Dimensional instability of LF21 aluminum alloy weldments at room temperature and after thermal cycles

The unstable dimensional distortion of LF21 aluminum alloy weldments at room temperature and after thermal cycles was studied by use of light interference and CMM. At the same time, distortion mechanism was analyzed from the viewpoint of mechanics and microstructure. Experimental results show that there exists obvious difference of unstable dimensional distortion between LF21 welded specimens under two conditions mentioned above. Under room temperature, dimensional variation of welded specimens will decrease gradually and finally tends to be stable during 130 h after welding. The relative elongation of welded specimen is 4.2×10 -5 . After thermal cycles, distortion of welded specimen is much larger than that at room temperature. After 11 thermal cycles, the dimension will tend to be stable. Dimensional unstable distortion of weldments mainly results from temperature condition, microstructure variation and relaxation of welding residual stress.

DIMENSIONAL INSTABILITY OF LD31 Al ALLOY WELDMENTS AT ROOM TEMPERATURE AND AFTER THERMAL CYCLES

The unstable dimensional distortion of LD31 aluminum alloy weldments at room temperature and after thermal cycles was studied by use of light interference and CMM(three-coordinate measuring machines). At the same time, distortion mechanism was analyzed from the viewpoint of mechanics and micro structure. Experimental results show that there exists obvious difference of unstable dimensional distortion between LD31 welded specimens under two conditions mentioned above. Under room temperature, dimensional variation of welded specimens will decrease gradually and finally tends to be stable during 200h after welding. The relative elongation of welded specimen is 3.0×10-5; After thermal cycles, distortion of welded specimen is much larger than that at room temperature. After 11 thermal cycles, the dimension will tend to be stable. Dimensional unstable distortion of weldments mainly results from temperature condition, microstructure variation and relaxation of welding residual stress.

Tailoring microstructure evolution and mechanical properties of a high-performance alloy steel through controlled thermal cycles of a direct laser depositing process

Direct laser deposition(DLD),as a popular metal additive manufacturing process,shows advantages of technical flexibility and high efficiency to gain a high-performance alloy steel component.However,during the processing of DLD,the deposited steel layer is affected by the subsequent layer depositing.The DLD block shows different microstructure and mechanical properties at the bottom,middle and top of the deposited parts.To date,there are few research works about the effects of inter-layer interval time and laser power on the microstructure evolution and mechanical properties of the deposited layers.In this study,the idle time and laser power layer by layer during DLD of 12CrNi2 steel were controlled to cause the deposited layers to maintain a high cooling rate,while the bottom deposited layer was subjected to a weak tempering effect.Results show that a high proportion of martensite is produced,which improves the strength of the deposited layer.Under the laser scanning strategy of laser power 2,500 W,scanning velocity 5 mm·s^(-1),powder feeding rate 11 g·min^(-1),overlap rate 50%,and a laser power difference of 50 W and a 2 min interval,the tensile strength of the deposited layer of 12CrNi2 steel is in the range of 873-1,022 MPa,and the elongation is in the range of 16.2%-18.9%.This study provides a method to reduce the tempering effect of the subsequent deposition layers on the bottom layers,which can increase the proportion of martensite in the low-alloy high-strength steel,so as to improve the yield strength of the alloy steel.

Assessment of gas tungsten arc welding thermal cycles on Inconel 718 alloy

Heat moving source models along with transient heat analysis by finite element method were used to determine weld thermal cycles and isothermal sections obtained from the application of a gas tungsten arc welding beads on Inconel 718 plates. Analytical (Rosenthal’s thick plate model) and finite element results show an acceptable approximation with the experimental weld thermal cycles. The isothermal sections determined by numerical simulation show a better approximation with the experimental welding profile for double-ellipse model heat distribution than Gauss model. To analyze the microstructural transformation produced by different cooling rates in the fusion and heat affected zones, Vickers microhardness measurements (profile and mapping representation) were conducted. A hardness decrement for the heat affected zone (~200 HV0.2) and fusion zone (~240 HV0.2) in comparison with base material (~350 HV0.2) was observed. This behavior has been attributed to the heterogeneous solubilization process of the γ″ phase (nickel matrix), which, according to the continuous-cooling-transformation curve, produced the Laves phase,δ and MC transition phases, generating a loss in hardness close to the fusion zone.

