Low Residual Stress Research Articles

Influence of accelerated corrosion on Al/steel RSW joints by in situ compression tests

The present work investigates the correlations between galvanic corrosion, intermetallic compound (IMC) formation, and the input of welding energy with respect to the initiation and propagation of micro-cracks in micropillars of resistant spot welding (RSW) joints between aluminum (Al) and steel. The present results indicate that, in the high welding energy region, Al was corroded first after 26 cycles of corrosion, and Al5Fe2 IMC was corroded after 72 cycles of corrosion due to its high corrosion potential predicted by the calculations based on density functional theory (DFT) in terms of the Nernst equation. In comparison with the high welding energy region, less corrosion was observed in the middle welding energy region due to the thinner Al5Fe2 IMC layer, a lower amount of Al13Fe4 IMC in the Al matrix, and lower residual stress. Mechanical properties at different locations after various corrosion conditions were obtained using in situ compression tests, including the stress-strain responses and the strain rate sensitivity. The micropillars from the high welding energy region have a higher average yielding stress due to the thicker IMC layer than those from the middle welding energy region. The yielding stress decreases gradually with increasing corrosion cycles. Three conditions for crack initiation and propagation have been identified: firstly, for pillars from the high welding energy region before corrosion or the middle welding energy region (before or under the minimal corrosion conditions), the cracks initiate within the IMC layer; secondly, for pillars from the high welding energy region after 26 cycles of corrosion (under the moderate corrosion condition), cracks propagate at the Al/IMC interface; finally, for pillars from the high welding energy region after 72 cycles of corrosion (under the severe corrosion condition), the cracks initiate at the IMC/steel interface.

Combining turning with slide burnishing to improve surface integrity and stress corrosion resistance

The surface characteristics play a pivotal role in stress corrosion cracking (SCC) resistance and fatigue life. Strengthening and modifying the material surface properties during manufacturing is a promising approach to improving the component service performance. Therefore, this study proposes a new surface finishing and strengthening mode that combines turning with slide burnishing (TSB) with a common turning tool. The core idea is that the cutting edge of the tool removes the material surface layer as a chip while the tool corner with a large radius performs a slide burnishing process on the manufactured surface. The machining-induced surface integrity in terms of residual stress, surface roughness, micro-hardness and microstructure changes was examined, and the SCC susceptibility was tested in a strong corrosion environment. The results showed that the surfaces processed by TSB suffered lower biaxial residual stresses, higher micro-hardness and deeper hardened layer when compared with the traditional turned surfaces. Besides, it was observed the TSB processed surfaces were less susceptible to SCC initiation which could be attributed to the lower residual stress introduced by TSB. Additionally, the multi-pass TSB treated surfaces showed significant low SCC susceptibility. Consequently, this work suggests that the surface integrity and the service performance like SCC resistance can be improved by TSB.

Oxidation resistance and mechanical properties of AlTiZrHfTa(-N) high entropy films deposited by reactive magnetron sputtering

(AlTiZrHfTa)1−xNx refractory high entropy films (RHEFs) were deposited by direct current magnetron sputtering in various nitrogen ratios (RN2 = N2/(Ar+N2)) on flat glass, silicon and sapphire substrates. The nitrogen-free film is amorphous while the nitrides are single-phased solid solutions with Face-Centered Cubic (FCC) structures. The hardness increases from 6.4 GPa for the nitrogen-free film to 25.3 GPa for the film obtained at RN2 = 20%. A preferred orientation from (111) to (200) occurs when RN2 increases from 5% to 50%. H/E and H3/H2 reports show high values for the film deposited at RN2 = 20% compared to that at 10%. However, this latter has the best compromise in terms of mechanical properties related to a lower residual stress value. The nitrides films obtained with RN2 ≥ 5%, are thermally stable at 800 °C for 3 h under vacuum. Compared to the metallic film they show an improved oxidation resistance. In fact, the Kp constant of the nitride (RN2=20%) is lower (1.22 10−7 g2 cm−4 h−1) than that of the nitrogen-free film (1.77 10−6 g2 cm−4 h−1). The corresponding activation energies (Ea) are respectively calculated at 94.8 KJ. mol−1 and 45.5 KJ.mol−1. After oxidation process, TEM analysis reveal the formation of homogeneous mixed oxide.

