Subsequent Thermal Cycling Research Articles

Additive-Manufacturing-Induced Cell Structure in Stainless Steel 316L: 3D Morphology and Formation Mechanism

AbstractThe 3D morphology of the additive-manufacturing-induced cell structure is characterized and its formation in austenitic stainless steel 316L fabricated by laser powder bed fusion is analyzed. The experimental results demonstrate that the cell structure has a 3D prism-like morphology with a crystallography-dependent spatial orientation. The formation of the cell structure is discussed. It is proposed that both the liquid–solid transformation and thermal strain contribute to the formation: the initial cells form during the liquid–solid transformation, and the final dislocation cell structure is shaped by thermal-stress-induced deformation during cooling and subsequent thermal cycles.

Real-Space Imaging of Intrinsic Symmetry-Breaking Spin Textures in a Kagome Lattice.

The electronic orders in kagome materials have emerged as a fertile platform for studying exotic quantum states, and their intertwining with the unique kagome lattice geometry remains elusive. While various unconventional charge orders with broken symmetry is observed, the influence of kagome symmetry on magnetic order has so far not been directly observed. Here, using a high-resolution magnetic force microscopy, it is, for the first time, observed a new lattice form of noncollinear spin textures in the kagome ferromagnet in zero magnetic field. Under the influence of the sixfold rotational symmetry of the kagome lattice, the spin textures are hexagonal in shape and can further form a honeycomb lattice structure. Subsequent thermal cycling measurements reveal that these spin textures transform into a non-uniform in-plane ferromagnetic ground state at low temperatures and can fully rebuild at elevated temperatures, showing a strong second-order phase transition feature. Moreover, some out-of-plane magnetic moments persist at low temperatures, supporting the Kane-Mele scenario in explaining the emergence of the Dirac gap. The observations establish that the electronic properties, including both charge and spin orders, are strongly coupled with the kagome lattices.

Dislocation structure evolution during metal additive manufacturing

Abstract Dislocation structures are abundantly present in any additively manufactured alloy and they play a primary role in determining the mechanical response of an alloy. Until recently, it was understood that these structures form due to rapid solidification during AM. However, there was no consensus on whether they evolve due to the subsequent solid-state thermal cycling that occurs with further addition of layers. In order to design alloy microstructures with desired mechanical responses, it is crucial to first answer this outstanding question. This question was answered in a recent work [1] involving a novel experiment employing high resolution reciprocal space mapping, a synchrotron based X-ray diffraction technique, in situ during AM of an austenitic stainless steel. The study revealed that dislocation structures formed during rapid solidification undergo significant evolution during subsequent solid-state thermal cycling, in particular during addition of the first few (up to 5) layers above the layer of interest. A summary of the findings of this study are presented in this work. A possible pathway (involving experiment and modelling synergy) to better understanding dislocation structure formation during AM is presented.

Mechanism of Grain Densification in Nano- and Poly-Crystalline Cu Films and Its Impact in Advanced Metallization Processes

We investigate the microstructural evolution of electrochemically deposited poly-crystalline Cu films during subsequent thermal process cycles at mild maximum temperatures, compatible with the integration in advanced metallization schemes for electronic device manufacturing. The modifications induced by the thermal budget have been characterized at different scales (from the film-substrate interface to the wafer scale) with different complementary techniques: X-ray Diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and dynamical warpage measurements. Moreover, the film internal grains’ evolution has been modelled by a tri-dimensional on-cell model, derived by the Pott-like multi-states configurational energy dependence, able to consider multiple orientation of the grains and densification kinetics in the canonical ensemble. Finally, a macroscopic model of the warpage dependence on the process conditions is discussed. The presented joint theoretical and experimental analysis provides a complete and consistent scenario of the grain densification phenomenon and its impact for the Cu film microstructure and the composite system morphology, indicating several strategies for the integration of the process in real device structures.

Initial study on thermal stability of cold spray tantalum coating irradiated with deuterium for fusion applications

Removal of neutral hydrogen atoms in the plasma edge reduces the number of charge exchange events and thus, the net energy losses in the plasma, significantly improving performance of fusion devices. Effective control of the residual pressure of hydrogen isotopes (HIs) in the plasma edge may be achieved by utilizing a hydrogen absorbing first wall interface capable of withstanding the harsh fusion environment. In this study, we have investigated tantalum (Ta) coating deposited by cold spray technology on 316L stainless steel substrate as a potential plasma-facing material surface. High fluence low energy deuterium plasma irradiation experiments and subsequent thermal annealing cycles associated with thermal desorption spectrometry (TDS) demonstrated superior structural stability of the Ta coating. TDS experiments revealed the outgassing of deuterium (as measure of its retention) for cold spray Ta coatings to be three times higher than bulk Ta and two orders of magnitude greater than bulk polycrystalline W. X-ray photoelectron spectroscopy revealed evolution of oxidation states upon deuterium irradiation and a partial recovery of the metallic signature of Ta after the thermal treatment at 1100 K.

