Generation Of Residual Stresses Research Articles

Smart-substrate: a novel structural design to avert residual stress accretion in directed energy deposition additive manufacturing

ABSTRACT Residual stresses, related distortions and cracks are detrimental in metallic Additive Manufacturing (AM). Previously developed stress-control strategies based on reducing thermal gradients hardly diminish the stress concentrations at the built basement and easily affect other physical phenomena involved in AM. To overcome this, a novel strategy, named as Smart-Substrate, consisting of optimising the inner structure and local stiffness of the substrate is proposed to avert stress accretion and related part deformations. To demonstrate its advantages, a coupled thermomechanical finite element model for AM, experimentally calibrated with in-situ temperature and displacement measurements, is employed to analyse the thermal and mechanical behaviour of three groups of different structures with increasing geometrical complexity (single-wall, rectangular and block parts) fabricated by Directed Energy Deposit (DED) on the standard and smart substrates, respectively. Through using Smart-Substrate, the generation of residual stresses, especially the stress concentrations at the bottom corner of DED-builds being highly sensitive to cracks, and the induced deflections, are fundamentally throttled, and contrariwise for the standard substrate. More importantly, the use of Smart-Substrate is almost without prejudice to the temperature field, metallurgy and resulting mechanical hardness. This provides a possibility for addressing different physical problems individually, enlarging the AM process window.

Residual stress associated with crystalline phase transformation of 3–6 mol% yttria-stabilized zirconia ceramics induced by mechanical surface treatments

Monolithic dental prostheses made of 3–6 mol% yttria-stabilized zirconia (3–6YSZ) have gained popularity owing to their improved material properties and semi-automated fabrication processes. In this study, we aimed to evaluate the influence of mechanical surface treatments, such as polishing, grinding, and sandblasting, on the residual stress of 3–6YSZ used for monolithic prostheses in association with crystalline phase transformation. Plate specimens were prepared from five dental zirconia blocks: Aadva Zirconia ST (3YSZ), Aadva Zirconia NT (6YSZ), Katana HT (4YSZ), Katana STML (5YSZ), and Katana UTML (6YSZ). The specimens were either polished using 1, 3, or 9 μm diamond suspensions, ground using 15, 35, or 55 μm diamond discs, or sandblasted at 0.2, 0.3, or 0.4 MPa. The residual stress, crystalline phase, and hardness were analyzed using the cosα method, X-ray diffraction (XRD), and Vickers hardness test, respectively. Additionally, we analyzed the residual stress on the surfaces of monolithic zirconia crowns (MZCs) made of 4YSZ, 5YSZ, and 6YSZ, which were processed using clinically relevant procedures, including manual grinding, followed by polishing using a dental electric motor on the external surface, and sandblasting on the internal surface. Residual stress analysis demonstrated that grinding and sandblasting, particularly the latter, resulted in the generation of compressive residual stress on the surfaces of the plate specimens. XRD revealed that the ground and sandblasted specimens contained a larger amount of the rhombohedral phase than that of the polished specimens, which may be a cause of the residual stress. Sandblasting significantly increased the Vickers hardness compared to polishing, which may possibly be due to the generation of compressive residual stress. In the case of MZCs, compressive residual stress was detected not only on the sandblasted surface, but also on the polished surface. The difference in the residual stress between the plate and crown specimens may be related to the force applied during the automated and manual grinding and polishing procedures. Further studies are required to elucidate the effects of the compressive residual stress on the clinical performance of MZCs.