A study of local brittle zone (LBZ) of 10Ni5CrMoV steel after double thermal cycles

The 10Ni5CrMoV steel examined was a 16 mm thick plate. Specimens measuring 12 mm×12 mm×120 mm were thermally cycled using DM-100A weld simulator with various parameters. The main results are summarized as follaws. In the coarse-grained austenitized region（ Tm = 1 300℃ ＋ 1300℃ ） ,the microstructure is in good toughness. At the condition of Tm = 1 300℃ ＋ 850℃ and t8/5=43 s, the toughness decreases heavily because M-A constituent and twin martensite appear at the prior aastenite grain boundaries. When Tm= 1300℃ ＋ 850℃ or 1300℃ ＋ 730℃ and t8/5 = 85 s, local brittle zone is formed because of relatively coarse granular bainite.

Effect of Multiple Thermal Cycles on Microstructure and Mechanical Properties of Cu Modified Ti64 Thin Wall Fabricated by Wire-Arc Directed Energy Deposition

This study investigated the effect of thermal cycles on Cu-modified Ti64 thin-walled components deposited using the wire-arc directed energy deposition(wire-arc DED)process.For the samples before and after experiencing thermal cycles,it was found that both microstructures consisted of priorβ,grain boundaryα(GBα),and basketweave structures containingα+βlamellae.Thermal cycles realized the refinement ofαlaths,the coarsening of priorβgrains andβlaths,while the size and morphology of continuously distributed GBαremained unchanged.The residualβcontent was increased after thermal cycles.Compared with the heat-treated sample with nanoscale Ti2Cu formed,short residence time in high temperature caused by the rapid cooling rate of thermal cycles restricted Ti2Cu formation.No formation of brittle Ti2Cu means that only grain refinement strengthening and solid-solution strengthening matter.The yield strength increased from 809.9 to 910.85 MPa(12.46%increase).Among them,the main contribution from solid solution strengthening(~51 MPa)was due to the elemental redistribution effect betweenαandβphases caused by thermal cycles through quantitative analysis.The ultimate tensile strength increased from 918.5 to 974.22 MPa(6.1%increase),while fracture elongation increased from 6.78 to 10.66%(57.23%increase).Grain refinement ofαlaths,the promotedα′martensite decomposition,decreased aspect ratio,decreased Schmid factor,and local misorientation change ofαlaths are the main factors in improved ductility.Additionally,although the fracture modes of the samples in the top and middle regions are both brittle-ductile mixed fracture mode,the thermal cycles still contributed to an improvement in tensile ductility.

Micro-macro evolution of mechanical behaviors of thermally damaged rock:A state-of-the-art review

The influence of thermal damage on macroscopic and microscopic characteristics of different rocks has received much attention in the field of rock engineering.When the rocks are subjected to thermal treatment,the change of macroscopic characteristics and evolution of micro-structure would be induced,ultimately resulting in different degrees of thermal damage in rocks.To better understand the thermal damage mechanism of different rocks and its effect on the rock performance,this study reviews a large number of test results of rock specimens experiencing heating and cooling treatment in the laboratory.Firstly,the variations of macroscopic behaviors,including physical parameters,mechanical parameters,thermal conductivity and permeability,are examined.The variations of mechanical parameters with thermal treatment variables(i.e.temperature or the number of thermal cycles)are divided into four types.Secondly,several measuring methods for microstructure,such as polarizing microscopy,fluorescent method,scanning electron microscopy(SEM),X-ray computerized tomography(CT),acoustic emission(AE)and ultrasonic technique,are introduced.Furthermore,the effect of thermal damage on the mechanical parameters of rocks in response to different thermal treatments,involving temperature magnitude,cooling method and thermal cycle,are discussed.Finally,the limitations and prospects for the research of rock thermal damage are proposed.

Effect of Thermal Cycles on the Thermal Expansion Behavior of T700 Carbon Fiber Bundles

The relationships between the coefficient of thermal expansion（CTE） of T700 carbon fiber bundles（CFBs） and the thermal cycles were investigated. The microstructure of T700 CFBs was analyzed with Raman spectra and XRD before and after the thermomechanical test. The results indicated that the T700 CFBs exhibited negative expansion in the direction of parallel fibers in the temperature range of-150-150 ℃ The thermal strain that occurred during the heating and the cooling thermal cycle had an unclosed curve that served as the loop. When the experimental load was the same, the position of strain loop tended to move upward, and the length of the specimen increased continuously with the thermal cycles increasing. The microstructural analysis suggested that the degree of structural order and the degree of orientation along the fiber axis were improved with the increase of thermal cycles. The change of microstructure parameters could be the primary cause of the negative CTE＇s variation within the T700 CFBs.

