The Effect of Aging on the Relaxation of Residual Stress in Cast Aluminum

Most cast aluminum-engineered components are water quenched after the solution-treatment cycle of the casting process. This rapid water quenching has the potential to induce high residual stresses in regions of the castings. Reducing the amount of residual stress can have a promising effect on the life of the component. This study was conducted to quantify how aging affects the amount of residual stress in an aluminum casting. An engineered high residual stress test sample and quenching technique has been developed, and a relaxation study has been completed. The study focused on four different temperatures: 463 K, 493 K, 513 K, and 533 K (190 °C, 220 °C, 240 °C, and 260 °C) and a range of aging times (0.3 to 336 hours). The aging data were used to verify a stress relaxation model. The results indicated that as the aging temperature increased, the amount of relaxation of the residual stress increased.     

Stress relaxation of shot-peened UDIMET 720Li under solely elevated-temperature exposure and under isothermal fatigue

The aim of this experimental study was to quantify the changes that occur to the residual-stress state and the near-surface work-hardened zone as a function of depth in a shot-peened UDIMET 720Li Ni superalloy following solely thermal exposure and in combination with strain-controlled fatigue loading at two strain amplitudes. Residual-stress measurements were performed using the sin² ψ method on a laboratory X-ray diffractometer as a function of depth by successive electrochemical removal of material. It was found that the as-peened variation in diffraction-peak width (X-ray) with depth correlated well with the recorded hardness profile. A hardness increase in excess of 50 pct was recorded close to the surface. Regarding solely thermal exposure, it was found that the near-surface compressive stresses were relieved to some extent at all temperatures (350 °C to 725 °C) after short-term exposure, being reduced by up to 50 pct at the highest temperatures (650 °C to 725 °C). The isothermally fatigued samples strained at 0.6 pct amplitude displayed similar stress-relaxation behavior to the thermally loaded samples, indicating that at such small strains, stress relaxation is controlled predominantly by thermal relaxation processes. In contrast, stress relaxation following strain-controlled fatigue at 1.2 pct strain is governed by a combination of thermal and mechanical processes. The mechanically induced relaxation component is anisotropic, being significantly greater along the loading axis than transverse to it. Unsurprisingly, with increasing temperature, the thermal contribution to stress relaxation becomes increasingly important and the degree of in-plane anisotropy of stress relaxation lessens.     

Thermal Relaxation of Residual Stresses in Nickel-Based Superalloy Inertia Friction Welds

This article describes an experimental study aimed at characterizing the extent of residual stress relaxation during thermal treatment of inertia friction-welded alloy 720Li nickel-based superalloy welded tubular rings. In the as-welded condition, yield level tensile hoop stresses were found by neutron diffraction in the weld region along with axial bending stresses (tensile toward the inner diameter (ID)/compressive toward the outer). The evolution of these residual stress levels during postweld heat treatment (PWHT) was mapped experimentally over the weld cross section. After 8 hours of PWHT, the axial stresses relaxed by 70 pct, whereas the hoop stresses reduced by only 50 pct. Some scatter of residual stress evolution was found between samples, particularly for the axial stress direction. This was attributed to substandard tooling to grip the rings. The results on subscale samples were transferred to a full-scale aeroengine (650-mm diameter) compressor drum assembly that was postweld heat treated for 8 hours. It was found that the residual stresses, particularly in the axial direction, were noticeably lower in this full-scale weld component compared to the subscale weld heat treated for the same time. The differences seem to be best rationalized by the different standards of jigging used during joining these two types of welds.     

Phase transformations and control of residual stresses in thick spray-formed steel shells

Large, thick steel shells for tooling applications have been produced using a robot manipulated electric arc spraying technique with steady-state temperatures ranging from 170 °C to 450 °C. Critical to these experiments has been the use of a real-time feedback control system for surface temperature based on infrared thermal imaging. There was a reproducible trend in net residual shell distortion as a function of temperature with residual tensile stresses in the shell for temperatures ≤210 °C and ≥390 °C, and net compressive stresses at intermediate temperatures. In-situ linear displacement sensor experiments have been used to investigate the dynamic distortion of sprayed steel shells on steel substrates, over the same range of surface temperatures. Residual and in-situ distortion measurements confirmed two manufacturing temperatures at which stresses in the steel shells were either minimized or eliminated. A numerical model has been developed to relate shell quench and transformations stresses to the shell dynamic distortion behavior. It is proposed that tensile quench stresses are balanced by the time- and temperature-dependent expansive austenite-to-bainite phase transformation.     