Influence of adhesive thickness and defect on mechanical property of adhesive bonded joint of flexible polymer substrate and cable for solar panel application

Flexible polymer substrate and flat polymer cable were adhesive bonded to fabricate the solar panel in the present study. The relationship between the adhesive thickness, mechanical properties and fracture morphologies of the joints were systematically investigated to figure out how the adhesive layers with different thickness affected the joints’ mechanical properties. The influence of the defects on the joint quality was also studied via an artificial defect introduction method. Adhesive failure accompanied with low tensile-shear strength was easy to occur to the joints with extremely thin adhesive thickness due to the weak boundary formation between the adhesive and the adherent. When the adhesive thickness increased, the joints started to show the cohesive failure with higher tensile-shear strength since the adhesive completely fill the surface crevices. The joints’ mechanical properties however decreased again with further increasing the adhesive thickness, owing to the generation of more defects and higher residual stress within the thicker adhesive layer. On the other hand, adhesive failure was more likely to occur around the defect, which significantly deteriorated the joints’ mechanical properties. The defect located in the center of the bonding area led to a larger adhesive fractured area, thereby having a greater impact on the mechanical properties in comparison to the same-size defect located in the corner or the marginal center.

Stability of forming induced residual stresses in stainless steel components under mechanical load

Cold forward extrusion leads to the generation of residual stresses, which can have an advantageous or detrimental effect on the operating behavior of the formed components. In industrial practice, energy-intensive annealing steps are often used to remove unknown stress states and thus prevent unexpected failures. Residual stress-related knowledge can therefore help to save energy and resources. To be able to estimate the effects, not only sign and magnitude need to be known, but also the residual stress stability under typical operating loads. In this paper, stainless steel X6Cr17 (DIN 1.4016, AISI 430) is formed by forward rod extrusion at room temperature. The forming process leads to residual stresses in the formed components, which are subsequently subjected to mechanical testing. To represent different initial states, components with varied die opening angles are produced and tested. A modified 3-point bending setup is used to generate a defined tensile load stress in the part surface. The results show a behavior of the residual stresses, which is depending on the load magnitude, the initial stress state and the work hardening distribution in the part. While the number of cycles has an influence, the largest changes occur in the first stroke.

On the Degree of Plastic Strain during Laser Shock Peening of Ti-6Al-4V.

Laser shock peening (LSP) is an innovative technique that is used to enhance the fatigue strength of structural materials via the generation of significant residual stress. The present work was undertaken to evaluate the degree of plastic strain introduced during LSP and thus improve the fundamental understanding of the LSP process. To this end, electron backscatter diffraction (EBSD) and nano-hardness measurements were performed to examine the microstructural response of laser-shock-peened Ti-6Al-4V alloy. Only minor changes in both the shape of α grains/particles and hardness were found. Accordingly, it was concluded that the laser-shock-peened material only experienced a small plastic strain. This surprising result was attributed to a relatively high rate of strain hardening of Ti-6Al-4V during LSP.

Improving the strength-ductility synergy and corrosion resistance of Inconel 718/316L dissimilar laser beam welding joint via post-weld heat treatment

Inconel 718/316L dissimilar welding is widely used in nuclear power plants, which suffers from the deterioration of mechanical properties and corrosion resistance after welding, attributed to the formation of brittle phase and the existence of residual stress. In this paper, the impact of post-weld heat treatment (PWHT) on Inconel 718/316L welding joints is systematically investigated. By conducting PWHT after welding, notable reductions in residual stress in as-welded joints are significantly reduced and the microstructure is improved, resulting in an enhanced strength-ductility synergy and corrosion resistance. The as-welded joints exhibit the generation of brittle Laves phase and welding residual stress in the fusion zone. After PWHT at 650 °C, a substantial increase in the quantity of fine-scale γ″ strengthening phases with a uniform dispersion distribution in weldment is observed, along with the release of residual stress. This combination yields an optimal strength-ductility synergy and corrosion resistance of the joint. However, the δ phase precipitates along in Laves phase boundary after PWHT at 850 °C, which exhibits a semi-coherent interface with γ matrix, and a non-coherent interface with the Laves phase, leading to severe stress concentration at the δ phase interface. This severe stress concentration could be a preferred location for crack initiation and propagation, resulting in a substantial loss in mechanical behavior. Additionally, the increase in the phase boundary of γ/γ″, γ/Laves, Laves/δ and γ/δ reduces the corrosion resistance of the joint. The findings of this study provide an effective strategy for improving the performance of Inconel 718/316L dissimilar laser beam welding joint.

