High Tensile Residual Stresses Research Articles

Experimental investigation of the effect of EDM parameters and dielectric type on the surface integrity and topography

The electrical discharge machining (EDM) process is mostly used in conditions that complex and intricate shapes need to be machined on very hard metals. However, the process leaves behind some undesirable properties such as high surface roughness, rough topography, high residual tensile stresses, micro-cracks, heat-affected zone (HAZ), and recast layer. This paper investigates the effects of different output parameters on the properties listed above. The input parameters studied include current, pulse-on time, and the type of dielectric; and the material being machined is AISI 1045 steel. Results show that the deionized water improved the output variables in EDM. The use of deionized water as the dielectric, not only improved the machined surfaces’ topography but also lowered the micro-cracks and HAZ thickness. The experiments also show how the pulse-on and current time affect the outcome of EDM machining of AISI 1045 steel.

Generation mechanism of residual stress at press-blanked and laser-blanking edges of 1.5 GPa ultra-high strength steel sheet

The generation mechanism of the residual stress at press-blanked and laser-blanked edges of a 1.5 GPa ultra-high strength martensite steel sheet was investigated. The residual stress at the press-blanked edges of the 1.5 GPa sheet is highly tensile, whereas the residual stress at the laser-blanked edges is compressive. The high tensile residual stress causes the occurrence of hydrogen-induced delayed cracking and the reduction in fatigue strength. In press blanking of the 1.5 GPa sheet, considerably high compressive stress was generated under the punch corner just before the separation due to a small critical punch stroke, and high tensile residual stress on the fracture surface of the blanked edge was induced by releasing this high compressive stress for the separation. Not only the high strength of the 1.5 GPa sheet but also the low ductility influences the generation of the high tensile residual stress on the fracture surface because of the small critical stroke. On the other hand, the compressive residual stress at the laser-blanked edges of the 1.5 GPa sheet was generated by the volume increase of the martensite transformation at a comparatively low temperature. In addition, an approach for determining the allowable amount of hydrogen for the occurrence of delayed cracking at the edges was proposed, and the relationship between the critical residual stress and the amount of hydrogen for the 1.5 GPa sheet was obtained.

Residual stress and mechanical strength of Ce0.8Gd0.2O2-δ-FeCo2O4 dual phase oxygen transport membranes

Ce0.8Gd0.2O2-δ-FeCo2O4 membranes, benefiting from their excellent chemical stability, exhibit a broad application potential in oxygen-consuming industrial processes running in harsh environments. For long-term reliable operation, the membrane needs to possess sufficient mechanical strength. This paper characterizes the typical aspects that challenge the mechanical stability of the sintered membrane, including composition and microstructural defects as well as residual stress and residual stress gradients. It is revealed that mechanical strengths of the sintered membranes increase with decreasing iron cobalt spinel content, that the high iron cobalt spinel content induces microcracks, and that high residual tensile stress gradually decreases from the as-sintered surface to the bulk. Although the residual tensile stress can be reduced by applying an extensive elevated temperature dwell time during cooling, it is suggested to limit the iron cobalt spinel content to a nominal value of 15 wt% to eliminate the residual tensile stress while maintaining a high mechanical strength.

Process-related influences and correlations in wire arc additive manufacturing of high-strength steels

High-strength fine-grained structural steels have great potential for weight-optimized, efficient structures in many modern steel applications. Further advances in efficiency can be achieved through additive manufacturing and bionic design. Commercial high-strength filler materials for wire arc additive manufacturing (WAAM) are already provided by the consumable producers. Today, application would be strictly limited due to absence of quantitative findings or any guidelines for the industry regarding welding-related stresses and component safety during manufacturing and service. Hence, process- and material-related influences and design-related restraint conditions associated with formation of residual stresses and cold cracking risk are investigated. The aim is the accessibility of special WAAM self-restraining cold cracking tests and easy applicable processing recommendations, enabling an economical, fit-for-purpose and crack-safe WAAM of high-strength steels. This first study focuses on determination of interactions between WAAM process parameters, resulting layer geometry, microstructure and residual stresses, analyzed via X-ray diffraction. Defined reference specimens are automated welded using a special WAAM solid wire (yield strength >820 MPa). Geometric properties can be specifically adjusted by wire feed and welding speed, but cannot be varied arbitrarily, since a high heat input causes local overheating, inadmissible changes of microstructure and mechanical properties, defects and comparable high tensile residual stresses.

