Additive manufacturing of Ni-based superalloys: Residual stress, mechanisms of crack formation and strategies for crack inhibition

The additive manufacturing(AM)of Ni-based superalloys has attracted extensive interest from both academia and industry due to its unique capabilities to fabricate complex and high-performance components for use in high-end industrial systems.However,the intense temperature gradient induced by the rapid heating and cooling processes of AM can generate high levels of residual stress and metastable chemical and structural states,inevitably leading to severe metallurgical defects in Ni-based superalloys.Cracks are the greatest threat to these materials’integrity as they can rapidly propagate and thereby cause sudden and non-predictable failure.Consequently,there is a need for a deeper understanding of residual stress and cracking mechanisms in additively manufactured Ni-based superalloys and ways to potentially prevent cracking,as this knowledge will enable the wider application of these unique materials.To this end,this paper comprehensively reviews the residual stress and the various mechanisms of crack formation in Ni-based superalloys during AM.In addition,several common methods for inhibiting crack formation are presented to assist the research community to develop methods for the fabrication of crack-free additively manufactured components.

Evaluation of the seismic moment of the April 20,2013 Lushan earthquake

The April 20, 2013 Lushan earthquake which occurred in Sichuan, China had only moderate thrust. However, the computed seismic moments （M0） for the Lushan earthquake calculated by several institutions differ significantly from 0.4 × 1019 to 1.69 ×019 Nm, up to four times difference. We evaluate ten computed Mos by using normal mode observations from superconducting gravimeters in China's Mainland. We compute synthetic normal modes on the basis of moment tensor solutions and fit them to the observed normal modes. Comparison of our results indicates that Mo is the main cause for some large differences between observations and synthetics. We sug- gest that a moment magnitude of Mw6.6, corresponding to a Mo of 0.97-1.08 × 1019 Nm, characterizes the size and strength of the seismic source of the Lushan earthquake.

One-year status of hepatic venous pressure gradient measurement from 85 hospitals in China

Aims:Surveys and research on the applications of the hepatic venous pressure gradient(HVPG)are important for understanding the current status and future development of this technology in China.This article aimed to investigate the status of hepatic venous pressure gradient measurement in China in 2022.Methods:We investigated the overall status of HVPG technology in China-including hospital distribution,hospital level,annual number of cases,catheters used,average cost,indications,and current challenges by using online questionnaire.By counting the number and percentages of cases of these results,we hope to clarify the current status of HVPG measurements in China.Results:According to the survey,85 hospitals in China used HVPG technology in 2022 distributed across 29 provinces.A total of 4989 HVPG measurements were performed in all of the surveyed hospitals in 2022,of which 2813 cases(56.4%)were measured alone.The average cost of HVPG measurement was 5646.8±2327.9 CNY.Of the clinical teams who performed the measurements(sometimes multiple per hospital),94.3%(82/87)used the balloon method,and the majority of the teams(72.4%,63/87)used embolectomy catheters.Conclusions:This survey clarified the clinical application status of HVPG in China and confirmed that some medical institutions in China have established a foundation for this technology.It is still necessary to continue promoting and popularizing this technology in the future.

A comparing study of defect generation in IN738LC superalloy fabricated by laser powder bed fusion:Continuous-wave mode versus pulsed-wave mode

Inconel 738 LC samples were fabricated using laser powder bed fusion under continuous-wave and pulsed-wave modes.Microstructure,surface quality and mechanical properties were compared to evaluate the printing quality between these 2 laser beam modes.The results show that the application of pulsed wave could effectively eliminate cracking in the as-fabricated sample,despite 0.046%porosity generated.Further microstructure analysis revealed that the refinement of grains by the pulsed-wave laser beam was the main contributor in eliminating the cracks.And this refinement was ascribed to the higher cooling rate under the discontinuous radiation of laser beam proofed by the numerical simulation.And the pore formation was related to Rayleigh instability and residual bubbles in the sample under the pulsed-wave mode,while pores were less detrimental to the mechanical properties than cracks.Therefore,the part under the pulsed-wave mode exhibited superior mechanical performance compared to that under the continuous-wave mode.

Stress corrosion cracking of L360NS pipeline steel in sulfur environment

Slow strain rate test(SSRT)and tensile fracture observation are applied to study the stress corrosion cracking(SCC)of L360NS pipeline steel.5.0 wt.%sodium chloride solution containing elemental sulfur and 0.5 wt.%glacial acetic acid solution containing elemental sulfur are used.The results show that brittle fracture mainly occurs on the samples in high sulfur environment.In the sulfur suspension solution,the SCC sensitivity of L360NS pipeline steel increases along elemental sulfur content.In sulfur melting cladding condition,obvious corrosion can be observed with a large amount of corrosion pits appearing on the gauge section of the sample.The corrosion products are Fe1txS,black easily detached,and H2.In sulfur suspension and sulfur melting cladding conditions,stress oriented hydrogen induced cracking(SOHIC)is the main cracking,which is caused by the joint effect of SCC and hydrogen induced cracking(HIC).