Ensuring the functional performance of the cylinder liner at the finishing processing stage

The article considers the use of a centrifugal ball burnisher in the hardening finishing operations of machining the cylinder liner of a marine small-sized diesel engine of the 4Ч9,5/11 type. The use of a burnisher provides high hardness and low surface roughness and residual stresses, which have a compressive positive effect on the surface layers of the product. Keywords: diesel engine, cylinder liner, roughness, hardness, burnisher. nurik909@mail.ru

In-situ monitoring of stress evolution in high power impulse magnetron sputtering-deposited Ti-Al-N films: Effect of substrate bias and temperature

The origin of residual stress during film deposition is a topic of strong interest in the coating engineering community; it requires appropriate investigation tools to explain the different mechanisms acting on the evolution of stress concerning process conditions and material characteristics. In the present work, we used in-situ curvature stress probing combined with ex-situ techniques such as X-ray diffraction cos2α*sin2ѱ applied to the deposition of model Ti0.42Al0.58N coatings. The films were prepared at room temperature and 300 °C, and four substrate bias strategies were studied, namely two constant values of substrate bias (0 V and −75 V), and two systems with alternating high bias and no bias. At room temperature, the application of bias changed the stress from slightly tensile to compressive, while at 300 °C, the use of high bias increased the compressive intrinsic stress by 3 GPa along with hardness. Linking in-situ and ex-situ stress measurements permitted a direct assessment of thermal stress, which varied considerably with bias. The combined analysis revealed the presence of different contributors to stress generation, particularly at the grain boundaries. The relevance of each contributor to the final stress state was found to depend on the energetic growth conditions. Stress monitoring revealed significant differences between the growth of dense films on the porous ones compared to the opposite sequence, suggesting a straightforward pathway to stress mitigation. This is documented by films produced in the alternating −75/0/−75 V bias sequence that presented comparable mechanical properties as films deposited at a constant high (−75 V) bias, but with substantially lower residual stress.

High crystal quality and low residual stress composite piezoelectric films prepared by a two-step method

The crystal quality and residual stress of piezoelectric films are critical factors that limit the performance of acoustic wave devices. To overcome this challenge, a new two-step method integrating metal-organic chemical vapor deposition (MOCVD) and physical vapor deposition (PVD) was presented. The lower part of the composite film was first grown by MOCVD, exhibiting monocrystalline quality and high residual stress. Subsequently, the upper part of the composite film deposited by PVD effectively compensates for the residual stress in the lower part of the composite film. The new two-step method was validated on the growth of AlN and AlScN on sapphire substrate. A composite film Al0.8Sc0.2 N/AlN was obtained with a full width at half maximum of 0.047° for AlN (002) of rocking curve, exhibiting a residual stress of +381 MPa and a surface roughness of 1.12 nm. It demonstrates the feasibility of preparing high-quality composite piezoelectric films for further acoustic applications.

Experimental and theoretical analysis of exhaust manifold by uncoated and coated ceramics (Al2O3, TiO2 and ZrO2)

This research work focuses on the experimental and theoretical analysis of the exhaust manifold with novel ceramic coatings (TiO2 Coatings; ZrO2 coatings; and Al2O3 coatings) and these coatings were applied on the manifold by the plasma arc technique with a temperature range of 14,000 οC and micron coatings ranging from 350 to 400μ. The implementation of these coatings over the exhaust manifold gives enormous benefits such as improvised endurance strength, higher heat dissipation, and flexible directional heat flux compared to uncoated materials. From the numerical analysis, it is found that the ceramic coatings give lower thermal residual stresses compared to the uncoated materials. Among the three coatings, Al2O3 coatings give optimal performance in terms of heat dissipation, heat flux, and directional heat flux and also lower thermal residual stresses. From the overall analysis; it is also clear that the implementation of ceramic coatings on the exhaust manifold gives enormous advantages in terms of corrosion resistance and lesser maintenance as compared to uncoated materials.