Adjusting the Residual Stress State in Wire Drawing Products via In-Process Modification of Tool Geometries.

After conventional forming processes, the residual stress distribution in wires is frequently unfavorable for subsequent processes, such as bending operations. High tensile residual stresses typically occur near the wire surface and normally limit further processability of the material. Additional heat treatment operations or shot peening are often used to influence the residual stress distribution in the material after conventional manufacturing, which is time- and energy-consuming. This paper presents an approach for influencing the residual stress distribution by modifying the forming process, especially regarding die geometry. The aim is to reduce the resulting tensile stress levels near the surface. Specific forming elements are integrated into the dies to achieve this residual stress reduction. These modifications in the forming zone have a significant influence on process properties, such as plastic strain and deformation direction, but typically do not influence product geometry. This paper describes the theoretical approach and model setup, the FE simulation, and the results of the experimental tests. The characterization of the residual stress states in the specimen was carried out through X-ray diffraction using the sin2Ψ method.

Influence of the groove shape on welding residual stresses in P92/SUS304 dissimilar metal butt-welded joints

Two dissimilar metal butt-welded joints between P92 steel and SUS304 stainless steel were fabricated with different groove shape, namely single V-groove and X-groove, by gas tungsten arc welding process. Based on Sysweld software, three-dimensional thermal-metallurgical-mechanical coupled finite element analyses were performed to investigate the welding-induced residual stresses in P92/SUS304 dissimilar metal butt-welded joints. Meanwhile, the hole-drilling strain gauge method was used to measure the distributions of residual stresses on the top and bottom surfaces of the two butt-welded joints. The results obtained indicate that the predicted welding residual stresses achieve good levels of agreement with the measurements. In the numerical model, the solid-state phase transformation in P92 steel, the strain hardening and annealing effect in SUS304 stainless steel, and the discrepancy in thermal expansion coefficients between dissimilar metals were carefully considered. The simulated results show that these three factors have significant influence on the formation of longitudinal residual stresses in P92/SUS304 butt-welded joints. In addition, the distribution of welding residual stresses in the butt-welded joint with X-groove was compared with that in the joint with single V-groove. The results suggest that the butt-welded joint with X-groove may be superior to that with single V-groove in reducing the region of high tensile residual stress.

Residual stress in electrical discharge coatings

Abstract Electrical discharge coatings (EDC) can be applied on complex metal components to repair or act as protective coatings. A variant of the EDM process, it can be used to make coatings up to several mm thickness on electrically conductive substrates using sacrificial electrodes. The technology can be classified under thin EDC which is a surface modification process or thick EDC, also known by the commercial name “MSCoating”, which is a surface coating process. Inclusion of a high current pulse in the discharge waveform results in high deposition efficiencies of between 40 and 200 μm/min, with a subsequent decrease in energy into the coating per volume of material resulting in reduced re-melting of deposited material; the thermal shrinkage phenomenon seen in standard EDM recast layers is not as significant here. This results in thick EDCs with consistent tensile residual stresses of between +70 and + 100 MPa. Exclusion of the high current pulse reduces the deposition efficiency to between 0.5 and 10 μm/min. The subsequent increase in energy into the coating increases the melting and quenching of material, resulting in a more nanostructured or amorphous layer. This results in thick EDCs with largely compressive residual stresses, with a range of between +6 and −140 MPa, explained by amorphisation induced volume expansion. In cases where the deposition efficiency is sufficiently low, below 0.1 μm/min, increased melting and quenching of coating material results in high tensile residual stresses due to the thermal expansion mismatch between the layer and substrate. This results in thin EDCs with average tensile residual stresses of between +440 and +490 MPa. It is proposed that the changeable and controllable directionality of the residual stress within these coatings is a positive attribute, ultimately allowing tailorable coatings with regions of favourable residual stress dependent on final application, e.g. for wear or fatigue.