Using defects as a ‘fossil record’ to help interpret complex processes during additive manufacturing: as applied to raster-scanned electron beam powder bed additively manufactured Ti–6Al–4V

AbstractDefects in parts produced by additive manufacturing, instead of simply being perceived as deleterious, can act as important sources of information associated with the complex physical processes that occur during materials deposition and subsequent thermal cycles. Indeed, they act as materials-state ‘fossil’ records of the dynamic AM process. The approach of using defects as epoch-like records of prior history has been developed while studying additively manufactured Ti–6Al–4V and has given new insights into processes that may otherwise remain either obscured or unquantified. Analogous to ‘epochs,’ the evolution of these defects often is characterized by physics that span across a temporal length scale. To demonstrate this approach, a broad range of analyses including optical and electron microscopy, X-ray computed tomography, energy-dispersive spectroscopy, and electron backscatter diffraction have been used to characterize a raster-scanned electron beam Ti–6Al–4V sample. These analysis techniques provide key characteristics of defects such as their morphology, location within the part, complex compositional fields interacting with the defects, and structures on the free surfaces of defects. Observed defects have been classified as banding, spherical porosity, and lack of fusion. Banding is directly related to preferential evaporation of Al, which has an influence on mechanical properties. Lack-of-fusion defects can be used to understand columnar grain growth, fluid flow of melt pools, humping, and spattering events. Graphical abstract

Enhanced two way shape memory effect in nanocrystalline NiTi shape memory alloy wires

Two-way shape memory effect (TWSME) has wide application prospect in the field of lightweight actuators. In the present study, an excellent TWSME with large magnitude and small cyclic degradation of NiTi wires is achieved by combining martensite deformation and nanocrystallization. The wires attain a maximum of 7.37% TWSME strain after pre-deformed to 11% and maintain a stable strain of 6.88% after 20 cycles. Microstructural characterization reveals that the excellent TWSME originates from both the dislocation structures induced by martensite deformation and nanocrystalline strengthened matrix. On one hand, the former provides adequate oriented stress field. On the other hand, the latter guarantees the availability of high-volume fraction of preferentially oriented martensite variations in subsequent thermal cycles through suppressing the formation of deformation twins, which are frequently discussed in coarse-grain NiTi.

Effect of post-annealing on thermal shock and steam corrosion of LaMgAl11O19/Yb2Si2O7 thermal/environmental barrier coatings

LaMgAl11O19/Yb2Si2O7 (LMA/YbDS) thermal/environmental barrier coatings (T/EBCs) on SiCf/SiC composites were annealed at 1200 °C for 5 h in air or Ar atmosphere. The effect of post-annealing in different atmospheres on the microstructure, thermal shock and steam corrosion of the LMA/YbDS T/EBCs was investigated. Results indicated heat treatment in air and argon eliminated the amorphous in the coatings, avoiding the aging stress associated with the crystallization. And the argon-annealed layers (LMA and YbDS) exhibited the less elastic moduli than the air-annealed ones, resulting in the lower thermal mismatch stress in the argon-annealed T/EBCs upon subsequent thermal cycling. Thus, the argon-annealed T/EBCs exhibited an improved thermal cycling lifetime than the as-sprayed and the air-annealed ones. In addition, the unbroadened vertical cracks in the argon-annealed LMA-TBC layer limited the reactant (water-vapor) access to the silica-TGO, leading to the greater resistance of the argon-annealed T/EBCs against steam corrosion than the other two systems.

