Microcrack healing in non-ferrous metal tubes through eddy current pulse treatment

Scientific Reports, Apr 2018

This study proposed a novel method to heal microcrack within Mg alloy tubes using high density eddy current pulse treatment (ECPT). Through electromagnetic induction inside a copper coil connected with a high density pulse power source supply, the high density (greater than 5 × 109 A/m2) and short duration eddy current was generated in tube specimens of Mg alloy. The results show that the microcracks in tube specimens was healed evidently and the mechanical properties of the tubes subjected to ECPT were improved simultaneously. The crack healing during ECPT was ascribed to not only the thermal stress around the microcrack tips and the softening or melting of metals in the vicinity of microcrack tips, but also the squeezing action acted by the Lorentz force. In the inward-discharging scheme, both the compressive radial stress and tangential stress induced by the Lorentz force contributed to more sufficient crack healing and thus better mechanical properties of tube specimens after the ECPT experiment, compared to the outward-discharging scheme. The ECPT can heal microcracks automatically without directly contacting tubular specimens and is not limited by the length of tubular workpieces, exhibiting great potential for crack healing in non-ferrous alloy tubes.

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Microcrack healing in non-ferrous metal tubes through eddy current pulse treatment

Abstract This study proposed a novel method to heal microcrack within Mg alloy tubes using high density eddy current pulse treatment (ECPT). Through electromagnetic induction inside a copper coil connected with a high density pulse power source supply, the high density (greater than 5 × 109 A/m2) and short duration eddy current was generated in tube specimens of Mg alloy. The results show that the microcracks in tube specimens was healed evidently and the mechanical properties of the tubes subjected to ECPT were improved simultaneously. The crack healing during ECPT was ascribed to not only the thermal stress around the microcrack tips and the softening or melting of metals in the vicinity of microcrack tips, but also the squeezing action acted by the Lorentz force. In the inward-discharging scheme, both the compressive radial stress and tangential stress induced by the Lorentz force contributed to more sufficient crack healing and thus better mechanical properties of tube specimens after the ECPT experiment, compared to the outward-discharging scheme. The ECPT can heal microcracks automatically without directly contacting tubular specimens and is not limited by the length of tubular workpieces, exhibiting great potential for crack healing in non-ferrous alloy tubes. Introduction Degradation, damage and failure are natural consequence of materials1. The degradation usually occurs in organic and inorganic materials, while the damage appears in metallic materials besides organic and inorganic materials. In terms of structural materials, the degradation and damage are prone to microcracks and then cause a failure. As is known, the failure of materials often leads to severe accidents and huge losses in engineering applications. If the cracks can be healed or repaired in intermediate or even micro scale, the reliability and lifetime of structural materials will be enhanced pronouncedly. Hence, the study on crack healing is garnering extensive interest from materials scientists. To date, there have been great advances in the crack healing of biological materials, polymers and ceramics, wherein the crack healing methods include vascular1 or hollow fiber2 method, microcapsule method3, SMA (shape memory alloy) method4, plasma coating method5, etc. But as far as the metals are concerned, it is much more difficult to heal cracks than for other materials owing to high bond strength, small volumes and low diffusion rates of metallic atoms6, so there has been very limited progress in damage healing of metallic materials until now. Olson G.B7. showed that shape memory alloy (SMA) wires could repair the cracks in off-eutectic metal matrices. Lumley R.N8. demonstrated that dynamic precipitation could reduce the cracks and voids in the supersaturated solid solution of Al alloy. However, SMA wires and dynamic precipitation techniques could not be used for pre-existing structures because the SMA wires and precipitation elements must be added into the materials in advance9. Yu H.L. et al.10 reported that the internal cracks in low-carbon steel could be healed by hot plastic deformation. Wei D. et al.11 indicated that crack healing in crack tips could be achieved after heat treatment at high temperature. But hot plastic deformation and heat treatment techniques may significantly change the microstructure and performance of materials. In recent years, the high density current pulse has been employed to heal the cracks in stainless steel12,13, 1045 steel14,15 and titanium alloy16. The healing effect can be ascribed to the melting and thermal compressive stress around crack tips, which is caused by the detouring of pulse current around the cracks. The temperature increase around the crack could also enhance the fracture resistance of specimen, which is known as the warm prestressing effect17,18 proposed as early as 1980s. The high density current pulse treatment can automatically detect the locations of cracks and quickly heal the crack without changing the microstructure of materials obviously19, which shows great application potential in metallic materials. Nevertheless, current researches on crack healing by high density current are mostly concentrated in simple sheet samples, while the crack healing in more complex-shaped components, such as tube specimens, has been seldom reported until now. As one kind of important structural components, thin-walled tubes are widely used in various fields, such as chemistry, power, oil and gas, aviation and aerospace industries. The damage and microcrack can decrease burst pressure, longitudinal and lateral load-capacity of tube components and even cause catastrophic failure in advance, so the study on the healing of cracks inside metal tubes is of great significance in engineering application. Because of their complex geometries, it is not convenient to directly apply electropulsing treatment to the tube components since the electrodes holding tube ends may bring about electric spar (...truncated)


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Xu, Wenchen, Yang, Chuan, Yu, Haiping, Jin, Xueze, Guo, Bin, Shan, Debin. Microcrack healing in non-ferrous metal tubes through eddy current pulse treatment, Scientific Reports, 2018, Issue: 8, DOI: 10.1038/s41598-018-24354-7