The effect of build direction and heat treatment on atmospheric stress corrosion cracking of laser powder bed fusion 316L austenitic stainless steel

Austenitic stainless steels are susceptible to stress corrosion cracking (SCC) when exposed to chloride ions and tensile stresses. For laser powder bed fusion (LPBF) components, high residual tensile stresses exist from the large thermal gradients during solidification. While post fabrication heat treatments can relieve residual stresses, their effectiveness in improving SCC resistance of LPBF material is undetermined. The effects of different heat treatments (i.e. temperature and cooling method) and build orientations on SCC susceptibility of LPBF 316L were explored. Samples were exposed to MgCl 2 and maintained at 75 °C and 70% RH for the test duration. Cracks were highly branched and propagated perpendicularly to build direction even in the presence of an applied perpendicular 60 MPa stress. Vertical as-built specimens exhibited higher cracking densities than horizontal ones, due to higher surface residual stresses. SCC and corrosion favoured one side for horizontal specimens while the distribution was random for vertical specimens. After 750 °C (2 h, furnace cooled) heat treatment, SCC susceptibility reduced for vertical 316L but increased for horizontal 316L. This was attributed to competing effects: reduction in residual surface tensile stresses (beneficial) and loss of non-equilibrium microstructure (detrimental). Encapsulation to prevent oxidation during heat treatment negatively impacted SCC resistance of LPBF 316L, likely due to changes to surface oxide condition. No SCC was observed after 900 °C (0.5 h) heat treatments within the test duration, however, pitting corrosion persisted.

Laser Beam as a Precision Tool to Increase Fatigue Resistance in an Eyelet of Undercarriage Drag Strut

The article contains the results of tests on a laser-processed eyelet of undercarriage drag strut to increase its fatigue strength. Laser processing concentrated on both sides around the hole of eye for connecting the undercarriage drag strut caused that the material in this area withstood more than twice the number of load cycles established for this material. In order to determine the reasons for the increase in fatigue strength, residual stresses in laser-treated LT areas and in the base metal BM located between melted paths were determined, using the nanoindentation test and Williamson Hall method. The modified Williamson Hall analysis of XRD patterns was also used to determine the dislocation density in both areas. The results indicate that high residual tensile stresses occur in melted areas and in base metal located between melted paths occur high residual compressive stresses. A large increase in hardness and elastic properties, dissolution of non-metallic inclusions, as well as large solid solution and sub-grain strengthening contributed to the high fatigue resistance of the melted areas.

A review on post processing techniques of additively manufactured metal parts for improving the material properties

Metal additive manufacturing (AM) processes are used to produce metal parts with greater design freedom and no material wastage from a CAD model, using powder or wire as feedstock with a layer by layer joining of materials by a high intensity energy source such as laser or electron beam. These processes include selective laser melting (SLM), electron beam melting (EBM) and wire arc additive manufacturing (WAAM) techniques which has wide usage in fabricating metal alloys such as aluminum and titanium for the aerospace and automotive applications. However, these AM parts show many limitations in the as built state like poor surface finish and high tensile residual stress, which will affect the mechanical properties and surface integrity because of the usage of high intensity energy-based melting for the fabrication of parts. This give rise to the need for post processing techniques such as heat treatment and additional machining and polishing operations such as shot peening (SP), laser shock peening (LSP), abrasive flow machining(AFM), friction stir processing (FSP) and hot isostatic pressing (HIP) to improve its surface finish, fatigue life and other material properties. This review paper aims to understand the defects of as built AM parts and focus on the post processing techniques employed by the researchers and its effects on the improvement of AM components.