The effect of multiple thermal cycles on Ti-6Al-4V deposits fabricated by wire-arc directed energy deposition: Microstructure evolution, mechanical properties, and corrosion resistance

Thermal cycles have an important effect on the microstructure and properties of the components fabricated by wire-arc directed energy deposition (wire-arc DED). In this study, a Gleeble thermal-mechanical simulator was adopted to create closer-to-reality thermal cycles with the assistance of a numerical simulation model and experimental Ti-6Al-4V deposition. Step-by-step microstructure evolution, including αm, retained β, and GB α, microhardness gradual variation, and the corrosion resistance change before and after the entire thermal cycle were investigated. Therefore, combining phase orientation and high-magnification morphology, transformed and untransformed α that occurred in low- and medium-temperature thermal cycles can be distinguished. After the entire thermal cycle, αm laths coarsened from ∼1 µm to ∼1.2 µm, and the content of retained β phase became more and more. The αm formed around grain boundaries partially disappeared and was occupied by α laths from the inner grain. GB α was more continuously distributed along prior β grain boundaries due to its lower formation temperature during the subsequent thermal cycles that were occurring incomplete α→β transformation. The severe preferential orientation of α phases formed after the deposition and high-temperature thermal cycle was also alleviated through the twice low-temperature thermal cycles. Besides, the microhardness decreased from 318.78 ± 7.5 HV to 285.17 ± 5.3 HV after the high-temperature thermal cycle but eventually increased significantly to 330.5 ± 6.4 HV after experiencing the final low-temperature thermal cycle. The corrosion resistance decreased after the entire thermal cycle, indicating a performance difference between the top and bottom regions of the Ti-6Al-4 V component fabricated by wire-arc DED.

Effect of Thermal Cycling on Grain Evolution and Micro-Segregation in Selective Laser Melting of FGH96 Superalloy

Extremely rapid heating and cooling rates during the additive manufacturing (AM) process generate complicated thermal cycles, which affect the microstructure evolution and ultimate mechanical properties of the alloy. In this paper, FGH96 blocks with a height of 6 mm were prepared by selective laser melting (SLM) and the microstructure was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Comparing specimens of varying heights, it was found that subsequent thermal cycles (STC) coarsened some solidified grains and accelerated the grain growth along the build direction, together with an increase in texture intensity and high-angle grain boundaries (HAGBs). After coarsening the grains in the middle portion of the built block, finer grains were observed near the top area due to a faster cooling rate. There were numerous dislocations in the grain because of the occurrence of unequal internal tension. In the middle of the sample with stable thermal cycles, the dislocations were both perpendicular to the grain growth direction and 45° off it. In spite of the texture characteristics, the segregation of elements was also found to be influenced by thermal cycling. Inherent reheating leads to the increase in the Laves phase and the decrease in the γ’ phase as subsequent deposition. This was also one of the reasons why the microhardness of the sample decreased as the building height and the other reason being the decrease in the solution treatment of the later sediments.

Microstructure and mechanical properties of GTA-based wire arc additive manufactured AZ91D magnesium alloy

Wire arc additive manufacturing offers advantages in producing large metal structures. The current research on GTA-based wire arc additive manufacturing (GTA-WAAM) of magnesium alloys is focused on deformed magnesium alloys, mainly on the Mg-Al alloy system. However, there is little research on GTA-WAAM for casting magnesium alloy. This study investigates the microstructural characteristics and mechanical properties of AZ91D magnesium alloy (AZ91D-Mg) deposited by GTA-WAAM. Single-pass multilayer thin-walled components were successfully fabricated. The results show that equiaxed grains dominate the microstructure of the deposited samples. During the remelting process, the precipitated phases dissolve into the matrix, and they precipitate and grow from the matrix under the thermal effect of the subsequent thermal cycle. The mechanical properties in the vertical and horizontal directions are similar, showing higher overall mechanical properties than the casting parts. The average yield strength is 110.5 MPa, the ultimate tensile strength is 243.6 MPa, and the elongation is 11.7%. The overall hardness distribution in the deposited sample is relatively uniform, and the average microhardness is 59.6 HV0.2.

An investigation into wire arc additive manufacturing of Mg-Y-RE-Zr alloy

• Mg-Y-rE-Zr alloy without obvious defects is fabricated by WAAM . • Microstructural homogeneity occurs along the deposited direction. • The precipitates of Mg 3 (Nd, Y) and Mg 14 Nd 2 Y are found in the1 st layer. • The bottom part of the deposited wall exhibits a higher hardness. This work fabricates Mg-4Y-3RE-0.5Zr alloy by wire arc additive manufacturing (WAAM). The oxidation inclusion, as one of the main defects in the Y-containing Mg alloys prepared by laser powder bed fusion (LPBF), has not been observed. However, microstructural inhomogeneity along the building height appears due to the combined effect of the different cooling rates of different deposited layers and the subsequent multiple thermal cycles (MTCs). The effect of the microstructural inhomogeneity on the hardness distribution of the deposited wall was also revealed. It is believed that this study provides some new findings to the additive manufacturing of high-performance magnesium rare-earth (Mg-RE) alloys.