Towards the Optimization of Post-Laser Powder Bed Fusion Stress-Relieve Treatments of Stainless Steel 316L

The use of post-processing heat treatments is often considered a necessary approach to relax high-magnitude residual stresses (RS) formed during the layerwise additive manufacturing laser powder bed fusion (LPBF). In this work, three heat treatment strategies using temperatures of 450 °C, 800 °C, and 900 °C are applied to austenitic stainless steel 316L samples manufactured by LPBF. These temperatures encompass the suggested lower and upper bounds of heat treatment temperatures of conventionally processed 316L. The relaxation of the RS is characterized by neutron diffraction (ND), and the associated changes of the microstructure are analyzed using electron backscattered diffraction (EBSD) and scanning electron microscopy (SEM). The lower bound heat treatment variant of 450 °C for 4 hours exhibited high tensile and compressive RS. When applying subsequent heat treatments, we show that stress gradients are still observed after applying 800 °C for 1 hour but almost completely vanish when applying 900 °C for 1 hour. The observed near complete relaxation of the RS appears to be closely related to the evolution of the characteristic subgrain solidification cellular microstructure.

     

Microstructural Evolution and Flow Analysis during Hot Working of a Fe-Ni-Cr Superaustenitic Stainless Steel

Hot compression tests were conducted in a temperature range of 1173 K to 1323 K (900 °C to 1050 °C) and strain rates of 0.001 seconds⁻¹ to 1 second⁻¹ to investigate the hot deformation behavior of the austenitic stainless steel type 1.4563. The results showed that hot deformation at low temperatures, i.e., 1173 K to 1223 K (900 °C to 950°C), and at low and medium strain rates, i.e., 0.001 seconds⁻¹ to 0.1 seconds⁻¹, results in the dynamic formation of worm-like precipitates on existing grain boundaries. This in turn led to the restriction or even inhibition of dynamic recrystallization. However, at higher temperatures and strain rates when either the time frame for dynamic precipitation was too short or the driving force was low, dynamic recrystallization occurred readily. Furthermore, at low strain rates and high temperatures, there was no sign of particles, but the interactions between solute atoms and mobile dislocations made the flow curves serrated. The strain rate sensitivity was determined and found to change from 0.1 to 0.16 for a temperature increase from 1173 K to 1323 K (900 °C to 1050 °C). The variations of mean flow stress with strain rate and temperature were analyzed. The calculated apparent activation energy for the material was approximately 406 kJ/mol. The hyperbolic sine function correlated the Zener-Hollomon parameter and flow stress successfully at intermediate stress levels. However, at low levels of flow stress a power-law equation and at high stresses an exponential equation well fitted the experimental data.     

Creep and Stress Relaxation Anisotropy of a Single-Crystal Superalloy

In this study, the TMF stress relaxation and creep behavior at 1023 K and 1223 K (750 °C and 950 °C) have been investigated for a Ni-based single-crystal superalloy. Specimens with three different crystal orientations along their axes were tested; 〈001〉, 〈011〉, and 〈111〉, respectively. A highly anisotropic behavior during TMF stress relaxation was found where the 〈111〉 direction significantly shows the worst properties of all directions. The TMF stress relaxation tests were performed in both tension and compression and the results indicate a clear tension/compression asymmetry for all directions where the greatest asymmetry was observed for the 〈001〉 direction at 1023 K (750 °C); here the creep rate was ten times higher in compression than tension. This study also shows that TMF cycling seems to influence the creep rate during stress relaxation temporarily, but after some time it decreases again and adapts to the pre-unloading creep rate. Creep rates from the TMF stress relaxation tests are also compared to conventional constant load creep rates and a good agreement is found.     

