Stress Relief Heat Treatment Research Articles

High cycle fatigue strength of additively manufactured AISI 316L Stainless Steel parts joined by laser welding

The present paper deals with the high cycle fatigue behavior of 316L stainless steel parts, obtained either by rolling or by LPBF additive manufacturing process, and their assemblies by autogenous laser welding. The aim is to study the effects of this assembly process on the fatigue strength, assessed at a 106 cycles, of additively produced parts, compared to that of raw rolled products. An original campaign of high cycle fatigue tests based on seven batches of specimens, manufactured additively or cold rolled, one-pieced or assembled, raw or polished, is carried out with a positive load ratio (R= 0.1). A stress relieving heat treatment is applied to all specimens to reduce residual stress levels. Particular attention is paid to identifying the damage mechanisms associated with each batch. Preliminary results allowed an original conclusion that laser welding does not degrade the fatigue strength of additively produced parts. In addition, the fatigue behavior of the studied system depends essentially on the stress concentration generated on the surface or a few micrometers in the underlayer, mainly by the roughness of the LPBF produced parts combined with the lack of fusion (LoF) defects, and in a second order by the geometry of the laser welding channel. Furthermore, additively produced parts showed higher sensitivity to the presence of defects than rolled parts.

Effect of a Stress Relief Heat Treatment of AlSi7Mg and AlSi10Mg Alloys on Mechanical and Electrical Properties According to Silicon Precipitation

Effect of a Stress Relief Heat Treatment of AlSi7Mg and AlSi10Mg Alloys on Mechanical and Electrical Properties According to Silicon Precipitation

Recrystallization behavior of selective laser melted Co–Cr–Mo alloys with several heat treatment times

For parts manufactured by selective laser melting (SLM), post-heat treatment is essential to relieve residual stress. However, recrystallization and grain growth processes upon the stress-relieving heat treatment in SLM-fabricated Co–Cr–Mo alloys are not well understood. In this study, Co–Cr–Mo alloy specimens were fabricated by SLM and then heat treated at 1150 °C for various time periods (10, 20, 30, 45, and 60 min) to evaluate the influence of heat treatment time on the recrystallization process. The alloy microstructure was analyzed via confocal laser scanning microscopy, scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) and X-ray diffraction analyses. The mechanical properties of the produced specimens were evaluated by tensile tests. When the heat treatment was conducted for up to 45 min, the 0.2% yield strength (YS) decreased, whereas elongation increased. In contrast, when heated for 60 min, both the 0.2% YS and elongation decreased. The EBSD images demonstrated that both columnar and equiaxed grains existed, and recrystallization partially initiated after heating for 10 min. When heated for 45 min or more, the samples consisted entirely of equiaxed grains; however, TEM images showed regions with remaining subgrain boundaries and dislocations. Fully recrystallized microstructures were not obtained even after heating for 60 min. In addition, heat treatments longer than 60 min induced coarse grains and precipitates along the grain boundaries, which decreased the strength and ductility of the alloys. Retaining the subgrains within the grains to some extent enables producing alloys with excellent strength and ductility.

Influence of Stress Relief Annealing Parameters on Mechanical Properties and Decomposition of Eutectic Si Network of L-PBF Additive Manufactured Alloy AlSi10Mg