Process chain simulation of laser powder bed fusion including heat treatment and surface hardening

Additive manufacturing (AM) has enabled the creation of geometrically complex parts for a range of industries. However, the nature of AM often requires multiple post processing techniques to be carried out to reach the desired material properties or required surface finish. This often involves heat treatment (HT), shot peening (SP) or laser shock peening (LSP). To date, hardly any process chain modelling has been carried out on manufacturing applications with AM. This investigation focuses on the finite element (FE) modelling of the complete manufacturing process chain of an AM impeller made of IN718, including the AM, HT, LSP and SP processes. The particular AM process applied to build the impeller is laser powder bed fusion (L-PBF). Each FE process is validated individually against experimental data before being applied to the impeller process chain. The validated data from each process is mapped to the next process in the chain to investigate the combined effects of manufacturing and post processing techniques. Results have shown that high tensile residual stresses induced by AM can be reduced by approximately 75% by applying HT. SP and LSP processes can further modify remaining tensile residual stresses after HT by inducing a layer of compressive stresses at the surface. In summary, this research work has demonstrated that the simulation of AM process chains using finite element techniques is sufficiently mature to support the product and process development of industrial AM components.

Analytical Modeling of Residual Stress in Laser Powder Bed Fusion Considering Volume Conservation in Plastic Deformation

Residual stress (RS) is the most challenging problem in metal additive manufacturing (AM) since the build-up of high tensile RS may influence the fatigue life, corrosion resistance, crack initiation, and failure of the additively manufactured components. While tensile RS is inherent in all the AM processes, fast and accurate prediction of the stress state within the part is extremely valuable and results in optimization of the process parameters to achieve a desired RS and control of the build process. This paper proposes a physics-based analytical model to rapidly and accurately predict the RS within the additively manufactured part. In this model, a transient moving point heat source (HS) is utilized to determine the temperature field. Due to the high temperature gradient within the proximity of the melt pool area, the material experiences high thermal stress. Thermal stress is calculated by combining three sources of stresses known as stresses due to the body forces, normal tension, and hydrostatic stress in a homogeneous semi-infinite medium. The thermal stress determines the RS state within the part. Consequently, by taking the thermal stress history as an input, both the in-plane and out of plane RS distributions are found from the incremental plasticity and kinematic hardening behavior of the metal by considering volume conservation in plastic deformation in coupling with the equilibrium and compatibility conditions. In this modeling, material properties are temperature-sensitive since the steep temperature gradient varies the properties significantly. Moreover, the energy needed for the solid-state phase transition is reflected by modifying the specific heat employing the latent heat of fusion. Furthermore, the multi-layer and multi-scan aspects of metal AM are considered by including the temperature history from previous layers and scans. Results from the analytical RS model presented excellent agreement with XRD measurements employed to determine the RS in the Ti-6Al-4V specimens.

Microstructure, mechanical behaviour and strengthening mechanisms in Hastelloy X manufactured by electron beam and laser beam powder bed fusion

Understanding the microstructure and mechanical properties associated with different metal additive manufacturing techniques is crucial for its wide-scale acceptance in industries. In this work, we aim to understand the microstructural variations and mechanical properties of Hastelloy X (HX) manufactured by electron beam powder bed fusion (PBF-EB) and laser-based powder bed fusion (PBF-LB) process. The size and shape of melt pool and grains were analysed and correlated to final microstructure. The elevated powder bed temperature during PBF-EB was found to promote the formation of grain-boundary precipitates. The higher thermal gradient and cooling rate in PBF-LB resulted in higher tensile residual stress (σmax = 447 ± 10 MPa) within the parts in comparison to PBF-EB parts (σmax = 16 ± 13 MPa). The as-fabricated PBF-EB HX specimens showed a lower tensile strength of 590 MPa but a higher elongation of 60%, whereas its PBF-LB counterparts demonstrated a significantly higher tensile strength of 825 MPa but a lower elongation of 38%. This notable difference in the mechanical behaviour of PBF-EB and PBF-LB built HX was attributed to the columnar microstructure,<100> crystallographic texture and underlying strengthening mechanisms. Furthermore, the mechanical properties of PBF-EB and PBF-LB built HX specimens were predicted using multiple strengthening mechanisms, which demonstrated a good agreement with that of experimentally measured.

