P91 Steel Research Articles

Compatibility of structural materials with AlSi12 alloys-based phase change material and increasing the corrosion resistance by ceramic coatings

Aluminum Silicon eutectic (AlSi12) alloy-based latent heat thermal energy storage can be integrated with concentrated solar power (CSP) to generate dispatchable power at an affordable cost. Despite AlSi12's favorable thermophysical properties, it reacts with steel at higher temperatures, posing a challenge for storage containers (structural materials). The use of ceramic materials as a protective coating for steel is possible due to their corrosion-resistant properties. Thus, this study examines the compatibility of steel structural materials (SS316, SS202, and P91) and ceramic-coated steel (Alumina – Al2O3 and Yttria Stabilized Zirconia – YSZ) with molten AlSi12. The experiment indicates that P91 steel and YSZ-coated steel have better corrosion resistance when measured over 120 h.

P92 steel and inconel 617 alloy welds joint produced using ERNiCr-3 filler with GTAW process: Solidification mechanism, microstructure, mechanical properties and residual stresses

The objective of the current study was to analyse the microstructure, mechanical characteristics, and residual stresses of a dissimilar welded joint (DWJ) made of P92 steel and the Inconel alloy 617 (IN617) using the gas tungsten arc welding (GTAW) method. The ERNiCr-3 filler was selected to produce the conventional V groove (VG) and narrow V groove (NVG) butt joint. The filler deficient zones in the weldments, such as the filler deficient beach, i.e. unmixed zone (UZ), peninsula, and island, as well as the distinct heat-affected zone (HAZ), were visible near the interface of ERNiCr-3 filler weld and P92 steel due to the distinct differences in the chemical composition, microstructure, and mechanical properties between the filler and P92 base metal (BM). A very narrow partial melted zone (PMZ) and almost negligible UZ and HAZ were noticed at the interface of IN617 and ERNiCr-3 weld metal and it occurred mainly due to the similarity in microstructure and melting point. The austenitic microstructure of ERNiCr-3 filler weld was accompanied by precipitates enriched with Ti and Nb along with the inter-dendritic space. At room temperature, the mechanical properties of both the groove joints were evaluated, and the test results indicated that the welded joint satisfied the standard requirements for AUSC power plants' boiler applications. The tensile test results showed the failure from ERNiCr-3 filler weld with a tensile strength of 627 ± 2 MPa and 636 ± 3 MPa for VG and NVG welded joints, respectively. A poor weld metal impact toughness in comparison to the BMs was attributed to the presence of the brittle Ti(C, N) and Nb(C) particles in the interdendritic space. The impact toughness for the NVG weld joint was measured higher than for the VG weld joint. A significant hardness deviation was measured along the weldments that might be due to heterogeneous microstructure, i.e. UZ, HAZ, delta ferrite, and weld metal. To impart the ductility and temper the martensite in P92 HAZ, post-weld heat treatment (PWHT) was also performed, and a studied their effect on microstructure evolution across the weldments and mechanical properties. Groove design also showed a significant effect on residual stress variation. The work highlights the groove geometry, welding procedure, evolution of the microstructure along the weldments, mechanical characteristics, and residual stress variation of DWJ of P92 steel and IN617 alloy. In comparison to conventional VG joints, the NVG joints exhibited superior mechanical properties and lower residual stress values.

Effect of post weld heat treatments on microstructure and mechanical properties of dissimilar P92 steel-AISI 304H ASS A-TIG weld joint

In present work, the efficacy of post weld heat treatment (PWHT) on microstructure and mechanical properties of dissimilar P92 steel-AISI 304H austenitic stainless steel (ASS) activated flux-tungsten inert gas (A-TIG) weld joint was studied. Based on the time temperature cycle, the post weld heat treatment was performed in two ways: (a) PWHT-1 (760 °C, 2 h) and (b) PWHT-2 (1160 °C, 2 h and 760 °C, 2 h). After PWHTs, the microstructure and mechanical properties (micro-hardness, tensile behavior and impact toughness) of weld joint were tested and compared with as-welded condition. PWHT-1 resulted in coarsening of existing precipitates and formation of new precipitates in weld zone. The average size of precipitates along the PAGB and inside the grain matrix was measured to be 294 ± 50 nm and 153 ± 50 nm, respectively. After PWHT-2, the microstructure of P92 FGHAZ, P92 CGHAZ and weld zone were observed to be quite similar due to the re- austenitisation during this PWHT regime. The hardness distribution shows the non-uniformity even after PWHT-1 however; PWHT-2 shows negligible variation in hardness of P92 side HAZs and weld zone. In tensile testing, the failure location has been changed to P92 steel base metal from AISI 304H ASS which suggests that P92 steel becomes weak after the post weld heat treatments. An increase in elongation % (PWHT-1: 37% and PWHT-2: 45%) was also obtained after PWHTs. Upon PWHTs the weld zone impact toughness also improved (PWHT-1: 59 ± 2 J and PWHT-2: 132 ± 2 J) and satisfy the minimum average impact toughness as per standard EN 1559:1999.