This paper evaluates the effect of stress-relieving heat treatment on the AlSi10Mg alloy prepared by additive manufacturing using the Laser Powder Bed Fusion (L-PBF) with print parameters: 370 W, 1400 m/s, and 50 μm. The as-built state and four different annealing modes (240 °C/2 h, 240 °C/6 h, 300 °C/2 h, and 300 °C/2 h/water-quenched) are investigated. To determine the effect of the annealing mode on the mechanical properties of the L-PBF AlSi10Mg alloy, heat-treated samples were compared with the as-built state and with each other. The mechanical properties of the samples were determined by tensile and hardness tests. The strength in the as-built state is 488 MPa, depending on the method of heat treatment, the strength values range from 296 MPa to 417 MPa, and the HV10 hardness values are in accordance with the measured strength values. Furthermore, the microstructure of the samples was investigated by scanning electron microscopy (SEM) analysis, which was then linked to the measured mechanical properties. The composition of the microstructure of the alloy and its influence on the mechanical properties were determined by energy dispersive spectroscopy (EDS) analysis. Furthermore, the differences between the individual heat treatments in comparison with the as-built state were analyzed and the phenomenon of decomposition of the silicon network after reaching specific temperatures was discussed and confirmed. The paper evaluates the effect of dwelling time on stress relief annealing. It was found that if annealing at intermediate temperatures of 240 and 300 °C is applied, changes in structure and mechanical properties are more temperature- than dwell-time-dependent.

Influence of Physical Vapor Deposition on High-Cycle Fatigue Performance of Additively Manufactured Ti-6Al-7Nb Alloy

Load-bearing permanent implants, such as hip or knee joint replacements, are permanently loaded in the human body and must withstand considerable high loading cycles. The characteristic properties of additively manufactured Ti-6Al-7Nb, manufactured by laser powder bed fusion (LPBF), such as a rough surface and high residual stresses, have a detrimental effect on the fatigue behavior of such components. Functional physical vapor deposition (PVD) coatings and heat treatments offer the possibility to influence these properties. For this reason, the effects of stress-relief heat treatment (SR; 600 °C/4 h) and three PVD coatings (titanium nitride (TiN), titanium carbonitride (TiCN), and silver-containing amorphous carbon (a-C:Ag)) on the mechanical properties, in terms of high-cycle fatigue, are identified. Wöhler curves are determined and the staircase procedure ascertains the fatigue strengths. The fatigue strengths increase compared to the as-built condition by 105.4% (SR), 44.2% (TiN), 31.1% (TiCN), and 2.6% (a-C:Ag). Fracture surfaces are analyzed by scanning electron microscopy and show LPBF characteristic defects such as pores. The surfaces are partially divided into forced and fatigue fracture, the latter characterized by fatigue striations. Overall, PVD coatings, and especially SR, lead to an improved high-cycle fatigue behavior.

Hardening mechanisms in a high wall thickness sour service pipe steel API 5L X65 before and after post-welding heat treatments

The strength of the pipe region opposite to the weld in a high wall thickness sour service pipeline steel API 5L X65, manufactured by the SAW UOE process, has been studied in detail. The fundamental hardening mechanisms that contribute to the yield strength of this pipe portion were determined according to its microstructural features before and after post-weld stress relieving heat treatment. The root mean square summation of the different strengthening mechanisms resulted in a reasonable estimate of the yield strength of this pipe portion before the heat treatment. The contributions of the different strengthening mechanisms proceeded at different length scales in this pipe portion before and after the heat treatment. The heat treatment at 650 °C (1.5 h) reduced the contributions of the strengthening mechanisms to the steel yield strength corresponding to this pipe portion. The dislocation strengthening contribution decreased by approximately 50%. The precipitation hardening and the solid solution and grain size strengthening reduced their contributions by approximately 14% and 3% respectively.

Influence of post-processing heat treatment on the cyclic deformation behaviour of AlSi10Mg aluminium alloy subjected to laser powder bed fusion

The effect of heat treatment on the cyclic deformation behaviour of AlSi10Mg aluminium alloy manufactured by laser powder bed fusion (LPBF) was investigated. Uniaxial fully-reversed strain-controlled fatigue tests were conducted on polished smooth specimens in the low-cycle and high-cycle fatigue regimes for three material states: no heat treatment, stress relief heat treatment and T6 heat treatment. The effect of the heat treatment on microstructure, ductility, hardness and tensile properties was also investigated. It was found that the microstructural transformation associated with the heat treatments increased the ductility, and reduced the hardness, the tensile strength, and the fatigue resistance, particularly in the low-cycle fatigue regime. Thus, these standard heat treatments used in conventional heat-treatable aluminium alloys are not recommended for LPBD aluminium alloys subjected to cyclic strain loading.