A Review of the Residual Stress Generation in Metal Additive Manufacturing: Analysis of Cause, Measurement, Effects, and Prevention.

Metal additive manufacturing (AM) is capable of producing complex parts, using a wide range of functional metals that are otherwise very difficult to make and involve multiple manufacturing processes. However, because of the involvement of thermal energy in the fabrication of metallic AM parts, residual stress remains one of the major concerns in metal AM. This residual stress has negative effects on part quality, dimensional accuracy, and part performance. This study aims to carry out a comprehensive review and analysis of different aspects of residual stress, including the causes and mechanisms behind the generation of residual stress during metal AM, the state-of-the-art measurement techniques for measuring residual stress, various factors influencing residual stress, its effect on part quality and performance, and ways of minimizing or overcoming residual stress in metal AM parts. Residual stress formation mechanisms vary, based on the layer-by-layer deposition mechanism of the 3D printing process. For example, the residual stress formation for wire-arc additive manufacturing is different from that of selective laser sintering, direct energy deposition, and powder bed fusion processes. Residual stress formation mechanisms also vary based on the scale (i.e., macro, micro, etc.) at which the printing is performed. In addition, there are correlations between printing parameters and the formation of residual stress. For example, the printing direction, layer thickness, internal structure, etc., influence both the formation mechanism and quantitative values of residual stress. The major effect residual stress has on the quality of a printed part is in the distortion of the part. In addition, the dimensional accuracy, surface finish, and fatigue performance of printed parts are influenced by residual stress. This review paper provides a qualitative and quantitative analysis of the formation, distribution, and evolution of residual stress for different metal AM processes. This paper also discusses and analyzes both in situ and ex situ measurement techniques for measuring residual stress. Microstructural evolution and its effect on the formation of residual stress are analyzed. Various pre- and post-processing techniques used to countermeasure residual stress are discussed in detail. Finally, this study aims to present both a qualitative and quantitative analysis of the existing data and techniques in the literature related to residual stress, as well as to provide a critical analysis and guidelines for future research directions, to prevent or overcome residual stress formation in metal AM processes.

Experimental and numerical study on the distribution of temperature field and residual stress in a multi-pass welded tube joint of Inconel 617 alloy

The purpose of this study is to compare the residual stress generation during hot wire gas tungsten arc welding (HW-GTAW) welding and conventional gas tungsten arc welding (GTAW) in a multi-pass pipe weld of a nickel-based superalloy. HW-GTAW yields better mechanical properties at low heat input than conventional GTAW, and decreases the accumulation of residual stress significantly. The distribution of residual stress developed in the through-thickness direction is determined experimentally using the Deep hole drilling method. As a result of the analysis, it was observed that the highest residual stress arises in Fusion Zone (FZ) and Heat Affected Zone (HAZ). In the case of conventional GTAW the hoop stress was observed to be tensile and distributed over 70% of the weld whereas only 20% of the weld is found to be under tensile hoop stress in the case of HW-GTAW along the weld center. The results show that shrinkage at the weld HAZ region affects the stresses produced at the cylinder's outer surface. The bending effect created during the welding process is primarily responsible for the distribution of residual stresses in the weld zone. A comprehensive numerical analysis has been carried out using ABAQUS software to predict the level of thermal and residual stresses in the weld zone, FZ, and HAZ in the case of both welding process. A 2D finite element models have been developed to simulate the welding process using the ABAQUS FE tool based on coupled thermal-mechanical analysis. A double ellipsoidal heat source model has been developed using a DFLUX subroutine file to simulate the welding process during the numerical analysis. The predicted residual stresses were validated with the experimentally obtained results.