Cr–Mn-diffusion coatings on VM12: In situ spinel formation with high solar absorptance for concentrated solar plants

Receiver systems in concentrated solar power plants have to withstand harsh desert conditions and high temperatures, while maintaining high efficiencies converting solar radiation to thermal energy within a heat transfer fluid. The need to improve high absorbing receiver coatings with respect to their lifetime is essential to enhance the economic efficiency of such plants. Diffusion coatings based on Cr and Mn are shown to provide intrinsic self-healing properties forming black Cr–Mn-oxides in situ. Oxidation tests in air are conducted at temperatures between 527 and 710 °C up to 100 h. Thereby, solar absorptances up to 93% and optical coatings efficiencies up to 80% are obtained on coated VM12 steel with the best coating performance shown at around 650 °C. Thermocyclic oxidation for up to 640 h at a maximum temperature of 650 °C proves that there is a slight drop in solar absorptance after around 50 h of exposure, but afterwards it remains stable. Additionally, it is found that coated VM12 performs better in comparison to P91 steel. A mechanism of oxide formation and coating consumption is proposed showing the importance of Mn diffusion on oxide formation. In the beginning of the exposure, Mn2O3 and cubic MnxCr3-xO4 (β-spinel) form, which transform to tetragonal MnxCr3-xO4 (α-spinel) and with continuing exposure also to Cr2O3. For analysis, metallographic investigations are combined with methods such as Raman-spectroscopy, XRD, spectrophotometry, qualitative and quantitative EPMA measurements.

A creep life prediction model of P91 steel coupled with back-propagation artificial neural network (BP-ANN) and θ projection method

In order to predict the long-term creep life of P91 steel, this work proposed a coupled model, combining the error-trained back-propagation artificial neural network (BP-ANN) and an improved θ model. Both the short-term rupture life obtained by creep experiments and the related creep data of National Institute for Materials Science (NIMS) database were used to validate the above model. In this model, creep parameters (temperature, stress) were used as the input and relative error of 5 θ model extrapolations as the output. Then, the prediction errors of BP-ANN method were introduced to correct the 5 θ model extrapolation results to achieve a more accurate prediction. Consequently, the long-term creep life of P91 steel was predicted. It demonstrated that with the proposed model, the prediction capability for long-term creep life of P91 steel with 200,000 h is much better than those of other θ methods, which has a prediction uncertainty less than 5%.

Selection of Electrode Material for Inconel 617/P92 Steel SMAW Dissimilar Welds

Abstract The present work aimed to investigate the microstructure and mechanical properties of the dissimilar shielded metal arc welded (SMAW) joint of Alloy 617 and P92 steel for advanced ultrasupercritical boilers. The SMAW joint was produced using three different types of electrodes, namely, ENiCrFe-3, ENiCrMo-3, and ENiCrCoMo-1. During microstructural observation of the welded joints, no possible cracking was detected in the weldments. The dissimilar metal welds (DMWs) obtained using the ENiCrCoMo-1 electrode showed the maximum tensile strength, while the minimum was obtained for the ENiCrFe-3 electrode. Among all the electrodes, the ENiCrFe-3 electrode exhibited the maximum impact toughness, while the minimum was obtained for ENiCrCoMo-1 electrodes. Based on microstructure and mechanical characterization, the ENiCrFe-3 electrode was selected as the best one among all to produce the DMW of Alloy 617 and P92 steel.

Multiaxial non-proportional fatigue behaviour and microstructural deformation mechanisms of 9%Cr steel at elevated temperature

High-temperature components utilised in practical service are often subjected to multiaxial non-proportional loading due to their complex geometry. In the present study, a set of strain-controlled fatigue tests were conducted on P92 steel at 600 °C, including uniaxial, multiaxial proportional, and multiaxial non-proportional loading conditions. Results reveal that the axial and shear stresses exhibit varied responses to the different non-proportionalities, with proportional loading resulting in the shortest fatigue life when the strain ratio is small. The non-proportional loading induces accelerated cyclic softening at axial direction while the shear stress is enhanced. Electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) analysis indicate that the increased role of shear stress promotes the dislocation motion from the planar slip to cross-slip under non-proportional loading, which in turn facilitates the microstructure evolution. It is also important to note that the evident development of microstructure and the combined effect of more crack initiations and more secondary cracks contribute to the lowest fatigue life under proportional loading.

