Causes and control of welding cracks in electron-beam-welded superalloy GH4169 joints

Welding joint of GH4169 alloy with a good formation was obtained. No macroscopic defects occurred in the joint. The weld had mainly a dendritic structure; the base metal was a solid solution of Ni, Cr, and Fe, and the strengthening-phase particles such as Ni3Nb were dispersively distributed along the grain boundary. The average tensile strength of the joint reached 743.7 MPa, and the Vickers hardness of the weld exceeded HV 300. Because of the segregation of the low-melting compound Ni3Nb at the grain boundary of the fusion zone, liquid cracks tended to occur as a result of welding stress. The formation of liquid cracks was inhibited by adding an alloying element, Mn, to the welding bath, because Mn diffused to the fusion zone and the high-melting phase Mn2Nb formed, and thus the overall properties of the joint were improved.

δ-phase precipitation regularity of cold-rolled fine-grained GH4169 alloy plate and its effect on mechanical properties

Cold rolling and heat-treatment were used for the grain refinement of GH4169 superalloy plate.The effects of cold rolling reduction ratio and heat-treatment time on the precipitatedδphase,and the effects ofδ-phase content and morphology on the mechanical properties of the GH4169 alloy plates,are studied.The results demonstrate that coldrolling can promote the precipitation of theδphase and its transformation from theδ-Ni3Nb phase to theδ-NbNi4 phase.The comprehensive properties of the alloy are better when the heat treatment time is 1 h,with 132 MPa increase in the tensile strength and only 2.9%decrease in the elongation relative to those of the original material.The mechanical properties of the alloy are shown to change greatly with the change in theδ-phase morphology.

Effects of different helium cooling conditions on the structures of GH4169 alloy vacuum arc remelting ingots

The effects of different helium cooling conditions on the molten pool depth,dendrite structure,and microsegregation of GH4169 alloy ?508 mm vacuum arc remelting( VAR) ingots were studied using an optical microscope and an electron probe. The results show that under different helium cooling conditions,the growth of columnar crystals in the VAR ingot is the same with a certain angle running from the edge to the center and the dendrites at the edges are relatively small whereas the dendrites near the center are large. As the helium cooling increased,the molten pool depth decreased from 137 mm to 120 mm. Observations of the microstructure showed that as the helium cooling increased,the secondary dendrite arm spacing( SDAS) decreased at the center and R/2 region. Also,the Laves phase content markedly decreased. Under the same helium cooling conditions,the SDAS and Laves phase content at the center were higher than that at the R/2 region. Thus,more intense helium cooling effectively reduced segregation in VAR ingots and improved the metallurgical quality.

Temperature dependence in tensile properties and deformation behavior of GH4169 alloy

The effect of temperature on the tensile properties and deformation mechanism of GH4169 alloy has been systematically studied over a wide range of room temperature(RT)to 1000℃.The results indicate that the stress–strain curve of the alloy shows serrations at 200–600℃,and the character of the serrations changes from type A to type B and then to type C at different temperatures.The ultimate tensile strength of the alloy decreases gradually from RT to 600℃.The yield strength decreases slowly from RT to 700℃ but decreases rapidly above 800℃.Transmission electron microscopy analysis relieves that the primary deformation mechanism of the alloy below 500℃ is Orowan bypass mechanism.At temperatures between 600 and 700℃,the coordinated deformation of twins and cross-slip of dislocations are activated.The transformation of\upgamma^{\prime\prime}phase toδphase above 650℃ will decrease the strength.The primary deformation mechanism above 800℃ transforms into the repeated shearing of\upgamma^{\prime\prime}by dislocations to form multiple stacking faults.Recrystallized grains were observed above 800℃,and continuous dynamic recrystallization and discontinuous dynamic recrystallization were observed.The stress concentration caused by Nb-rich carbides is the cause of intracrystalline crack nucleation.At 700℃,grain boundary crack sprouting is caused by the combined effect of slip band impact on grain boundaries and grain boundary dislocation plugging.The relationship between the serrated flow behavior and the deformation mechanism has been discussed based on the experimental results.