A review on the fatigue performance of micro-arc oxidation coated Al alloys with micro-defects and residual stress

Unavoidable issues of micro-arc oxidation (MAO) coated Al alloys are the micro-defects and residual stress generation. Some scientists have reported the effects of micro-defects and residual stress on the fatigue performance of MAO coated samples. However, the influence of the residual stress sign in the MAO coating and substrate beneath the interface on the fatigue damage behavior of the substrate was controversial. Moreover, the coupling mechanism between micro-defects and residual stress on the fatigue performance of the MAO coated samples was nebulous. These present a challenge for the collaborative design of micro-defects and residual stress to optimize fatigue performance. Aiming at the issues, this paper summarized the influencing factors of MAO coatings on the fatigue performance of the substrate. From two aspects of microstructure characterization and stress analysis, the coupling mechanism between micro-defects and residual stress in fatigue failure was elucidated. And the mechanism of the micro-defects and residual stress on the fatigue damage behavior was developed. Finally, strategies to optimize the fatigue performance of MAO coated Al alloys were proposed. The results of this study have great guiding significance in regulating micropores, cracks, interface defects, and residual stress in MAO coatings in order to extend fatigue life.

Effects of annealing on the interfacial microstructure and mechanical properties of explosively welded AZ80 magnesium alloy and A6005C aluminium alloy

Effects of annealing on the interfacial microstructure, mechanical properties, and residual stress of the explosively welded AZ80 magnesium alloy and A6005C aluminium alloy cladding plate were investigated. By explosive welding, a thin interlayer composed by an intermetallic compound, i.e. γ-Mg17Al12 phase, was formed at the interface of the cladding plate. After annealing at both 373 K and 473 K, the thickness of the interlayer increased. After annealing at 473 K, the interlayer changed from a single layer of γ-Mg17Al12 phase to a double layer of γ-Mg17Al12 phase and β-Al3Mg2 phase, resulting in a decrease in shear strength. As a result of nanoindentation measurement at the interface, the hardness was remarkably high in the β-Al3Mg2 phase. It was suggested that this phase became the crack initiation site for brittle fracture and the shear strength is decreased. Measurements of the residual stress using synchrotron radiation X-rays at the interface of cladding plate revealed the tendency of the generation of tensile residual stress on AZ80 magnesium alloy side and compressive residual stress on A6005C aluminium alloy side. After annealing at 473 K, residual stress in AZ80 magnesium alloy side and A6005C aluminium alloy side changed to compressive and tensile stresses, respectively, and these stress values became smaller in both cases. On the other hand, after annealing at 373 K, compressive residual stress was observed in both AZ80 magnesium alloy side and A6005C aluminium alloy side.

Bearing Inner Ring Raceway Low-Temperature Aerosol Cooling Lubrication Grinding

A low-temperature aerosol cooling and lubrication grinding test bench was built to evaluate the cooling and lubrication performance of the low-temperature aerosol. The mechanism of grinding residual stresses in the inner ring raceway under the coupling effect of grinding force and grinding heat was studied, and the effects of the grinding process parameters and cooling and lubrication conditions on the distribution state of the grinding residual stresses in the surface layer of the inner ring raceway were revealed. The results showed that reducing the initial temperature of the grinding fluid could reduce the residual tensile stresses on the raceway surface. The results also showed that the low-temperature aerosol cooling lubrication method is conducive to the generation of the axial residual compressive stress of −500 MPa and tangential residual compressive stress of −300 MPa in the inner ring raceway surface layer of the bearing. Compared with the convective heat transfer coefficient, the effect of the initial temperature of the grinding fluid is smaller.

Numerical simulation of the 3D propeller repair process by laser cladding of SUS316L on SUS304