Attainment of favorable microstructure for residual stress reduction through high-temperature heat treatment on additive manufactured inconel 718 alloy

Mitigation of residual stress in additive manufactured metal parts through post-heat treatment helps enhancement of life during service. The present work aims to investigate the effect of stress-relieving heat treatment (SRHT) at 1065 °C on microstructural changes, residual stress patterns, and mechanical properties of Inconel 718 (IN718) blocks fabricated through use of the laser-based powder bed fusion (L-PBF) process. The X-ray diffraction technique known as cos α method was used to capture the Debye ring for the measurement of surface residual stress. Dissolution of the columnar dendrites along with laves phases in the interdendritic region was seen after SRHT. The resultant microstructure showed partially recrystallized grains along with the formation of annealing twins and strengthening phases such as γ’, γ’’, and tiny metal carbides. The residual stress on the top surface of as-built sample was found to be 77% higher on the corners compared to the center region. The distribution of residual stress was seen as not uniform in the as-built condition. The SRHT sample showed uniform distribution of compressive residual stresses in all the locations. This phenomenon was due to the diffusion of atoms present in the high-stress regions migrated to low-stress regions until attaining their equilibrium. The combination of face centered cubic (FCC) and hexagonal lattices in as-built microstructure led to misfit strain between the grains. After SRHT, microstructure of IN718 experienced relaxation of residual stresses due to the transformation of hexagonal structured laves phase into the body-centered tetragonal (BCT) γ’’and FCC-structured γ’ phases. Furthermore, SRHT significantly alters the mechanical properties of IN718 alloy causing a 23.03% increase in average hardness and a 21.04% increase in tensile strength.

Effects of ER308L buttering and post-buttering heat treatment on the microstructure and mechanical properties of API 5L X65/AISI304 dissimilar joint

This study investigates the effects of ER308L butter layer on the microstructure and mechanical properties of API 5L X65/dissimilar joint. High strength low alloy API 5L X65 and AISI 304 steel grades are widely used in a wide range of industrial applications, and more specifically in the oil and gas sector. Dissimilar welds between API 5L X65 pipeline and austenitic stainless steel joints have long been used in the oil/gas and petrochemical sectors. Using buttering layer to improve properties of these types of dissimilar joints has lately received significant attentions. In the present work, the ER308L butter layer was applied on the API 5L X65 side through gas tungsten arc welding, followed by a stress-relief heat treatment at 600 °C. Buttered API 5L X65 samples were welded to AISI 304 samples through gas tungsten arc welding in 5 passes. Microstructures of joints were evaluated by scanning electron microscope (SEM) and optical microscope (OM). Mechanical properties were also assessed by tensile, impact, and Vickers micro-hardness tests. Fractography and phase identifications were carried out using SEM and energy dispersive spectroscopy (EDS). Results showed that a martensite layer on the fusion zone with a thickness of about 30 μm was observed at the interface of API 5L X65/butter layer. It appears that compositional changes played a significant role in the formation of martensitic structures and type-II grain boundaries. Results also showed that the fracture positions in both heat treated and as-welded samples were in the heat affected zone (HAZ) of API 5L X65. The implications of buttering and post-buttering stress-relief heat treatment for the microstructure and mechanical properties are comprehensively discussed.