Additive manufacturing of copper-tool steel dissimilar joining: Experimental characterization and thermal modeling

In high-pressure die casting applications, efficient cooling of the die/mold is critical to reduce cycle time. Tool steel such as H13 is widely adopted to make these components due to its excellent mechanical properties. However, the low thermal conductivity of tool steel largely impedes its cooling capability. Therefore, joining tool steel with copper to enhance the overall thermal conductivity is critical. In this study, pure copper was deposited on H13 substrates using a laser-aided additive manufacturing process. Depositing Cu on H13 directly and via interlayers of a nickel-based alloy Deloro 22 (D22) were employed. Both experimental characterization and computational analysis were conducted to study the properties of these structures. The results highlight the cracking failure in direct depositing copper on H13 while these cracks have been successfully eliminated when the D22 interlayers were introduced. Evidence of strong bonding at the Cu/D22 and D22/H13 interfaces was obtained. A sharp drop of hardness was observed over the Cu/H13 direct joint interface, while this descend was largely smoothed when D22 was inserted as interlayers. The directly joined Cu/H13 structure has a high longitudinal tensile residual stress (~385 MPa) at the Cu/H13 interface. By adding D22 interlayers, a reduced longitudinal tensile residual stress (~192 MPa) is obtained at the Cu/D22 interface. D22 interlayers have mitigated residual stress at the dissimilar joints.

Investigation of thermal residual stresses during laser ablation of tantalum carbide coated graphite substrates using micro-Raman spectroscopy and COMSOL multiphysics

Laser ablation of high-temperature ceramic coatings results in thermal residual stresses due to which the coatings fail by cracking and debonding. Hence, the measurement of such residual stresses during laser ablation process holds utmost importance from the view of performance of coatings in extreme conditions. The present research aims at investigating the effect of laser parameters such as laser pulse energy, scanning speed and line spacing on thermal residual stresses induced in tantalum carbide-coated graphite substrates. Residual stresses were measured using micro-Raman spectroscopy and correlated with Raman peak shifts. Transient thermal analysis was performed using COMSOL Multiphysics to model the single ablated track and residual stresses were reported at low, moderate and high pulse energy regimes. The results showed that the initial laser conditions caused higher tensile residual stresses. Moderate pulse energy regime comprised higher compressive residual stresses due to off centre overlapping of the laser pulses. Higher pulse energy (250 μJ), higher scanning speed (1000 mm/s) and moderate line spacing (20 μm) caused accumulation of tensile residual stresses during the final stage of laser ablation. The deviation of experimental residual stresses from COMSOL numerical model was attributed to unaccounted additional stresses induced during thermal spraying process and deformation potentials in the numerical model.

Prevention of delayed cracking of punched 1.5 GPa ultra-high strength steel sheets by ironing with punched slug

Hydrogen-induced delayed cracking of punched 1.5 GPa ultra-high strength steel sheets was prevented by ironing with a slug ejected from punching, because the risk of delayed cracking for sheared 1.5 GPa sheets is very high due to high tensile residual stress and large plastic deformation. To prevent the occurrence of delayed cracking, the sheet was punched, and then, the punched hole was ironed by passing the punched slug. Although the residual stress around the punched edge of the 1.5 GPa sheet was tensile, the stress was turned to compressive stress by slug ironing. In addition, the fracture surface of the sheared edge was changed to a smooth ironed surface. A cathode hydrogen charging test of the punched edge for delayed cracking was performed. Although delayed cracks were caused at the punched hole by hydrogen charging, no cracks occurred at the ironed hole for charging. It was found that slug ironing of the punched hole is effective in preventing delayed cracking.