Conductivity and Oxidation Behavior of Fe-16Cr Alloy as Solid Oxide Fuel Cell Interconnect Under Long-Stability and Thermal Cycles

Conductivity and oxidation behavior of Fe-16Cr alloy were investigated under long-term stability operation at 750℃and thermal cycles from room temperature to 750℃.The results showed that the area specific resistance(ASR)of Fe-16Cr alloy increased over time and reached about 56.29 mΩcm~(2)after 40,000 h of long-term stability operation at 750℃by theoretical calculation.The ASR of Fe-16Cr remained about 11 mΩcm~(2)after 52 thermal cycles from room temperature to750℃.The analysis of structure showed that the oxidized phase on the surface of Fe-16Cr was mainly composed of Cr_(2)O_3and Fe Cr_(2)O_(4)spinel phase under long-term stability operation at 750℃.While the Cr_(2)O_(3)phase was mainly observed on the surface of Fe-16Cr alloy after 52 thermal cycles,the oxidation rates of Fe-16Cr alloy were 0.0142μm h~(-1)and 0.06μm cycle~(-1)under long-term stability operation and under thermal cycle,respectively.The property of Fe-16Cr alloy with 2.6 mm thickness met the lifespan requirement of interconnect for solid oxide fuel cell(SOFC)stacks.The Cr element all diff used onto oxidation surface,indicating that it was necessary to spray a coating on the surface to avoid poisoning cell cathode of SOFCs.Two 2-cell stacks were assembled and tested to verify the properties of Fe-16Cr alloy as SOFC interconnect under long-term stability operation and thermal cycle condition.

Effect of thermal cycles on microstructure of reduced activation steel fabricated using laser melting deposition

Reduced activation steel was successfully fabricated by laser melting deposition employing a Gaussian and a ring-shaped laser.The microstructure evolution of the reduced activation steel was investigated using the scanning electron microscope,transmission electron microscope and electron backscatter diffraction.The experimental results showed that the grains close to the substrate were smaller than the grains in the upper layers.Compared to those deposited using a Gaussian laser,the samples deposited using a ring-shaped laser showed a more homogeneous microstructure.Furthermore,a finite element analysis(FEA)model was applied to reveal the thermal history during laser melting deposition.The simulation results were well validated with the experimental results.FEA results indicate that the peak temperature increases and the cooling rate decreases,as the layer gets further from the substrate.Additionally,the temperature and the cooling rate resulting from the Gaussian laser model were higher at the midline of the samples and lower around the edges,whereas those of the ring-shaped laser model were consistent with both at the center and around the edges.

Performance Evaluation of a Molten Carbonate Fuel Cell-Graphene Thermionic Converter-Thermally Regenerative Electrochemical Cycles Hybrid System

A combined system model is proposed including a molten carbonate fuel cell(MCFC),a graphene thermionic converter(GTIC)and thermally regenerative electrochemical cycles(TRECs).The expressions for power output,energy efficiency of the subsystems and the couple system are formulated by considering several irreversible losses.Energy conservation equations between the subsystems are achieved leaned on the first law of thermodynamics.The optimum operating ranges for the combined system are determined compared with the MCFC system.Results reveal that the peak power output density(POD)and the corresponding energy efficiency are 28.22%and 10.76%higher than that of the single MCFC system,respectively.The effects of five designing parameters on the power density and energy efficiency of the MCFC/GTIC/TRECs model are also investigated and discussed.