Rapid method for comparative studies on stress relief heat treatment of additively manufactured 316L

The additive manufacturing method laser powder bed fusion (L-PBF) is known to introduce large residual stresses in the built component. Optimization of process parameters and subsequent heat treatment is crucial to relieve these residual stresses. However, many of the available tools used to analyze these residual stresses are either prohibitively expensive, or too time consuming for initial prototyping stages.A qualitative method for rapid evaluation of the effectiveness of stress relief heat treatment of L-PBF manufactured 316L has been tested. Residual stress induced distortion has been measured with contact and non-contact methods to study the effect of different stress relief heat treatment temperatures (600–950 °C, fixed holding time: 1 h). Over the examined temperature interval, at which deformation was measured, distinct differences were observable at each temperature with both methods. Based on the distortion, shape stability was considered reached after subjecting the test geometry to a heat treatment temperature of 900 °C for 1 h. Complementary mechanical testing and microstructural characterization were carried out to provide a more general understanding of the implications of each heat treatment temperature. Microstructural characterization revealed that complete dissolution of the cellular sub-grain features occurred at the same temperature as where the minimum magnitude of distortion was obtained.

Effect of stress-relief heat treatments on the microstructure and mechanical response of additively manufactured IN625 thin-walled elements

Metallic structures fabricated with laser powder-bed fusion (LPBF) often have elemental segregations that are far from equilibrium and high levels of residual stresses due to the localized melting and rapid solidification that occur during printing. These unique characteristics result in mechanical property debits that make it difficult to employ such printed parts in critical applications. However, post-processing heat treatments can minimize the negative effects of residual stresses and reduce segregation. In this study, we measure the effect of two stress-relief heat treatments, 870°C for 1 h (industry-recommended) and 800°C for 1 h (potential alternative), on the microstructure and room temperature, monotonic quasi-static mechanical response of LPBF-processed thin-walled Inconel 625 T-elements. Electron backscatter diffraction analysis revealed an overall columnar microstructure with a strong <001> texture that is closely aligned with the build direction. The well-developed dislocation cell walls in both stress-relieved cases were found to be decorated with two types of blocky precipitates, namely, (Nb, N)-rich MX and (Nb, Mo, Si, N)-rich -M6X with scant evidence of the deleterious δ phase. The slightly larger size and higher volume fraction of precipitates in the 870°C case was found to have a small effect on the mechanical response of the T-elements under the current loading conditions and the difference in mechanical response of the two heat treatments was only significant at the very later stages of deformation. These findings make 800°C for 1 h a viable stress-relief alternative for our IN625 samples under the loading conditions studied here.

Hybrid Manufacturing of Aluminium Parts Combining Additive and Conventional Technologies—Mechanical and Thermal Properties

Metal additive-manufacturing technologies enable the production of complex geometries. However, high manufacturing costs hinder these technologies being employed in some industries. In this sense, a hybrid strategy is presented in this paper, to achieve the best of additive and subtractive technologies, offering economic advantages. AlSi10Mg aluminium powder was deposited on AW-6082 pre-machined substrates and mechanical and thermal properties of these specimens were evaluated considering the application of a stress relief heat treatment. The results were especially good in the compressive mechanical properties and in the thermal properties: compressive properties were improved by up to 27%, and the specific heat capacity and coefficient of thermal expansion were reduced by up to 38%, compared to additively manufactured AlSi10Mg. Therefore, hybrid manufacturing can be a profitable solution (i) in thermal management applications, (ii) when compressive loads are presented, or (iii) to repair damaged parts, providing a circular economy, as presented in a case study of this paper.

The effect of shot peening on corrosion performance of anodized laser powder bed fusion manufactured AlSi10Mg

Anodizing is commonly used on aluminium to improve the surface properties of the product for better corrosion resistance and wear resistance. In this research, the anodizing of laser powder bed fusion (LPBF) manufactured AlSi10Mg parts which were subjected to prior shot peening (SP) were investigated. Anodized specimens were analysed using SEM imaging and the corrosion properties were examined considering the effect of residual stress relief heat treatment and the SP process. The results showed that the SP deteriorates the corrosion performance of the material as such, but together with the subsequent anodizing it resulted to the lowest corrosion current of all investigated structures. As the SP can be used to lower the surface roughness of the LPBF parts and to increase fatigue life as well, the results of this work further encourage the use of SP with anodizing.