Influence of Cryoprocessing on Mechanism of Carbide Development in Cobalt-Bearing High-Speed Steel (M35)

Cryogenic treatment has been used commonly to high-speed tool steels to enhance the wear resistance of the materials. In the current research study, specimens of complex alloyed high-speed tool steel (M35) were hardened at 1473 K (1200 °C), triple tempered at 673 K (400 °C) and then cryogenically treated at 88 K (?185 °C) for varying lengths of period starting from 16 to 48 hours of cryosoaking followed by soft tempering at 373 K (100 °C). These treated specimens were studied as a function of cryosoaking period for their electrical resistivity, residual compressive stress, and its correlation with carbide density was established. TEM analysis indicated carbide size 0.156 to 1 μm, which confirms that the cryogenic treatment enhances the precipitation of finer carbides. Lower residual stresses in the higher carbide density regimes identified in 2D contour map were explained by the stress relaxation in the matrix through precipitation of incoherent carbides.     

Evolution of Precipitate Microstructure During Creep of an AA7449 T7651 Aluminum Alloy

Creep forming is a process where plastic deformation is applied at the material’s aging temperature. It enables to obtain parts of complex shape with reduced internal stresses and finds applications, for instance, in the aerospace industry. In this article, we report in-situ small-angle X-ray scattering measurements during creep experiments carried out on an AA7449 Al-Zn-Mg-Cu alloy in the T7651 temper. In the range of temperatures of 413 K to 453 K (140 °C to 180 °C), we show that the initial microstructure is not stable with respect to the applied stress/strain. Accelerated precipitation coarsening is shown to occur, clearly related to the plastic deformation. This strain-induced microstructure evolution is shown to happen even at temperatures well below the aging temperature that has led to the initial temper.     

Effect of Welding and Post-weld Heat Treatment on Tensile Properties of Nimonic 263 at Room and Elevated Temperatures

Nimonic 263 has been developed for the improved ductility in welded assemblies and is a candidate material for gas turbine combustor and transition pieces along with its good weldability and mechanical properties at room and elevated temperatures. In this study, the tensile behavior of an as-welded Nimonic 263 specimen at room temperature and 1053 K (780 °C) was examined in conjunction with microstructural evolution during welding and postweld heat treatment (PWHT). With the welding and the PWHT, the yield strength (YS), ultimate tensile strength (UTS), and tensile elongation of Nimonic 263 varied in a complex manner. It was observed that the PWHT of resolutionization at 1423 K (1150 °C) for 2 hours gave the highest YS and UTS values, whereas the tensile elongation was the lowest, at both testing temperatures. With increasing resolutionization time, the YS and UTS tended to decrease along with the increase in tensile ductility. The tensile behaviors of as-welded Nimonic 263 specimens was affected by several factors, including grain size, residual stress, possible microsegregation of γ′ forming elements, a tendency for interdendritic or intergranular fracture and a morphological change in both M₂₃C₆ and MC type carbides, depending on the testing temperature and the PWHT. The complex changes in tensile properties of Nimonic 263 with welding and PWHT at room temperature and 1053 K (780 °C) were discussed based on the micrographic and fractographic observations.     

Reduction of Residual Stress and Improvement of Dimensional Accuracy by Uphill Quenching for Al6061 Tube

The purpose of this study is to reduce the residual stress and machining distortion of an Al6061 tube by using uphill quenching. During uphill quenching, solid-solution heat-treated aluminum parts are usually immersed in LN₂ at 77 K (?196 °C), followed by the rapid heating of the parts, to produce a new residual stress that is opposite in nature to the original. The uphill quenching method used in this study employed two types of heating methods: boiling water at 373 K (100 °C) and high-velocity steam at 448 K (175 °C). First, FE-simulation coupled with a CFD analysis was performed to predict the residual stress of the backward hot-extruded Al6061 tube with the following dimensions: Ø200 mm × h200 mm × t10 mm. Experiment of uphill quenching was also conducted to measure the residual stress using the boiling water and high-velocity steam uphill quenching methods. The predicted residual stresses were compared with the experimental results obtained via micro-indentation and saw-cutting tests, and a deviation of about 10.4 pct was found. In addition, the experimental results showed that uphill quenching could relieve up to 91 pct of the residual stress induced by water quenching. Finally, the dimensional accuracy of uphill quenched tubes was evaluated by measuring the roundness after the machining process, which showed that the uphill quenching method could improve the dimensional accuracy of an Al6061 tube by reducing the residual stress.     