Effects of supersonic fine particle bombarding on thermal cyclic failure lifetime of thermal barrier coating

Thermal barrier coating ( TBC) consisting of a NiCoCrAlY bond coat ( BC) and a ZrO2-8 wt. % Y2O3 topcoat ( TC) was fabricated on the nickel-base superalloy by air plasma spray ( APS) . The BC was treated by supersonic fine particle bombarding ( SFPB) . Thermal cyclic failure and residual stress in thermally grown oxide ( TGO) scale were studied by SEM with EDS and ruby fluorescence spectroscopy ( RFS) . As shown in the results,after treated by SFPB,thickening of TGO was relatively slow,which reduced the level of growth stress. The TBC with SFPB treatment was still remained well undergoing 350 times of thermal cycle. However,after thermal cycle with the same times,the separation of TC was observed in TBC without SFPB treatment. The residual stress analysis by RFS showed that the residual stress of SFPB-treated TBC increased with the increasing number of thermal cycle. The residual stress of conventional TBC reached a value of 650 MPa at 350 times of cycle and that of SFPB-treated TBC only reached 532 MPa at 400 times of cycle. The BC with SFPB treatment after 400 times of cycle was analyzed by RFS,the high stress value was not observed in local thickened region of TGO. Thermal cycling resistance of TBC can be improved by the SFPB technology.

Microstructure instability of fully lamellar TiAl alloy containing high content of Nb after long-term thermal cycling

Microstructure instabilities of the fully lamellar Ti-45Al-8.5Nb-（W,B,Y） alloy were investigated by SEM and TEM after long-term thermal cycling（500 and 1000 thermal cycles） at 900 °C. Two major categories of microstructure instability were produced in the alloy after the thermal cycling： 1） The discontinuous coarsening implies that grain boundary migrations are inclined to occur in the Al-segregation region after the long-term thermal cycling, especially after 1000 thermal cycles. Al-segregation can be reduced during the process of long-term thermal cycling as a result of element diffusion; 2） The α2 lamellae become thinner and are broken after 1000 thermal cycles caused by the dissolution of α2 lamellae through phase transformation of α2→γ. The γ grains nucleate within the α2 lamellae or（α2＋γ） lamellae in a random direction.

Valve Dynamic and Thermal Cycle Model in Stepless Capacity Regulation for Reciprocating Compressor

The existing researches of stepless capacity regulation system by depressing the suction valve for reciprocation compressor always adopt hypothesis that the compressor valves are open or close instantaneously, the valve dynamic has not been taken account into thermal cycle computation, the influence of capacity regulation system on suction valves dynamic performance and cylinder thermal cycle operation has not been considered. This paper focuses on theoretical and experimental analysis of the valve dynamic and thermal cycle for reciprocating compressor in the situation of stepless capacity regulation. The valve dynamics equation, gas forces for normal and back flow, and the cylinder pressure varying with suction valve unloader moment during compression thermal cycle are discussed. A new valve dynamic model based on L-K real gas state equation for reciprocating compressor is first deduced to reduce the calculation errors induced by the ideal gas state equation. The variations of valve dynamic and cylinder pressure during part of compression stroke are calculated numerically. The calculation results reveal the non-normal thermal cycle and operation condition of compressor in stepless capacity regulation situation. The numerical simulation results of the valve dynamic and thermal cycle parameters are also verified by the stepless capacity regulation experiments in the type of 3L-10/8 reciprocating compressor. The experimental results agree with the numerical simulation results, which show that the theoretical models proposed are effective and high-precision. The proposed theoretical models build the theoretical foundation to design the real stepless capacity regulation system.

Thermal cycle and its influence on the microstructure of laser welded butt joint of 8 mm thick Ti-6Al-4V alloy

Ti-6Al-4V alloy is extensively used in the manufacture of components in aviation.In the current study,the laser welding process is adopted to joint the Ti-6Al-4V alloy plate which has the thick of 8 mm.A three-dimensional finite element model is established to simulate the temperature distribution of laser welding process.The thermal cycle curves are produced on the strength of the simulation results.Meanwhile,the microstructure characteristics of the welded joint are investigated combined with simulation results.The results show that weld zone,heat affected zone and based metal experience similar thermal cycles process and the cooling rate has an important influence on the formation of microstructure.Moreover,the simulation results are well matched with experiment results.