Experimental study on ultrasonic vibration assisted grinding of GCr15SiMn bearing steel

Many problems of GCr15SiMn high-carbon chromium bearing steel have been found in the ordinary grinding (OG) process. Aiming at the problems of high grinding force, low residual compressive stress and low surface quality, ultrasonic machining technology is combined with OG. The test of ultrasonic vibration assisted grinding (UVAG) GCr15SiMn high carbon chromium bearing steel is carried out. The residual stress, surface roughness, grinding force, and surface morphology are analyzed by changing machining parameters. The research showed that as the grinding depth and feed speed become larger, the grinding force increased, when the spindle speed rises, the grinding force becomes smaller. In the test range, with the ultrasonic amplitude from small to large, the grinding force from large to small. The ultrasonic grinding force is 27.8% lower than that of OG. The surface roughness increases with increasing grinding depth and feed speed and decreases with increasing spindle speed and ultrasonic amplitude, and at an amplitude of 6 μm, the surface roughness is reduced by 27% compared to OG. When the feed speed and grinding depth are lowered or the ultrasonic amplitude is increased, the surface residual compressive stress is increased. Compared to OG, it is increased by more than 13.5% under the same parameters. At the same time, the phenomenon of burrs and surface tears can be reduced, and the surface quality of the workpiece becomes better.

Innovative powder-based wettability evaluation of HfB2-ZrB2-SiC-B4C-CNT composite: Effect of surface roughness and ambient conditions

Active metal brazing, the most common method for joining UHTCs, necessitates the wettability of diborides in contact with filler layer, which is evaluated in present work using a powder-based approach. Ni powder is placed on pre-spark-plasma-sintered HfB2-ZrB2-SiC-B4C-CNT (HZSBC) composites with varying surface roughness (0.1 and 0.7 μm) and heated to 1450°C in Ar and Ar+H2 atmosphere. Formation of gas-liquid-Ni interface for rough surface (Ra:0.7 µm) restricted the spreading of molten Ni on HZSBC surface and increased contact angle (by ∼30°) compared to smooth surface (Ra:0.1 µm). Addition of 5% H2 into Ar atmosphere suppressed the oxide formation and improved molten Ni spreading at a contact angle of ∼10°. Cross-sectional analysis of droplets supports dissolutive wetting where enhanced surface roughness has reduced the dissolution depth (by ∼50% for Ar and ∼12% for Ar+H2 atmospheres). Furthermore, brazing of HZSBC composites with Ni interlayer is performed at 1450°C, in Ar and Ar+H2 atmosphere. Lower wettability of Ni with HZSBC in Ar resulted in detachment of composites during polishing, thus, indicating poor interfacial bonding. Successful brazing of these composites in Ar+H2 atmosphere resulted in microstructural inhomogeneity with decreased hardness and Young's modulus by ∼68% and ∼58%, respectively, from HZSBC region to Ni-Si rich filler layer. Lower interfacial residual stresses (∼0.4 GPa) have provided the structural integrity between ceramic and metal layer, thus making them suitable for applications involving complex shapes.

Effect of support structure design toward residual stress of part build using selective laser melting

AbstractSelective laser melting process required post‐processing of support structure and it is compulsory to use it at overhanging part. There is certain condition where support structure cannot be remove using wire cut machine because of limited access to part and need to be remove manually. This is where the removability of support structure plays important role because support structure needed to be design so that it can be easily remove using manual way. In addition, when comparing contour type of support structure with the most commonly use block type of support structure it shows that contour type is easier to be remove. Furthermore, no research has been done on optimizing the contour type of support structure. A research experiment is conducted in this work to optimize the parameter of contour support structure which is contour offset, teeth height, teeth top length and teeth base length. The result indicates amount of residual stress and the time required for support removal are significantly influenced by contour offset, while all parameters are significant for support volume. The optimum parameter to get low residual stress is contour offset is 0.6 mm, teeth height is 1.4 mm, teeth top length is 0.75 mm and teeth base length is 1.55 mm.

