricated samples with similar geometry using one of the most common additive manufacturing methods, i.e. selective laser melting. Microstructural characteristics, distribution of residual stresses, porosity and structural integrity of the cold spray deposited samples were compared with those obtained by selective laser melting before and after different heat treatments. The results of the first time study of axial fatigue strength of cold spray deposited freeform samples indicate the notable efficiency of cold spray for fabrication of freestanding objects for structural components, with similar characteristics to those obtained from laser based additive manufacturing technique and even comparable to bulk material properties. The low working temperature of the cold spray method, suggests it as a promising additive manufacturing technique with a high potential to address many challenges regarding laser based approaches.

Cold spray deposition for additive manufacturing of freeform structural components compared to selective laser melting

The fatigue and fracture behavior of nickel-based superalloy Inconel 718 was investigated up to the very high cycle regime under rotary bending tests at room temperature. It was found that this superalloy can still fracture after exceeding 107 cycles. Fractographic analysis revealed that there was a transition from fatigue crack initiation at multi-sites to single initiation with decreasing stress levels. The fracture surface can be divided into four areas according to the appearance, associated with fracture mechanics analysis of the corresponding stress intensity factors. The fracture mechanism dominant in each area was disclosed by scanning electron microscope examination and analyzed in comparison with those obtained from the crack growth tests. Subsequently, life prediction modeling was proposed by estimating the crack initiation and propagation stage respectively. It was found that Chan (2003)’s model for initiation life and the Paris law for growth life can provide comparable predictions against the experimental life.

Fatigue and fracture behavior of nickel-based superalloy Inconel 718 up to the very high cycle regime *

There is provided a taper coupling structure in which a taper shaft can easily be fitted in and removed from a hub. In a taper coupling structure 10, a groove 34 serving as a hollowed part is provided in a rear end surface HB of a hub 30. The groove 34 is formed continuously in the circumferential direction and uniformly in the radial direction. In the state in which the taper shaft 20 is fitted in the hub 30, the groove 34 interferes with the taper shaft 20 in the radial direction. An inner shell part 35, which is a portion on the inside of the groove 34 in the radial direction, has a low radial rigidity because of the presence of the groove 34. For this reason, if a force is applied from the hollow portion of a flange part 32 toward the outside in the radial direction, the diameter of the inner shell part 35 is liable to be increased in the radial direction by elastic displacement (deflection). Therefore, the surface pressure at a position corresponding to the portion in which the groove 34 is formed decreases, so that the taper shaft 20 can easily be fitted in and removed from the hub 30.

Taper coupling structure and rotating machine

PROBLEM TO BE SOLVED: To obtain an effective fracture toughness value by sampling an actual member from an actual machine, preparing an auxiliary member of the same or different material, welding the auxiliary members to the opposite side parts of the actual member to prepare a test piece with auxiliary member and then performing a three point bending test. SOLUTION: A micro test piece made of an actual member 1a is inserted at a predetermined fatigue precrack 4a part, on the opposite sides thereof, with a hard part and a fatigue precrack 4a, and then a three point bending test is performed in order to calculate the fracture toughness value from a load and a crack opening displacement thus obtained. The actual member 1a to be tested is welded, at the opposite ends thereof, with auxiliary members 11 of same or different material as the actual member 1a so that a three point bending test can be performed for the actual member 1a. CO2 laser welding or electron beam welding is employed in order to suppress thermal effect on the actual member 1a. Since deformation constraint similar to that of an ordinary test piece is attained at the forward end of fatigue precrack 4a in the test piece, a hard part two times or more as hard as the actual member 1a is formed at a welding bead part 10. Consequently, triaxial constraint is increased.

Method for testing fracture toughness using micro test piece

Gigacycle fatigue behaviors of two SNCM439 steels with different tensile strengthes were experimentally studied by rotating bending tests, to investigate the effects of the tensile strength obtained by different heat treatment processes on very high cycle fatigue failure mechanisms. The material with higher tensile strength of 1 710MPa exhibited typical gigacycle fatigue failure characteristics, whereas one with lower tensile strength of 1 010MPa showed only traditional fatigue limit during the tests and no gigacycle failure could be found even when the specimen ran up to more than 108 cycles. Metallographic and fractographic analysis were carried out by an optical microscope (OM) and scanning electron microscope (SEM). It showed two different crack initiation mechanisms that for the specimen with lower tensile strength the crack prefers surface initiation and for that with higher strength the crack initiates from subsurface inclusions revealed by a fish-eye like microstructure.

Gigacycle fatigue behaviors of two SNCM439 steels with different tensile strengths

A method of manufacturing an impeller including a substantially disk-shaped disk, a cover facing the disk, and a blade provided between the disk and the cover, the method includes: forming a groove in a blade attachment surface of the disk or the cover so as to correspond to the shape of an edge tip of the blade; and bonding an inner surface of the groove to the edge tip of the blade through a bonding agent after inserting the edge tip of the blade into the groove.

Eiichi Yanagisawa

Papers

Fatigue and fracture behavior of nickel-based superalloy Inconel 718 up to the very high cycle regime *

Taper coupling structure and rotating machine

Method for testing fracture toughness using micro test piece

Gigacycle fatigue behaviors of two SNCM439 steels with different tensile strengths

Method of manufacturing impeller, impeller, and compressor having impeller