Study on Microstructure and Properties of P92 Steel Joint with Narrow Gap Automatic Welding

The microstructure and mechanical properties of automatic welding joint of P92 steel are studied by means of optical microscopy and mechanical property testing. The results show that: the welded joint of P92 steel with narrow gap automatic welding has good fusion, and no defects such as unfusion, porosity and slag inclusion are found in the weld metal. The microstructure of weld and HAZ is tempered martensite. The hardness distribution in different areas of the welded joint is uniform, the impact toughness of the weld and HAZ is good, and exceed that of manual welding, the strength of the joint meets the standard requirements, and the residual stress distribution of the whole joint is low. Therefore, P92 steel welded joint obtained by narrow gap automatic welding process has good performance and obvious advantages.

Influence of Different Weld Heat Input on Microstructure and Properties of New Martensitic Heat-resistant Steel Joints by Automatic Welding

In this article, the influence of different automatic weld heat input about the microstructure and properties of the joint of new martensitic heat resistant steel of P92 is studied. Based on this, the key factors to obtain excellent quality of P92 joint are proposed. The results show that the weld is tempered martensite microstructure, the grain of Heat-affected zone (HAZ) is finer with the weld heat input lower. The width of the slats and the crystalline size of the coarse-grained zone (CGHAZ) in the weld metal microstructure is gradually larger with the heat input higher. Meanwhile, as the welding heat input increase, the hardness of weld bead in the welded joint of P92 steel has a gradual downward trend, the impact toughness has a significant decrease, and the impact fracture morphology changes from ductile fracture to brittle fracture. Therefore, when the automatic welding technology is used to repair P92, so as to obtain better performance of welded joints, appropriate gas protection flow and welding parameters should be ensured according to welding thickness, and heat input should be strictly controlled.

An insight into the creep-fatigue damage localization in welded joints based on crystal plasticity finite element method

In the present work, a numerical simulation based on the crystal plasticity finite element (CPFE) method is conducted to capture the different mechanical responses and damage in different micro areas of the P92 steel welded joint at 650 °C. A robust Matlab-Abaqus-Python interactive parameter optimization procedure is proposed to identify the constitutive parameters for heat affected zone (HAZ). Good agreement between experimental and simulated stress–strain curves verifies the accuracy of the present approach. Three fatigue indicator parameters are incorporated into the CPFE to capture the damage distribution under low cycle fatigue (LCF) and creep-fatigue interaction (CFI) loadings. The damage accumulation at grain boundaries is observed in the FE simulation. The damage captured by the simulation under both LCF and CFI loadings shifted from HAZ to base zone.

Electron beam welding of P91 steel and incoloy 800HT and their microstructural studies for advanced ultra super critical (AUSC) power plants

This study characterizes an electron beam welded dissimilar joint between P91 ferritic/martensitic steel and Incoloy 800HT Nickel-based alloy. Such dissimilar metal welds (DMW) have applications in nuclear and thermal powerplants where steam-carrying pipes lead to header sections operating at around 600–650 °C. The study employed an advanced welding process to obtain a clean, high-quality dissimilar metal. The microstructure characterization of the DMW involving Nickel-based alloys is prone to solidification cracking in the weld fusion zone (WFZ), which affects the overall weld properties and leads to failure. In the present study, the WFZ microstructure revealed a mixed solidification mode with a homogenous presence of equiaxed dendritic and cellular-type structures. Advanced analyses of WFZ microstructure were done to account for microsegregation. Nb, Ti, Cr, and Mo microsegregation was found in the WFZ. The Ti-rich grain boundaries in the WFZ provided the pinning action during plastic deformation. The evolution of Cr, Mo-based carbide phases affected the microhardness distribution. Microsegregation under rapid solidification of the WFZ resulted in the formation of secondary phases in the interdendritic region. The effect of the evolution of precipitates deteriorated the mechanical strength. High and uneven dislocation distribution was observed in the WFZ near the IN 800HT weld interface. Under tensile loading, the DMW failed from the IN 800HT weld interface due to coarsening of precipitated in the IN 800HT HAZ region near the weld interface and grain boundary thickening due to the deposition of Ti over them. The tensile strength of WFZ was 592 MPa. The tensile strength of WFZ was 592 MPa. The post-weld heat treatment (PWHT) homogenized the microhardness across the weld. Impact toughness after PWHT slightly increased from 63 J to 69 J. The dissimilar joint obtained using the EBW process had sufficient mechanical strength to be considered for Generation IV power plants under the American Society for Testing and Materials (ASTM) standards. The study concludes that the dissimilar metal welded joint was highly prone to failure despite sufficient strength, as IN 800HT weld interface was susceptible to grain boundary liquation.