Residual elastic strain evolution due to thermal cycling of a ceramic-metal composite (WC-Cu) via high energy X-ray diffraction and analytical modeling

Residual stress, when superimposed with in-service loading, can significantly reduce the lifetime and performance of a component. Ceramic-metal composites are susceptible to residual stresses due to the thermal expansion mismatch of the ceramic and metallic phases. The WC-Cu composite explored in the present study provides a promising combination of thermal conductivity and strength properties, while exhibiting counterintuitive improvements in strength and ductility after thermal cycling. This work quantifies the evolution of the residual elastic strains as a result of processing and cyclic thermal loading in a co-continuous WC-Cu composite through experimental high energy X-ray diffraction and kinetics-based modeling. Both analyses indicate that processing-induced residual tensile stress in the copper phase is relieved upon subsequent thermal cycling, with kinetics modeling revealing the cyclic-dependent nature of the active power-law creep mechanisms. The results indicate that, through stress relaxation, this material system maintains structural stability during thermal cycling. The illustrated kinetics of relaxation can inform general material processors and designers of ceramic-metal composites to minimize detrimental residual stress and improve performance of these material systems.

In-situ grain structure control in directed energy deposition of Ti6Al4V

In metal additive manufacturing (AM), long columnar grains along the building direction due to highly directional thermal gradients often lead to a strong texture and severe anisotropy in mechanical properties of the deposits, which significantly impair part qualification and targeted applications. Here, a novel depositing strategy by periodically alternating processing parameters is designed to partially preserve the equiaxed grains resulted from the columnar-to-equiaxed transition at the top of each deposited layer and, at the same time, to interrupt the epitaxial growth of columnar grains in AM titanium alloy. With the help of the competitive growth of the new grains and the potential coarsening effect during subsequent thermal-cycles, a microstructure of full equiaxed prior-β grains in Ti6Al4V fabricated by laser directed energy deposition (DED) is finally obtained without either using any auxiliary equipment or adjusting alloy chemistry like previous researches. This further contributes to a superior mechanical property and a remarkable reduction on both the crystallographic textures and the property anisotropy.

Grain refinement by dynamic recrystallization during laser direct energy deposition of 316L stainless steel under thermal cycles

During direct energy deposition (DED) process, the deposited material undergoes complex thermo-mechanical processing from subsequent thermal cycles. Dynamic recrystallization (DRX) can be activated under suitable strain and temperature conditions, which can induce grain refinement and homogenization. In this work, the evolution of temperature and thermal strain are experimentally measured, and the corresponding evolution of grain morphology, geometric dislocation density (GND), dislocation morphology and misorientation map under multiple thermal cycles during laser-DED of 316L stainless steel are investigated. Results show that DRX is activated during the 8th to 85th thermal cycles, during which temperature evolves in the range of 730 K – 800 K – 700 K and the corresponding strain evolves in the range of 4.5% – 7.0% – 5.3%. Nucleated grains grow by consuming grain boundary energy, with original columnar grains swallowed. Equiaxed grains are formed at the end of 90 thermal cycles. Dynamic recovery (DRV) would occur during the thermal cycling process, with the formation of sub-grains, the increased GNDs and the random grain misorientations through dislocation movement and entanglement promoted by high temperature and accumulated strain.

Ar-implanted vanadium dioxide thin film with the reduced phase transition temperature

VO2 layers with a low metal–insulator phase transition (MIT) temperature TMIT below 40 °C were prepared using magnetron sputtering, rapid thermal annealing, and Ar2+ ion implantation. The implantation-induced strains and point defects effectively lower the metal–insulator transition temperature, while the subsequent thermal cycling (cooling-heating cycles) improves the temperature coefficient of resistance (TCR) up to the values >10%/°C within the temperature range 30–40 °C. These approaches can be used to extend the range of applications of VO2 thin films, especially for uncooled microbolometers. A methodology for studying the influence of defects and strains on electrical parameters of MIT in VO2 films has been proposed.