High Temperature Tensile and Fatigue Behaviors of Additively Manufactured IN625 and IN718

Recent advancement and research efforts in additive manufacturing (AM) have made it a promising technology to fabricate nickel-base superalloy parts in near net shapes. IN625 and IN718 are the commonly used superalloys in elevated-temperature applications in the aerospace and energy sectors. However, the fatigue performance of the additively manufactured (AM) components often depends on the defects like porosity, micro-cracks, and lack-of-fusions, etc. In addition, the presence of columnar grains and residual stresses also affects their fatigue performance. In this study, IN625 and IN718 specimens were fabricated via both laser powder bed fusion (L-PBF) and laser powder directed energy deposition (LP-DED) followed by stress-relief and standard heat treatments. Both alloys exhibited similar defect characteristics and grain morphologies. High-temperature fatigue properties of both IN625 and IN718 were tested at two strain amplitudes of 0.005 and 0.01 mm/mm and at two elevated temperatures of 427 and 625 °C. The fatigue lives of both alloys decreased with increasing testing temperatures. AM IN718 showed inferior fatigue performance at both test temperature and strain amplitudes in L-PBF conditions while in LP-DED condition both alloys have shown comparable fatigue performance.

Effect of heat treatment on fatigue crack growth performance of AlSi10Mg aluminum alloy submitted to LPBF

Aluminum alloys are used in several industries, namely in the aeronautic and aerospace industries due to its good corrosion resistance and good mechanical strength/weight ratio. The optimization of fatigue crack growth resistance in AlSi10Mg aluminum alloy is a vital issue for safety-relevant components, which are designed to work for a large number of loading cycles before periodic inspections. An unclear subject is the benefit effect of heat treatments in the fatigue crack growth of AlSi10Mg aluminum alloy submitted to LPBF. Therefore, the current work purposes to analyze the effect of a heat treatment at low temperature on fatigue crack growth of AlSi10Mg aluminum alloy submitted to LPBF. Fatigue crack growth tests in Mode I are performed in compact tension (CT) specimens at two different conditions: as-build and heat treated at low temperature. The possible befits effect is analyzed, as well as the effect of the average stress on the fatigue crack growth. The crack closure parameter is studied in order to analyze the different fatigue crack growth behaviors, as well as the microstructure, hardness, and fractography. From this work, the main achievement is the microstructural transformations associated with the standard heat treatments (T6 and stress relived at 300 °C) are unfavorable to fatigue strength in the AlSi10Mg aluminum alloy submitted to LPBF, while the non-standard stress relief heat treatment, performed at a lower temperature, cause a benefic effect in the fatigue crack growth resistance.

Effect of Stress Relief Heat Treatment on Low Cycle Fatigue Behaviours of Lpbf Stainless Steel 316l

Effect of Stress Relief Heat Treatment on Low Cycle Fatigue Behaviours of Lpbf Stainless Steel 316l

Microstructure and corrosion properties of PBF-LB produced carbide nanoparticles additivated AlSi10Mg parts

Due to its good corrosion resistance, demand for PBF-LB-produced Al alloys increases rapidly in industrial applications. However, recent studies showed that corrosion resistance critically depends on the microstructure of as-built Al parts. This study showed that nano-additivation with carbides (SiC and TiC) could gradually tune the microstructure of as-built AlSi10Mg parts, which directly affects their corrosion resistance. In this study, the microstructure of the as-built and heat-treated parts are examined by SEM-EBSD, and corrosion properties are measured with a potentiodynamic polarization test. Furthermore, we found that both SiC and TiC nanoparticles acted as a grain refiner, reduced columnar grain formations in the AlSi10Mg matrix, and improved corrosion properties after stress-relief heat treatment of as-built parts with TiC outperforming SiC.