Parametric Optimization of Cutting Force and Temperature in Finite Element Milling of AISI P20 Steel

Multiple manufacturing methods are used in the manufacturing industry. The most commonly used method is machining methods. With machining methods, production can be made from both raw materials and finishing processes can be applied to products produced with different production methods. However, the initial cost of machining operations is quite high due to factors such as the machining parameters used during the process, the rigidity of the machine, and the machining conditions. Today, the finite element method (FEM) has been widely used in order to reduce the initial cost of machining. For this reason, in our study, the machining of AISIP20 material was carried out with the FEM. This study, four different cutting speeds, feed rates and two different cutting depths were used and the lateral feed rate was kept constant. As a result of the study, the cutting force values and temperature values that occurred depending on the machining parameters were evaluated by finite element analysis. Consequently, in the study, an increase in the cutting force occurred in general with the increase of feed, cutting depth and cutting speed parameters, and a decrease in temperature values occurred with an increase in cutting speed and feed and constant cutting depth. The lowest cutting force was 36.11 N, while the highest was 1951.42 N. The lowest and highest temperature values that occur during the process are 448.98 and 593.14 °C, respectively. In this regard, for the optimization of the parameters, the proportional change between the parameters and the finite elements as well as the physical processes can be performed for the desired final product.

High-Temperature Corrosion Behavior of Selected HVOF-Sprayed Super-Alloy Based Coatings in Aggressive Environment at 800 °C.

This study is focused on the high-temperature corrosion evaluation of selected thermally sprayed coatings. NiCoCrAlYHfSi, NiCoCrAlY, NiCoCrAlTaReY, and CoCrAlYTaCSi coatings were sprayed on the base material 1.4923. This material is used as a cost-efficient construction material for components of power equipment. All evaluated coatings were sprayed using HP/HVOF (High-Pressure/High-Velocity Oxygen Fuel) technology. High-temperature corrosion testing was performed in a molten salt environment typical for coal-fired boilers. All coatings were exposed to the environment of 75% Na2SO4 and 25% NaCl at the temperature of 800 °C under cyclic conditions. Each cycle consisted of 1 h heating in a silicon carbide tube furnace followed by 20 min of cooling. The weight change measurement was performed after each cycle to establish the corrosion kinetics. Optical microscopy (OM), scanning electron microscopy (SEM), and elemental analysis (EDS) were used to analyze the corrosion mechanism. The CoCrAlYTaCSi coating showed the best corrosion resistance of all the evaluated coatings, followed by NiCoCrAlTaReY and NiCoCrAlY. All the evaluated coatings performed better in this environment than the reference P91 and H800 steels.

Validation of applicability of induction bending process to P91 piping of prototype Gen-IV sodium-cooled fast reactor (PGSFR)

The application of the induction bending process to pipe systems in various industrial fields is increasing. Recently, efforts have also been made to apply this bending process to nuclear power plants because it can innovatively reduce welded parts of the curved pipes, such as elbows. However, there have been no cases of the application of induction bending to the piping of nuclear power plants.In this study, the applicability of the P91 induction bending piping for the sodium-cooled fast reactor PGSFR was validated through high temperature low cycle fatigue tests and creep tests using P91 induction bending pipe specimens. The tests confirmed that the materials sufficiently satisfied the fatigue life and the creep rupture life requirements for P91 steel at 550 °C in the ASME B&PV Code, Sec. III, Div. 5. The results show that the effects of heating and bending by the induction bending process on the material properties were not significant and the induction bending process could be applicable to piping system of PGSFR well.