Investigation into thermo-mechanical reliability of copper trace lines in stacked dies ball grid array packaging

The persistent trend toward device miniaturization in the semi-conductor field has led to an ever finer line width and spacing of the copper trace patterns in the organic substrate used to connect the chip and printed circuit board. Thus, copper trace fracture has emerged as a major concern during the manufacturing process or subsequent thermal cycle reliability testing stage. Accordingly, the present study develops a three-dimensional (3D) computational model to analyze the reliability of the copper trace lines in a stacked ball grid array (BGA) containing two dies under thermal cycling between −65 °C and 150 °C. The simulation model takes account of both the tensile mechanical strength of the copper trace and its creep behavior at elevated temperatures. The validity of the simulation model is confirmed by comparing the predicted distribution of the plastic work density (PWD) in the copper trace/substrate layer following thermal cycling with the experimental observation. In general, the results show that the maximum average PWD of the copper trace should not exceed 400 kJ/m3 in order to minimize the risk of trace fracture. Taguchi analysis experiments are performed to determine the optimal design of the stacked BGA package which minimizes the PWD and hence maximizes the fatigue life of the copper trace. The results indicate that the thermo-mechanical reliability of the stacked BGA package depends mainly on the choice of substrate core material and core trace thickness.

Effect of dimensionless heat input during laser solid forming of high-strength steel

Laser solid forming (LSF) technology can be used to rapidly manufacture and repair high-strength steel parts with superior performance, but the value of the heat input during operation is difficult to quantify, which has a substantial impact on the microstructure and mechanical properties of the parts. A promising method to improve the forming efficiency and quality of LSFed parts is to accurately control the heat input and explore its relationship with the microstructure and mechanical properties. To remove the interference of other variables from the experiment, the dimensionless heat input Q* was introduced. The Q* values were designed in advance to calculate the experimental parameters used to perform the LSF experiment. The microstructure was observed at different regions of the sample, and its mechanical properties were analyzed. From the results, the following conclusions were drawn. The Q* value was directly related to the cooling rate and heat accumulation in the top structure, leading to the formation of different microstructures; it also modified the original structure at the bottom, affecting the subsequent thermal cycle and indirectly changing the tempered martensite morphology. The heat input also affected the mechanical properties of the sample. The hardness of the stable zone decreased with increasing Q* value, and the lowest value was 190 HV. Similarly, the tensile strength and yield strength of the LSFed samples decreased considerably with increasing Q* value, and the lowest values were 735 and 604 MPa, respectively. Only the elongation and reduction in the area increased after a slight decrease. The Q* value had a significant effect on heat treatment. When Q* = 2.9, the increase in tensile strength and yield strength after heat treatment was the largest (29% and 44%, respectively).

Residual stress and distortion in thick-plate weld joint of AF1410 steel: finite element simulations and experimental studies

The quantification of the residual stress and distortion in the thick-plate weld joint of AF1410 steel has great significance, which is the basis for the safety and process design of the component. Therefore, this paper systematically investigated the welding residual stress and distortion distribution in the 10 mm thick welded plate for AF1410 steel by tungsten inert gas welding. The residual stress measurements were carried out by the X-ray diffraction method and advanced contour method, and the distortion was determined by coordinate measuring machine. The developed thermo-metallurgical-mechanical finite element model was applied to predict the residual stress and distortion in multi-pass welding considering solid-state phase transformation (SSPT) and transformation plasticity. The measured residual stress and distortion distribution were in good agreement with the finite element simulation results. The comparison results revealed that the residual stresses in the upper and lower parts of the welding plate were small, and there was apparent stress concentration in the center of the welding joint. SSPT and the subsequent thermal cycle had a significant influence on the magnitude of final residual stress. The accurate finite element model and the understanding of stress formation mechanism are helpful to control the residual stress and distortion distribution of aviation parts reasonably.

Mechanical Properties of the Epoxy Resin Composites Modified by Nanofiller under Different Aging Conditions

The effects of aging conditions on the mechanical properties of epoxy resin (EP) with halloysite nanotube (HNT) were studied. The aging conditions include soaking in water at various temperatures, soaking in acid solution, soaking in alkali solution, thermal shock cycling (TSC), and soaking coupled with subsequent thermal shock cycling (i.e., composite aging conditions). Under aging conditions, the EP is degraded, plasticized, and swelled, resulting in different degrees of cracks and pores in EP. The tensile and bending properties of EP and HNT/EP nanocomposites decreased after aging, indicating that the durability of the EP decreased under aging conditions. The addition of HNT could improve the immersion aging resistance and delay the immersion aging behavior of EP. Under TSC conditions, the reduction in mechanical properties of HNT/EP nanocomposites with HNT is slightly more than that of the neat EP due to different thermal expansion coefficients between HNT and EP. The fracture morphology and chemical change were studied to reveal the aging degradation mechanisms in the presence of HNT addition.