Effect of residual stress and phase constituents on corrosion-cavitation erosion behavior of 304 stainless steel by iso-material manufacturing of laser surface melting

Different from additive manufacturing and subtractive manufacturing, iso-material manufacturing (also named isovolumetric manufacturing) is applied to 304 stainless steel (304SS) by laser surface melting (LSM) at four energy density (51 J/mm2, 44 J/mm2, 39 J/mm2, 37 J/mm2), aiming at improving the resistance to corrosion and cavitation erosion without any addition of precious metals. The experimental results show that the microstructure presents the transformation from plane crystal → columnar crystal → equiaxed crystal from interface to top surface. All the samples exhibit austenite+martensite+Cr23C6, and the martensite content is increased from 9.3% to 40.8% with the energy density decreasing after iso-material manufacturing. In addition, low angle grain boundary is increased, and nano-mechanical properties also show an increasing trend. The decreasing of energy density also enhances the residual tensile stress of the melting layer. The sample obtained at energy density of 51 J/mm2 displays the highest corrosion resistance in 3.5 wt% NaCl solution, which can be attributed to its low galvanic corrosion degree and small residual tensile stress. The sample obtained at energy density of 39 J/mm2 exhibits the highest cavitation erosion resistance (Re) and its passivation film displays better self-repairing and re-passivation ability as indicated by synergistic cavitation erosion-corrosion behavior. The high cavitation erosion resistance can be attributed to the proper compromise of the constituent phases and higher self-repairing of the passivation film for the melting layer. Moreover, the 304 stainless steel prepared in this study exhibits a high or comparable corrosion and cavitation erosion resistance in comparison with the samples prepared by other fabrication method reported previously.

Microstructure of <111>‐Textured Cubic Boron Nitride Film Deposited under Oxygen‐Containing Atmosphere

The cross‐sectional and planar microstructures of a cubic boron nitride (cBN) thin film with a <111>‐preferential orientation are observed using transmission electron microscopy. The cBN films are deposited by unbalanced magnetron sputtering under the condition that oxygen is added to a 20 sccm Ar–N2(25%) gas mixture. Thecross‐sectional view of the cBN film deposited with the addition of 0.4 sccm of oxygen shows a dual‐phase structure: turbostratic boron nitride (tBN) layers are filled between cBN columns, and the planar view shows that cBN crystals were surrounded by a tBN matrix. The films deposited under less than 0.4 sccm of oxygen addition show a single‐phase structure with no tBN layers between the cBN columns. The difference between dual‐ and single‐phase structures is that they have preferred <111> and <220> orientations. This texture variation is interpreted as being due to the low residual stress of the dual‐phase‐structured cBN film and the low surface energy of the cBN (111) plane. The residual stress of the dual‐phase structure is significantly lower than that of the single‐phase structure. This is attributed to the compressive residual stress relieved by the tBN layers formed between the cBN columns.

Optimization of Segmented Thermal Barrier Coatings (s-TBCs) for High-Temperature Applications

AbstractHot section components of stationary gas turbines, such as turbine blades and vanes, are coated with thermal barrier coatings (TBCs) to increase the component life. TBCs provide thermal insulation to the metallic components from hot gas in the gas turbines. The TBCs represent high-performance ceramics and are mainly composed of yttria-stabilized zirconia (YSZ) to fulfill the thermal insulation function. The microstructure of the TBCs should be porous to decrease heat conduction. Besides the porous TBCs, the subsequently developed vertically segmented thermal barrier coatings (s-TBCs) feature outstanding thermal durability. For the formation of this segmented coating microstructure, the YSZ should be deposited under high thermal tensile stress during the coating process. Therefore, substrates are heated just before the coating by plasma or in an oven in recent research. In this work, the development of process parameters for s-TBCs produced by atmospheric plasma spray (APS) without pre-heating is presented. Within the experiments, the relevant process parameters, such as plasma gases, powder feed rate, surface speed, and pathway strategy, have been optimized to achieve the segmented coating microstructure with high deposition efficiency by a conventional plasma torch. Furthermore, YSZ powders used in this study are characterized, and the effect of powder characteristics on the coating microstructure is investigated. The coating microstructure in this work aims to achieve the formation of a high number of vertical cracks with a combination of low internal residual stress and high adhesive tensile strength for the s-TBCs.