Effect of B and N Content and Austenitization Temperature on the Tensile and Impact Properties of Modified 9Cr-1Mo Steels

The present study investigates the relative effect of B and N concentrations and the austenitization temperature on the microstructure and mechanical properties (tensile and Charpy impact) of modified 9Cr-1Mo (P91) steels. Initially, a B-free P91 steel (with 500 ppm N) and four different B-containing steels (25–100 ppm) with varying N concentrations (20–108 ppm) were hot-rolled, normalized from different austenitization temperatures (1000–1100 °C/1 h) and finally tempered at 760 °C for 1 h. A Charpy impact test shows that the ductile–brittle transition temperature (DBTT) of all the B-added steels decreases with an increase in the austenitization temperature, where the 100 ppm B steel offers the lowest DBTT (−85 °C). Similarly, the strength increases with the increase in the austenitization temperature (1100 °C), with a slight drop in ductility. The influence of precipitates on the microstructure and mechanical properties is explained considering the B enrichment at the precipitates and the thermodynamic stability of the precipitates. The 100 ppm B steel (containing the maximum B and minimum N), normalized from 1100 °C austenitization, shows the best combination of tensile and Charpy impact properties, owing to the effective dissolution of coarse M23C6 and MX precipitates during the normalization treatment and the formation of fine B-rich (Fe,Cr)23(B,C)6 precipitates during the subsequent tempering.

Investigation on post-weld heat treatment (PWHT) inputs for controlling average fusion zone hardness in fibre laser-welded modified 9Cr–1Mo steel

Post-weld heat treatment (PWHT) generally relieves stress and enhances weldments’ mechanical properties. Optimisation of PWHT parameters is very much needed to control the process parameters for achieving the desired property requirements. In this research, the PWHT parameters for laser-welded P91 steel were optimised using the response surface methodology-based Box–Behnken methodology. Tempering temperature, heating rate and holding time are considered the input parameters and using analysis of variance (ANOVA), their impact on average fusion zone hardness has been analysed. The results indicate that the hardness values are highly influenced by temperature followed by holding time. A parametric combination of 780°C tempering temperature, 6 hours holding time, and 200°C/h heating rate have evolved as the optimal solution. The autogenous nature of the welding process resulted in a heterogeneous microstructure with untempered lath martensite structure showing precipitate dissolution in the as-welded fusion zone, ultimately resulting in a high average fusion zone hardness of 502 HV0.3, whereas PWHT with the optimal solution resulted in an average fusion zone hardness value of 252 HV0.3. The decrease in hardness value followed by heat treatment is primarily due to the tempering process, which resulted in precipitation within and over the grain boundaries. The validation experiment result using the optimum combination of parameters has been observed to be aligned significantly with the expected values, indicating the adequacy of the developed model.

The Synergistic Effects of Boron and Impression Creep Testing during Paced Controlling of Temperature for P91 Steels

This article induces the quadruple knowledge on effects of 100 ppm (P91B) and 22 ppm (P91) boron in as‐received 9Cr steels in normalized and tempered conditions (NT) and impression crept conditions (ICC). Herein, impression creep testing is improved through paced temperature to fast evaluate creep performance. P91B performs the utmost by advancing activation energy and 30% better creep life, leading to same targeted creep life to 574 °C than P91 at 550 °C. Further, it incisively discusses variations in microstructure, precipitate size, geometrically necessary dislocations (GNDs), pseudo‐rocking curve, grain size (GS), several types of grain boundaries (GBs), effective GB energy, and elastic stiffness. Novel phenomenological mechanisms like GB hardening/softening; antithetical mechanisms; boron‐dependent mosaicity; grain refinement/fragmentation/coarsening/homogenization; interaction among various types of GBs, triggering re/degeneration of dislocations and Σ boundaries as Σ reactions; super‐continuous and geometric dynamic recrystallizations; enhanced creep immunity of like‐oriented GBs than unlike oriented GBs; primary/secondary recrystallization; specialness; boron/NT/ICC‐dependent stiffness; high stiffness is dependent on low GNDs and big GS, and vice versa are observed. The synergistic effects of [100 ppm boron in NT/ICC] and ICC in manifestation of 22/100 ppm boron are ascertained. However, dependency [proportionally and antithetically] and independency of GB character on creep‐microstructural reciprocity discourse and converse into the mathematical relationship.