Experimental and numerical investigation of asperities and indents with respect to rolling contact fatigue

Rolling contact experiments with slip were performed on artificial asperities and indents with pile-up. Micro-pits arose at the leading edge of the asperities and classic rolling contact fatigue (RCF) cracks initiated behind the trailing edge of the indents. The elastic-plastic run-in process and the thermal elastohydrodynamic lubrication (TEHL) load cycles were studied numerically. The run-in process caused high tensile residual stresses at the leading edge of the asperities while the TEHL load cycle caused high tensile stresses at the trailing edge of both the asperities and the indents. The conclusion was thus drawn that the classic RCF cracks behind the indents were caused by the TEHL load cycle while the micro-pits at these artificial asperities were caused by the tensile residual stresses from plastic deformation.

Effect of surface mechanical attrition treatment on high cycle and very high cycle fatigue of a 7075-T6 aluminium alloy

Effects of surface mechanical attrition treatment (SMAT) on fatigue performance of a 7075-T6 aluminium alloy were studied in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes. Fatigue test results showed that SMAT can improve fatigue strength in HCF regime, but decrease it in VHCF regime. Fracture surface observations indicated that the low SMAT intensity is quite optimal, which can protect the surface from crack initiation. However, increasing SMAT intensity could lead to over-peening by inducing deep impact dimples, micro-cracks at the surface and high tensile residual stresses in the subsurface, which deteriorated fatigue performance in VHCF regime.

Residual Stresses Induced by Surface Working and Their Improvement by Emery Paper Polishing

In many of machine parts and structural components, materials surface would be worked. In this study, residual stresses on the surfaces were measured by X-ray diffraction method, and effects of surface working on the residual stresses were examined. In case of lathe machining of type 304 stainless steel bar, the residual stresses in circumferential directions are tensile, and those in axial directions are almost compressive. Highly tensile residual stresses in the circumferential directions were improved by emery paper polishing. 10 to 20 times of polishing changes high tensile residual stresses to compressive residual stresses. In the case of shot peening on a type 304 stainless steel plate, the compressive residual stress inside is several hundred MPa lower than that on the surface. By applying the emery paper polishing to the shot peened surface 10 or 20 times, the residual stress on the surface is improved to −700 MPa. While fatigue strength at 288 °C in the air of the shot peened material is 30 MPa higher than solution heat treated and electro-polished material, the fatigue strength of the shot peened and followed by emery paper polished material is 60 MPa higher. Thus, the emery paper polishing is simple and a very effective process for improvement of the residual stresses.

Impacts of laser cladding residual stress and material properties of functionally graded layers on titanium alloy sheet

Laser cladding induces high tensile residual stress (RS), which can compromise the quality of a specimen. Therefore, it is critical to accurately predict the RS distribution in cladding and understand its formation mechanism. In this study, functionally graded material (FGM) layers were successfully deposited on the surface of a titanium alloy Ti6Al4V sheet by laser cladding technology. A corresponding thermo-mechanical coupling simulation model of the laser cladding process was developed to investigate the formation mechanism of RS in the laser cladding FGM layers. The results show that high tensile RS forms in cladding components. Subsequent cladding can effectively alleviate the RS in cladding components although the position of maximum RS remains unchanged. The measurement results of the longitudinal RS on the top and bottom surfaces of cladding components by the X-ray diffraction (XRD) method agreed with the simulation results, thereby proving the accuracy of the simulation. In addition, the formation mechanism of RS in the laser cladding FGM layers was revealed by discussing the individual impact of each material property on RS. It was indicated that the RS distribution in the laser cladding FGM layers was significantly affected by material properties (in particular, coefficient of thermal expansion and Young’s modulus), except for the temperature gradient induced by the laser cladding process.