Construction of a ceramic coating with low residual stress on C/CA composites for thermal protection at ultra-high temperatures

Carbon fiber reinforced carbon aerogel (C/CA) composites are promising candidates for thermal protection at ultra-high temperatures. However, the poor anti-oxidative performance and difficulties in preparing a dense and crack-free coating without damaging the substrate have limited their use in oxidizing environments. Here, we have prepared a dense SiC coating on C/CA by chemical vapor deposition under the assistance of a SiC gradient layer derived from in-situ ceramization of the CA matrix. By courtesy of the gradual compaction and composition, the SiC bilayer coating with 440 μm thickness is crack-free and has low residual stress. Resultantly, the coating has a high interfacial bonding strength of 6.79 MPa due to the difficult crack initiation and propagation as well as the limited strain localization. The coated C/CA shows good ablation resistance with a linear ablation rate of 0.62 μm/s at 1900 °C for 600 s and remarkable thermal insulation with a large temperature drop of 930 °C in 10 mm thickness. The strategy of a combination of dense CVD and gradient ceramization layers can be extended to other components with excellent high temperature properties such as ZrC, HfC, which will facilitate the improvement of the ablation performance of C/CA in ultra-high temperatures.

Finite element analysis and experimental verification of residual stress in brazed diamond with Ni-Cr filler alloy

Residual stress is the main factor affecting the processing performance and service life of brazed diamond tools. Due to the high melting point of Ni-Cr filler alloy, the difference in thermal expansion coefficient between diamond and brazing layer is large during high brazing temperature, and excessive residual stress tends to be generated at the brazed joint. The existing process parameters are difficult to meet the demand for the lower residual stress under high brazing temperature. Suppressing effectively excessive residual stress has become the focus of current researches for process optimization of brazed diamond. In this paper, the conventional spherical finite element model of diamond is replaced by a six-octahedral finite element model of diamond, and a 3D finite element model of brazed diamond is established. The effects of brazing temperature, brazing layer thickness and holding time on the residual stresses in brazed diamond are analyzed. The results show that higher brazing temperatures result in larger differences in thermal expansion coefficients between the diamond and brazing layer, resulting in larger residual stress. The higher brazing layer thickness will enlarge the stress affected area inside diamond and thus generate higher stress. The residual stress decreases with rising holding time, however, the subsequent thermal damage to the diamond increases. Experimental measured tendency of residual stress for brazed diamond through Raman spectroscopy is well consistent with that of calculated results. After observing of the surface morphology for brazed diamond through super-depth of field microscope, the brazed diamond presents the low residual stress, weak graphitization and excellent processing performance at the condition of brazing temperature of 1010 °C, brazing layer thickness of 0.3 mm and holding time of 5 min.

Efficacy of TiCrN/DLC coatings for service life enhancement of stamping dies

The present study examines the applicability of wear-resistant TiCrN/DLC coatings to dies used in stamping, for extending the service life. The TiCrN/DLC coatings were deposited via the Cathodic Arc Physical Vapor Deposition (CAPVD) route on D3 tool steel. The thickness of the TiCrN layer was tailored to achieve the desired properties, such as high adhesion strength, toughness, wear resistance, and low residual stress. The tribological properties of the TiCrN coating with best properties was assessed by universal tribometer and dry sand rubber wheel tester. Further, the tribological response of the identified TiCrN coating was investigated by adding a thin layer of low friction coefficient diamond-like carbon (DLC). Finally, the emphasis was also placed on determining the real time life of stamping dies with TiCrN/DLC coating. From the surface relief data of the finished product, measured at regular intervals pertaining to both coated and uncoated dies, a two-fold increase in the productivity with enhanced flowability was observed. The enhancement in the service life of the stamping dies clearly indicates that the CAPVD TiCrN/DLC coating can be a good alternative to hard chrome coatings already in use.

A study of fracture toughness and thermal property of nanostructured Yb2SiO5 environmental barrier coatings

The objective of this study was to examine the characteristics of a nanostructured Yb2SiO5 coating and a conventional Yb2SiO5 coating, both prepared using atmospheric plasma spraying (APS). The investigation focused on the phase composition, microstructure, thermal properties, and fracture toughness of the two types of Yb2SiO5 coatings. The findings revealed that both nanostructured and conventional coatings consisted of Yb2SiO5, Yb2O3, and amorphous phases. The nanostructured Yb2SiO5 coating exhibited lower residual tensile stress compared to the conventional Yb2SiO5 coating, resulting in fewer crack defects in the former. The thermal conductivity of the nanostructured Yb2SiO5 coating ranged from 0.71 to 1.22 W/(m·K) between 200 and 1200 °C, while the conventional Yb2SiO5 coating exhibited a thermal conductivity range of 0.81–1.32 W/(m·K) within the same temperature range. Consequently, the thermal conductivity of the nanostructured Yb2SiO5 coating was lower than that of the conventional counterpart. Furthermore, the presence of nanograins significantly enhanced the fracture toughness of the nanostructured Yb2SiO5 coating, which was found to be 1.7 times greater than that of the conventional Yb2SiO5 coating.