Fatigue crack growth of SM-1240/TIMETAL-21S metal matrix composites at elevated temperatures

A series of high-temperature fatigue crack growth experiments was conducted on a continuous-fiberreinforced SM1240/TIMETAL-21S composite using three different temperatures, room temperature (24 °C), 500 °C, and 650 °C, and three loading frequencies, 10, 0.1, and 0.02 Hz. In all the tests, the cracking process concentrated along a single mode I crack for which the principal damage mechanism was crack bridging and fiber/matrix debonding. The matrix transgranular fracture mode was not significantly influenced by temperature or loading frequency. The fiber debonding length in the crack bridging region was estimated based on the knowledge of the fiber pullout lengths measured along the fracture surfaces of the test specimens. The average pullout length was correlated with both temperature and loading frequency. Furthermore, the increase in the temperature was found to lead to a decrease in the crack growth rate. The mechanism responsible for this behavior is discussed in relation to the interaction of a number of temperature-dependent factors acting along the bridged fiber/matrix debonded zone. These factors include the frictional stress, the radial stress, and the debonding length of the fiber/matrix interface. In addition, the crack growth speed was found to depend proportionally on the loading frequency. This relationship, particularly at low frequencies, is interpreted in terms of the development of a crack tip closure induced by the relaxation of the compressive residual stresses developed in the matrix phase in regions ahead of the crack tip during the time-dependent loading process.     

Effect of Postweld Heat Treatment on the Microstructure and Cyclic Deformation Behavior of Laser-Welded NiTi-Shape Memory Wires

NiTi wires of 0.5 mm diameter were laser welded using a CW 100-W fiber laser in an argon shielding environment with or without postweld heat-treatment (PWHT). The microstructure and the phases present were studied by scanning-electron microscopy (SEM), transmission-electron microscopy (TEM), and X-ray diffractometry (XRD). The phase transformation behavior and the cyclic stress–strain behavior of the NiTi weldments were studied using differential scanning calorimetry (DSC) and cyclic tensile testing. TEM and XRD analyses reveal the presence of Ni₄Ti₃ particles after PWHT at or above 623 K (350 °C). In the cyclic tensile test, PWHT at 623 K (350 °C) improves the cyclic deformation behavior of the weldment by reducing the accumulated residual strain, whereas PWHT at 723 K (450 °C) provides no benefit to the cyclic deformation behavior. Welding also reduces the tensile strength and fracture elongation of NiTi wires, but the deterioration could be alleviated by PWHT.     

<Emphasis Type="Italic">In Situ</Emphasis> Neutron-Diffraction Studies on the Creep Behavior of a Ferritic Superalloy

Precipitate strengthening effects toward the improved creep behavior have been investigated in a ferritic superalloy with B2-type (Ni,Fe)Al precipitates. In situ neutron diffraction has been employed to study the evolution of the average phase strains, (hkl) plane-specific lattice strains, interphase lattice misfit, and grain-orientation texture during creep deformation of the ferritic superalloy at 973 K (700 °C). The creep mechanisms and particle-dislocation interactions have been studied from the macroscopic creep behavior. At a low stress level of 107 MPa, the dislocation-climb-controlled power-law creep is dominant in the matrix phase, and the load partition between the matrix and the precipitate phases remains constant. However, intergranular stresses develop progressively during the primary creep regime with the load transferred to 200 and 310 oriented grains along the axial loading direction. At a high stress level of 150 MPa, deformation is governed by the thermally activated dislocation glide (power-law breakdown) accompanied by the accelerated texture evolution. Furthermore, an increase in stress level also leads to load transfer from the plastically deformed matrix to the elastically deformed precipitates in the axial direction, along with an increase in the lattice misfit between the matrix and the precipitate phases.     

Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.     

Cold Cracking Development in AA7050 Direct Chill–Cast Billets under Various Casting Conditions

Cold cracking is a potentially catastrophic phenomenon in direct chill (DC) casting of 7xxx series aluminum alloys that leads to safety hazards and loss of production. The relatively low thermal conductivity and wide solidification temperature range in these alloys results in accumulation of residual thermal stress under nonuniform cooling conditions of the billets. In addition, such alloys show a severe loss in ductility below a critical temperature of 573 K (300 °C). This brittleness along with high stress concentration at the tips of voids and microcracks can lead to catastrophic failure. Casting process parameters affect the magnitude and distribution of stresses in the billet and increase the susceptibility of the material to cold cracking. In order to investigate the effect of casting process parameters such as casting speed, billet size, and water flow rate, thermomechanical simulations were applied using ALSIM5 casting simulation software. Among the studied casting process parameters, the increased billet size and high casting speed resulted in the most dramatic increase in residual stress level. Critical crack sizes that led to catastrophic failure were also calculated and are reported against process parameters.     

Solid Solution Strengthening for Mg-3.0 Mass?Pct (2.71 At.?Pct)Al and Mg-0.06 Mass?Pct (0.036 At.?Pct)Ca Alloys

Tensile tests were carried out at 123 K to 373 K (–150 °C to 100 °C) on pure Mg, Mg-3.0 mass pct (2.71 at. pct) Al alloy, and Mg-0.06 mass pct (0.036 at. pct) Ca alloy. Little decrease occurred in the yield stress of the pure Mg and the Mg-Ca alloy with increasing temperature from 223 K to 373 K (–50 °C to 100 °C). For the Mg-Al alloy, however, its yield stress decreased with increasing temperature from 223 K to 373 K (–50 °C to 100 °C). Analyses based on the existing solid-solution strengthening theories, focusing on the athermal component of stress, revealed that the dominant strengthening mechanism is the shear modulus effect for the Mg-Ca alloy and the chemical interaction for the Mg-Al alloy. It is suggested that the shear modulus effect is dominant at a low concentration and the chemical interaction is dominant at a high concentration for Mg alloys.     

Microstructure and Impression Creep Characteristics of Cast Mg-5Sn-xBi Magnesium Alloys

The microstructure and creep behavior of a cast Mg-5Sn alloy with 1, 2, and 3 wt pct Bi additions were studied by impression tests in the temperature range 423 K to 523 K (150 °C to 250 °C) under punching stresses in the range 125 to 475 MPa for dwell times up to 3600 seconds. The alloy containing 3 wt pct Bi showed the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the favorable formation of the more thermally stable Mg₃Bi₂ intermetallic compound, the reduction in the volume fraction of the less stable Mg₂Sn phase, and the dissolution of Bi in the remaining Mg₂Sn particles. These particles strengthen both the matrix and grain boundaries during creep deformation of the investigated system. The creep behavior of the Mg-5Sn alloy can be divided into the low- and high-stress regimes, with the respective average stress exponents of 5.5 and 10.5 and activation energies of 98.3 and 163.5 kJ mol⁻¹. This is in contrast to the creep behavior of the Bi-containing alloys, which can be expressed by a single linear relationship over the whole stress and temperature ranges studied, yielding stress exponents in the range 7 to 8 and activation energies of 101.0 to 107.0 kJ mol⁻¹. Based on the obtained stress exponents and activation energies, it is proposed that the dominant creep mechanism in Mg-5Sn is pipe-diffusion controlled dislocation viscous glide the low-stress regime and dislocation climb creep with back stress in the high-stress regime. For the Mg-5Sn-xBi alloys, however, the controlling creep mechanism is dislocation climb with an additional particle strengthening effect, which is characterized by the higher stress exponent of 7 to 8.