The laser cladding (LC) process is widely used for repair and surface modification; however, it generates not only high residual stress, which can negatively impact fatigue life, but also large distortions. Therefore, this study aimed to predict and minimize the residual stress and distortion of the LC process. Three numerical analyses were conducted using different finite element models to simulate the 3D propeller repair process; 3D finite element models involving conductive, convective, contact, and radiative heat transfer and requiring an element birth and death technique for expressing the additive process of LC were developed and validated experimentally. The numerical simulation results for the temperature history, distortion, and residual stress were consistent with the experimental results. Furthermore, a mismatch in the thermal contraction due to high thermal gradients and the gap in the coefficient of thermal expansion between the substrate and LC coating primarily contributed to the generation of residual stress. In addition, the effects of preheating, fixing, scanning patterns, and cooling conditions on reducing residual stress and distortion were investigated. First, preheating reduced the distortion and residual tensile stress of the LC coating owing to a decrease in the thermal gradients. Second, fixing both ends suppressed deformation without generating significantly high tensile stresses in the LC coating. Finally, optimizing the scanning pattern and allowing a cooling time between the deposition tracks to lower the interpass temperature reduced the distortion and residual stress in the entire substrate. Thus, numerical simulation can help in the prediction and minimization of the residual stress and distortions during the LC process.

Residual stress generation and evaluation in milling: a review

Residual stress generation and evaluation in milling: a review

Mechanism of Residual Stress Generation in Quenched Fe-Ni Alloy Cylinders Using Simulated Strains-Based Approach*

Abstract To elucidate the mechanisms of residual stress generation in quenched steel parts has been a longstanding problem. Qualitative explanations for the changes in the stress distribution in quenched cylinders appeared in the 1930s when it became possible to measure residual stresses in specimens. The explanation at that time used the concept of thermal and transformation stresses, which is still included in current textbooks. This concept is referred to here as the estimated stresses-based approach. To simplify the explanation of stress generation due to the combined effects of temperature change and phase transformations, quenching experiments were devised using Fe-Ni alloys in which only martensitic transformation occurs. On the other hand, since heat treatment simulations provide strains resulting from thermal, phase transformation, plastic, transformation plastic, and creep phenomena, a method to elucidate the mechanism using these strains was devised in the early 2000s and named the simulated strains-based approach. This paper contrasts the estimated stresses-based and simulated strains-based approaches to the mechanism of residual stress generation in quenched Fe-Ni alloy cylinders and highlights the superiority of the latter.

Finite Element Simulation of Residual Stress in Laser Additive Manufactured Functionally Gradient Materials Based on Bessel Heat Source Model.

Laser additive manufacturing (LAM) technology has the advantages of short manufacturing cycles, low material waste rate, and design ability. It is especially suitable for preparing functionally gradient materials (FGM). However, due to the large temperature gradient and the change in material composition, the residual stress is very high, which will seriously affect the mechanical properties and manufacturing accuracy of the structure. In this study, the thermomechanical coupled finite element model based on the Bessel heat source is established, and the residual stress in LAM TC4/TC11 FGM is obtained. The results show that the Bessel heat source can effectively suppress the generation of residual stress in the additive manufacturing process, and the finite element results are consistent with the experimental results. Compared with the traditional Gaussian heat source, the maximal residual tensile stress is reduced by an average of 28.1%. The value of residual stress increases with the increase in the number of printing layers, and it increases with the increase of the laser power and decreases with the increase of the scanning speed. The overall trend is that the two sides are compressive stress and the middle is tensile stress. The research has important reference significance for the reasonable suppression of the residual stress in FGM produced by LAM.

Effect of synchronous induction heating on residual stress for laser-based directed energy deposition of thin-walled structures

To reduce the high residual stress during directed energy deposition (DED), induction heating is always integrated. However, traditional pre-heating method has limits to applied in some important fields like repairing and hybrid manufacturing. Synchronous induction-assisted laser deposition (SILD) is a new technology, which have great potential to in-situ control the thermal stress flexibly. Nevertheless, the complex dynamic deposition process resulting from two heat sources complicates the understanding of the underlying mechanism for the generation of residual stress in SILD. This study established a 3D finite-element model based on the indirect coupling thermo-mechanical method to investigate the stress behavior during SILD. The experimental and simulation results were in good agreement and confirmed that the integration of synchronous induction heating significantly reduced the thermal stress during the deposition process, but increased the temperature gradient and thus thermal stress in the cool-down phase. The residual stress of the finished SILD-fabricated part is determined by these two competing mechanisms. As a result, the distribution of residual stress is counterintuitive. Although the maximum residual stress is reduced, residual stress in some part increased with the integration of synchronous induction heating. This study will provide new insight for the in-situ reduction of residual stress in DED.