Oxidation properties of complex concentrated alloys FeAlCrV and FeAlCrMo

Complex concentrated alloys (CCAs) are attracting considerable interest due to their potential applications under extreme conditions. This study focuses on two complex concentrated alloys, the FeAlCrV and the FeAlCrMo alloys, which already exhibited exceptional mechanical properties at high temperatures. In this regard, room temperature corrosion resistance and high-temperature oxidation were studied to investigate their potential applicability in harsh environments. It is shown that the corrosion resistance of both CCAs is much higher than that of AISI 304 and P91 steels in 0.5 M H2SO4 solution, while in 3.5% NaCl solution was comparable. On the contrary, high-temperature oxidation of CCAs was unsatisfactory, especially exceeding 700 °C. The intensive analysis of the formed oxide scales revealed that the protective oxide layer is not being formed at temperatures above 700 °C, primarily because of the occurrence of vanadium corrosion (FeAlCrV) and evaporation of Mo oxides (FeAlCrMo). The results of this study unambiguously showed the importance of studying oxidation properties at high temperatures parallel with the mechanical properties for the development of CCAs for cutting-edge technical applications.

Structural integrity assessment of Inconel 617/P92 steel dissimilar welds for different groove geometry

The work is focused on examining the effect of the weld groove geometry on microstructure, mechanical behaviour, residual stresses and distortion of Alloy 617/P92 steel dissimilar metal weld (DMW) joints. Manual multi-pass tungsten inert gas welding with ERNiCrCoMo-1 filler was employed to fabricate the DMW for two different groove designs: Narrow V groove (NVG) and Double V groove (NVG). The microstructural examination suggested a heterogeneous microstructure evolution at the interface of the P92 steel and ERNiCrCoMo-1 weld, including the macrosegregation and element diffusion near the interface. The interface structure included the beach parallel to the fusion boundary at the P92 steel side, the peninsula connected to the fusion boundary and the island within the weld metal and partially melted zone along Alloy 617 fusion boundary. An uneven distribution of beach, peninsula and island structures along the fusion boundary of P92 steel was confirmed from optical and SEM images of interfaces. The major diffusion of the Fe from P92 steel to ERNiCrCoMo-1 weld and Cr, Co, Mo, and Ni from ERNiCrCoMo-1 weld to P92 steel were witnessed from SEM/EDS and EMPA map. The Mo-rich M6C and Cr-rich M23C6 phases were detected in inter-dendritic areas of the weld metal using the weld’s SEM/EDS, XRD and EPMA study, which formed due to the rejection of Mo from the core to inter-dendritic locations during solidification. The other phases detected in the ERNiCrCoMo-1 weld were Ni3(Al, Ti), Ti(C, N), Cr7C3 and Mo2C. A variation in the microstructure of weld metal from top to root and also along the transverse direction in terms of composition and dendritic structure and also due to the composition gradient between dendrite core and inter-dendritic areas, a significant variation in hardness of weld metal was observed from both top to root and also in the transverse direction. The peak hardness was measured in CGHAZ of P92 while the minimum was in ICHAZ of P92 steel. Tensile test studies of both NVG and DVG welds joint demonstrated that failure occurred at P92 steel in both, room-temperature and high-temperature tensile tests and ensured the welded joint’s applicability for advanced ultra-supercritical applications. However, the strength of the welded joint for both types of joints was measured as lower than the strength of the base metals. In Charpy impact testing of NVG and DVG welded joints, specimens failed in two parts with a small amount of plastic deformation and impact energy of 99 ± 4 J for the NVG welds joint and 91 ± 3 J for the DVG welded joint. The welded joint met the criteria for boiler applications in terms of impact energy (minimum 42 J as per European Standard EN ISO15614-1:2017 and 80 J as per fast breeder reactor application). In terms of microstructural and mechanical properties, both welded joints are acceptable. However, the DVG welded joint showed minimum distortion and residual stresses compared to the NVG welded joint.

Study on Friction Characteristics of AA7075 Aluminum Alloy under Pulse Current-Assisted Hot Stamping

Friction during contact between metals can be very complex in pulse current-assisted forming. Based on stamping process characteristics, a reciprocating friction tester was designed to study the friction characteristics between AA7075 aluminum alloy and P20 steel under different current densities. Origin software was used to process the experimental data, and a current friction coefficient model was established for the pulse current densities. The results show that the friction coefficient of the aluminum alloy sheet decreased with the increase in the pulse current density (2–10 A/mm2). After that, the friction mechanism was determined by observing microscopic morphology and SEM: some oxide cracked on the friction surface when the current was large. Finally, finite element simulations with Abaqus software and a cylindrical case validated the constant and current friction coefficient models. The thickness distribution patterns of the fixed friction coefficient and the current coefficient model were compared with an actual cylindrical drawing part. The results indicate that the new current friction model had a better fit than the fixed one. The simulation results are consistent with the actual verification results. The maximum thinning was at the corner of the stamping die, which improved the simulation accuracy by 7.31%. This indicates the effectiveness of the pulse current friction model.