Effect of Thermal Cycle on Microstructure and Fracture Morphology in HAZ of HQ130 Steel

The effect of different peak temperature(Tp) and cooling time (t8/5) on microstructure, hardness, impact toughness and fracture morphology in the heat-affected zone (HAZ) of HQ130 steel was studied by using weld thermo-simulation test. Experimental results indicate that the impact toughness and hardness decrease with the decrease of Tpor increase of t8/5 under the condition of a single thermal cycle. There is a brittle zone in the vicinity of Tp= 800℃, where the impact toughness is considerably low. There is a softened zone in the vicinity of Tp=700℃, where the hardness decreases but the toughness increases. In the practical application of multi-layer and multipass welding, the welding heat input should be strictly limited (t8/5≤20s) so as to reduce the softness and brittleness in the HAZ of-HQ130 steel.

Effect of Thermal Cycling on Properties and Microstructure of SnAgCuCe Soldered Joints in QFP Devices

Increasing global concern about the environment is bringing regulatory （European directives） and consumer （＂green products＂） pressure on the electronics industry in Europe and Japan to reduce or completely eliminate the use of lead （Pb） in products. Among all lead-free solder alloys, SnAgCu solder system, which has better thermo-mechanical properties compared with those of SnPb solder, is proven to be one of the promising candidates for electronic assembly. Previous work also revealed that adding a small amount of rare earth Ce into SnAgCu solder can visibly improve the properties and inhibit the excessive growth of the intermetallic compound layer. Thermal fatigue properties of SnAgCuCe soldered joints in QFP devices under thermal conditions have been investigated by finite element method and experiments. Based on creep model of low stress and high stress, corresponding creep subroutine was established for simulating the stress and strain response of SnAgCuCe soldered joint from -55 ℃ to 125 ℃, and fatigue life was calculated using creep fatigue life prediction equation. Moreover, thermal cycling experiments were conducted, the experimental results were found to be close to the simulated results. In addition, the tensile force of SnAgCuCe soldered joints decreased with increasing number of thermal cycles, and the fracture mechanism transformed from toughness fracture to brittle intergranular fracture. Moreover the tensile force changes and fracture microstructure evolution could benefit the quantitative evaluations of the mechanical performances of lead-free soldered joints under thermal cycling loadings.

Effects of Shot Peening Process on Thermal Cycling Lifetime of TBCs Prepared by EB-PVD

Conventional two-layered thermal barrier coatings （TBCs） are prepared by electron beam physical vapor deposition （EB-PVD） with ZrO2-8 wt% Y2O3 （8YSZ） as top coat and CoCrAlY as bond coat on disk-shaped Ni based super-alloy. In this paper, three kinds of shot peening process with different lengths of operating time were adopted for bond coating. As a result, changes took place in its surface roughness and the surface micro-hardness. A thermal cycling test at 1 273 Kx55 rain and another at room temperature for 5 min were performed to study the effects of shot peening process on the thermal cycling lifetime of TBCs. It is found that a moderate shot peening process will be able to prolong the life time. The oxidation dynamic of the as-processed TBCs basically accords with the parabolic rule, and the oxidation test also attests to the spallation between YSZ and thermal growth oxide （TGO） responsible mainly for the failure of TBCs.

Effects of Composition and Thermal Cycle on Transformation Behaviors,Thermal Stability and Mechanical Properties of CuAlAg Alloy

The phase transformation behavior, mechanical properties, and the thermal stability of CuAlAg alloy were studied and minor rare earth (0.1 wt pct La+Ce) was added to improve the mechanical property of the studied alloy. It was found that Ag addition in the CuAl binary alloy can improve the stability of martensitic transformation and high Al content leads to the disappearing of martensitic transformation. The tensile strength and strain of the Cu-10.6AI-5.8Ag (wt pct) alloy were measured to be 383.5 MPa and 0.86%, respectively. With rare earth addition, the tensile strain increased from 0.86% to 1.47%. The CuAlAg alloy did not exhibit martensitic transformation on the second heating process. Its poor thermal stability still needs to be improved.

The effect of thermal cycle on joint of Ti/stainless steel phase transformation diffusion bonding

The effects of thermal cycle parameters on the tensile strength and fracture characteristics of phase transformation diffusion bonding(PTDB) joint of titanium and stainless steel (Ti/SS) were studied in this paper. With the maximum cyclic temperature of 1 173～1 223 K , the minimum cyclic temperature of 1 073～1 093 K , the heating velocity of 30～50 K/s , the cooling velocity of 15～20 K/s , the cycle numbers of 15～20 and bonding pressure is 13 MPa , the tensile strength of joint is more than 380 MPa , exceeding 80% of that of Ti.