A comparative assessment of the effects of laser shock peening and ultrasonic shot peening on surface integrity and ratcheting fatigue performance of HSLA steel

In this work, a comparative assessment of the effects of laser shock peening (LSP) and ultrasonic shot peening (USP) on surface integrity and ratcheting fatigue performance of ASTM A588 Grade D high strength low alloy steel was done. Surface roughness, microhardness, microstructure, residual stress, and micro-texture analyses were used to describe the unpeened and peened surfaces. Three sets of tension-compression asymmetric cyclic loading tests were carried out at room temperature on unpeened and peened specimens to evaluate the effects of the applied treatment on strain accumulation and fatigue life. The quantitative dislocation densities of selected specimens were calculated in order to examine the mechanism underlying the softening behavior of USP-treated specimens during fatigue testing. The results are presented in light of the substantial surface roughness, lower compressive residual stress, and weakening of texture intensity revealing an unexpectedly large drop in fatigue strength following USP treatment compared to the unpeened and laser-peened specimens.

Bottom-up micromachined PZT film-based ultrasonic microphone with compressible parylene tube

This paper reports on a micromachined ultrasonic microphone using a bottom-up fabrication scheme. Starting with a 4 μm-thick titanium foil as the substrate, each functional film and key element was added to the foil substrate to complete the ultrasonic microphone. The piezoelectric lead zirconate titanate film hydrothermally grown on the patterned substrate with low residual stress effectively deflected the unimorph-sensing cantilever array of the microphone under ultrasound pressure. The created cantilever array structure secured on a 250 μm-thick SU8 hollow plate formed an ultrasonic microphone plate that was tested with a sensitivity of −60 dBV Pa−1 at 21 kHz (with 0 dB gain amplification) and an operation bandwidth of 5–55 kHz. Different thicknesses of parylene films ranging from 0.5 to 2 μm overlaid over the entire sensing region and converted the cantilever-to-diaphragm-structured microphone for further investigation. An enhanced result was observed when the deposited parylene film thickness was in the submicron range. The sensitivity of the microphone can be further enhanced by up to 33% by adding a parylene-film-made compressible tube to act as a Helmholtz resonator (HR). The HR model was discussed and compared with the experimental results. The output amplitude of the developed microphone assembled with the compressible tube demonstrates a 15 dB increase compared to that of a commercial capacitive MEMS ultrasonic microphone.

Effect and mechanism analysis of free carbon on the oxidation behaviour of SiC fibres

Continuous SiC fibres have received widespread attention in the field of aero-engines due to their excellent mechanical properties and oxidation resistance. Free carbon is one of the main factors affecting the oxidation behaviour of SiC fibres, but its role and influence are still unclear. The oxidation behaviour of two types of SiC fibres with similar oxygen contents and crystallite sizes but different carbon-silicon ratios was investigated in this study. Free carbon did not significantly influence strength retention, but positively affected elastic modulus retention. The oxidised carbon-rich SiC fibres exhibited a thinner, smoother, and denser oxide scale. The oxidation rate of the carbon-rich fibres was lower than that of near-stoichiometric fibres after oxidising at 1200 °C. The oxidation activation energies of both were 170.44 ± 15.80 and 234.45 ± 24.94 kJ/mol, respectively. Free carbon can inhibit oxidation scale crystals at the beginning of oxidation while promoting crystallisation scale as oxidation intensifies. The oxidation scale crystals of carbon-rich fibres are mainly controlled by the interface, whereas those of near-stoichiometric fibres are mainly controlled by diffusion. A stress expansion zone was formed at the interface of the oxidised SiC fibres. Free carbon can enhance the internal stress of the stress expansion zone, resulting in lower residual compressive stress on the surface of carbon-rich SiC fibres. These results are significant for improving the oxidation resistance of SiC fibres by retaining free carbon.