Oxidation damage zone formed in creep fatigue crack growth of GH4169 alloy at 650℃

In the creep fatigue crack growth of GH4169 alloy,oxidation is a prominent damage source,which is mainly manifested as the oxidation damage zone in front of crack tip.In order to investigate the property of the oxidation damage zone formed in the creep fatigue crack growth,crack growth tests of directly aged GH4169 alloy were conducted at 650℃ in air under various load conditions.Interrupted tests were performed to observe the damage characteristics at crack tip.Block tests were systematically executed to quantify the dependency of oxidation damage zone size on load and holding time.The crack propagation of the GH4169 alloy has a close relationship with grain boundary oxidation at 650℃.An oxidation damage zone in front of crack tip includes intergranular microcracks and oxidised but uncracked grain boundaries.Its size has been calculated from transient crack growth rate and described as a function of maximum stress intensity factor and holding time.Based on oxidation damage zone size,a novel model has been developed to predict the creep fatigue crack growth rate of the GH4169 alloy at 650℃.

Simulation and experimental investigation of inner-jet electrochemical grinding of GH4169 alloy

GH4169 alloy is one of the most commonly used materials in aero engine turbine blades,but its machinability is poor because of its excellent strength at high temperatures. Electrochemical machining（ECM） has become a common method for machining this alloy and other difficult-tomachine materials. Electrochemical grinding（ECG） is a hybrid process combining ECM and conventional grinding. In this paper, investigations conducted on inner-jet ECG of GH4169 alloy are described. Two types of inner-jet ECG grinding wheels were used to machine a flat bottom surface.The machining process was simulated using COMSOL software, and machining gaps under different machining parameters were obtained. In addition, maximum feed rates and maximum material removal rates under different machining parameters were studied experimentally. The maximum sizes and the uniformity of the distributions of the gaps machined by the two grinding wheels were compared. The effects of different applied voltages on the machining results were also investigated.

Effect of δ Phase on Mechanical Properties of GH4169 Alloy at Room Temperature

Tensile tests of GH4169 alloy were performed at room temperature. Different fractions, distributions and shapes of δ phase was prepared by aging treated at 880 ℃, 930 ℃ and 980 ℃ for 5 h or 10 h. The effect of δ phase on the mechanical pro- perties of GH4169 alloy was investigated. The results show that 0.2% yield strength and ultimate tensile strength of GH4169 alloy increase by 61 MPa and 78 MPa respectively when the fraction of δ phase increases from 2.20% to 5.21%. Then, the ultimate tensile strength remains at 1 012 MPa even when the fraction of δ phase reaches 7.56%. The fraction effect of δ phase on the strength improvement of GH4169 alloy is more significant than morphology, and the critical fraction value is 5.21%. In addition, the elongation decreases by 14.1% when the fraction of δ phase increases from 2.20% to 7.56%. Excessive needle or short rod shaped 8 ohase is resoonsible for the reduction of eloneation.

A New Strategy for Restraining Dynamic Strain Aging in GH4169 Alloy During Tensile Deformation at High Temperature

The dynamic strain aging(DSA) behavior was investigated in GH4169 alloy during tensile deforming with electric-pulse current(EPC) at 750 ℃.The results show that DSA is restrained in the alloy when deformed with 40 Hz-EPC.The size ofγ " phase inner grains increases obviously and δ phase is facilitated to precipitate on grain boundary in the alloy applied with EPC,due to the promotion effect of EPC on the diffusion and segregation of atoms.Transmission electron microscopy(TEM)results indicate that dislocations can cut through small γ" precipitate with the size of less than 10 nm,while dislocations can only bypass dislocations when γ " precipitate grow up over 20 nm.The growth of precipitates consumes large amounts of atoms as well as the velocity of dislocation increase,which makes dislocations difficult to be pinned.Therefore,when γ" precipitates grow up to a large size more than the critical size of dislocation pinning,DSA is significantly restrained in the alloy after necking deformed with EPC.

Effect of Cold Rolling Parameters on the Longitudinal Residual Stress Distribution of GH4169 Alloy Sheet

For plastic deformed parts, the dimensional accuracy is significantly affected by residual stresses and so does the performance in service. Therefore, the rolling process of GH4169 alloy sheet at room temperature was investigated by finite element method. The effects of rolling reduction, friction coefficient, rolling velocity and initial stress on the longitudinal residual stress distribution over the thickness of GH4169 alloy sheet were analyzed. The results show that the values of longitudinal residual stress can be slightly reduced by increasing the rolling reduction and velocity. The longitudinal residual stress over the thickness distributes as ‘‘V＇＇ type or weak ‘‘W＇＇ type. The initial stress mainly has an effect on the longitudinal stress in the backward slip area. But the friction coefficient has remarkable influence on longitudinal residual stress. With the friction coefficient increasing from 0.1 to 0.5, the value of longitudinal residual stress on the sheet surface is reduced by 282 MPa. Simultaneously, the tensile stress turns into compressive stress with a strong‘‘W＇＇ type distribution.