A short review on thermal treatments of Titanium & Nickel based alloys processed by selective laser melting

Selective laser melting (SLM) has been considered as a well-researched technology used to produce near net shape metallic components with superior mechanical and physiochemical characteristics. Recently SLM has recently been used for the production of components made by high-performance alloys such as Titanium and Nickel. However, the microstructural and physical anisotropy along with the defects have negative impact on the performance and behavior of SLM-produced alloys. Therefore, to improve the material characteristics, extra attention must be paid to the source(s), identification, and removal of these deviations. Thermal heat treatments are often considered as an effective approach to deal with aforementioned issues. stress-relieving, quenching/hardening, annealing/recrystallization, and thermo-mechanical heat treatments are among prominent procedures adopted for the post processing of SLM fabricated metallic components and hence considered in this article. Since these procedures have substantial impact on the microstructure and mechanical behavior therefore an assessment of their influence and resulting changes is significantly important. Therefore, this short review provides the said knowledge on thermal treatments used to improve the SLM fabricated titanium and nickel-based alloys. This review can significantly help in designing and selecting appropriate post processing thermal treatments keeping in mind the expected outcome.

Helium effects on tensile properties of powder metallurgical-processed tungsten for fusion reactor applications

• Effects of He concentration on tensile properties of SR-W were clarified. • Suppression of recovery and recrystallization by He caused the mechanical properties. Powder metallurgical processing of Tungsten (W) followed by thermomechanical treatment, including rolling and stress relief (SR) heat treatment, is a candidate fabrication process for W materials for fusion applications. Helium (He) is insoluble in almost all metals, and grain boundary embrittlement occurs on the accumulation of He on their grain boundaries. In this study, the effects of He concentration, test temperature, and strain rate on the mechanical properties of SR-W were investigated using tensile tests. The He effects were studied by the uniform implantation of high-energy He-ions into specimens by cyclotron accelerator irradiation and by tensile tests from 300 °C to 1300 °C. The examined He concentration was below 20 appm. Vickers hardness tests on an isochronal annealed specimen up to 1500 °C showed that the threshold concentration of He for suppressing the recovery and recrystallization of the microstructure of SR-W by annealing was between 2 and 20 appm. The tensile test showed a slight increase in the ultimate tensile strength (UTS) at He concentrations up to 20 appm; however, no clear effect on the elongation or fracture mode was observed. The results of the tensile test conducted at 1300 °C on a 20-appm He-implanted specimen showed that its softening and ductility by recrystallization were suppressed; however, there was no observation of grain boundary embrittlement by He implantation. The results suggest that the mechanism of the mechanical response of He-implanted SR-W was He suppression of the recovery and recrystallization of the layered structure unique to SR-W up to 1500 °C. Moreover, at the level of He concentration introduced by nuclear transmutation in the divertor of a fusion reactor without displacement damage, no severe effect occurred to promote the embrittlement of SR-W.

Correlation between pre- and post-treatments of additively manufactured 316L parts and the resulting low cycle fatigue behavior

PurposeThe currently existing restrictions regarding the deployment of additively manufactured components because of poor surface roughness, porosity and residual stresses as well as their influence on the low-cycle fatigue (LCF) strength are addressed in this paper.Design/methodology/approachThis study aims to evaluating the effect of different pre- and post-treatments on the LCF strength of additively manufactured 316L parts. Therefore, 316L specimens manufactured by laser powder bed fusion were examined in their as-built state as well as after grinding, or coating with regard to the surface roughness, residual stresses and LCF strength. To differentiate between topographical effects and residual stress-related phenomena, stress-relieved 316L specimens served as a reference throughout the investigations. To enable an alumina coating of the 316L components, atmospheric plasma spraying was used, and the near-surface residual stresses and the surface roughness are measured and investigated.FindingsThe results have shown that the applied pre- and post-treatments such as stress-relief heat treatment, grinding and alumina coating have each led to an increase in LCF strength of the 316L specimens. In contrast, the non-heat-treated specimens predominantly exhibited coating delamination.Originality/valueTo the best of the authors’ knowledge, this is the first study of the correlation between the LCF behavior of additively manufactured uncoated 316L specimens in comparison with additively manufactured 316L specimens with an alumina coating.