Surface composition, structure and corrosion properties of a ZrTiNb alloy: Effect of impact treatment energy

A comparative study of the microstructure, surface chemical and phase composition, microtopography, surface roughness, microhardness, and corrosion properties of Zr31Ti18Nb alloy in as-prepared state and after ultrasonic impact treatment under two loading schemes has been carried out by using XRD, XPS, SEM/EDX and TEM/SAED. The surface of as-prepared alloy was considerably enriched with Zr as compared to the nominal composition, covered by a thick native oxide and comprised carbide and nitride precipitates. The processing of the alloy for 2 min in (i) a multi-pin impacting mode combined with a reciprocating movement of the sample and (ii) single-pin normal impacting mode caused deformation-enhanced uptake of oxygen promoting further oxidation of the alloy components. Besides, the impact treatment brought about the accumulation of structural defects, texturing, generation of compressive residual stresses, a decrease in surface roughness, an increase in microhardness, and a significant increase in the corrosion resistance of the alloy in a 0.9% NaCl solution. The highest effect of surface modification of the alloy was obtained after treatment in the single-pin impacting mode. This was related to a larger energy density and power density deposited in the impact area during processing in this mode as evaluated from measurements of pins’ energetics.

Evaluation of Thin Wall Milling Ability Using Disc Cutters

In some cases, industrial practice requires the production of walls or parts with a thickness of less than one millimeter from a metal workpiece. Such parts or walls can be made by milling using disc cutters. This machining method can lead to the generation of residual stresses that determine the appearance of a form deviation characterized by bending the part or the thin wall. To evaluate the suitability of a metallic material for the manufacturing of thin walls by milling with disc cutters, different factors capable of exerting influence on the deviation generated by the residual deformation of the walls were taken into account. A test sample and an experimental research program were designed for the purpose of obtaining an empirical mathematical model. The empirical mathematical model highlights the magnitude of the influence exerted by different input factors on the disc cutter milling process regarding the size of the deviation from the form, and the correct position of the wall or thin part, in the case of a test sample workpiece made of an aluminum alloy. Input factors considered were cutting speed, feed rate, cutter thickness, wall or part thickness, thin wall length, and height. To rank the input factors whose increase leads to an increase in shape deviation, the values of the exponents attached to the factors in question in the empirical mathematical model of the power-type function were taken into account. It was found that the values of the exponents are in the order 0.782 > 0.319 > 0.169 for wall height, feed rate, and wall width, respectively. It was thus established that the strongest influence on the residual deformation of the thin wall is exerted by its height.

A study on residual stress generation versus recrystallisation in FSW joint of dissimilar alloy pair of the AA5083 and HSLA steel

The distribution of residual stresses across the joint interface of dissimilar joining between AA5083 and HSLA steel by friction stir welding technique was investigated through 2D XRD method and a numerical simulation model. The measured and simulated residual stress distributions are compared and discussed in the context of active recrystallisation mechanism and the possible presence of intermetallic compound (IMC) layers across the joint interface. The active dynamic recrystallisation (DRX) and multiple Fe/Al series IMCs as evident from the EBSD and 2D XRD, respectively critically demonstrated the variation in residual stresses between the experimental and theoretical values. The highest residual stress values of 375 and 281 MPa were recorded for the HSLA steel side and AA5083 side, respectively.

3D Numerical modelling of residual stresses induced in finish turning of a fillet radius on a shaft made of 15-5PH

The objective of this paper is to model the residual stress generation mechanisms induced during the finish turning of a fillet radius on a 15-5PH steel shaft. The model, based on the 3D thermo-mechanical equivalent loading method, is used to predict the continuous variation of residual stresses resulting from the continuous variation of the machined sections in the fillet radius. Finally, experimental tests are carried out to compare the numerical results with the residual stress measurements made by X-ray diffraction in some positions.