A recent increase in the use of ecofriendly, natural fibers as reinforcement for the fabrication of lightweight, low cost polymer composites can be seen globally. One such material of interest currently being extensively used is basalt fiber, which is cost-effective and offers exceptional properties over glass fibers. The prominent advantages of these composites include high specific mechano-physico-chemical properties, biodegradability, and non-abrasive qualities to name a few. This article presents a short review on basalt fibers used as a reinforcement material for composites and discusses them as an alternative to the use of glass fibers. The paper also discusses the basics of basalt chemistry and its classification. Apart from this, an attempt to showcase the increasing trend in research publications and activity in the area of basalt fibers is also covered. Further sections discuss the improvement in mechanical, thermal and chemical resistant properties achieved for applications in specific industries.

A short review on basalt fiber reinforced polymer composites

We employ the high-speed synchrotron hard X-ray imaging and diffraction techniques to monitor the laser powder bed fusion (LPBF) process of Ti-6Al-4V in situ and in real time. We demonstrate that many scientifically and technologically significant phenomena in LPBF, including melt pool dynamics, powder ejection, rapid solidification, and phase transformation, can be probed with unprecedented spatial and temporal resolutions. In particular, the keyhole pore formation is experimentally revealed with high spatial and temporal resolutions. The solidification rate is quantitatively measured, and the slowly decrease in solidification rate during the relatively steady state could be a manifestation of the recalescence phenomenon. The high-speed diffraction enables a reasonable estimation of the cooling rate and phase transformation rate, and the diffusionless transformation from β to α
 ’
 phase is evident. The data present here will facilitate the understanding of dynamics and kinetics in metal LPBF process, and the experiment platform established will undoubtedly become a new paradigm for future research and development of metal additive manufacturing.

Real-time monitoring of laser powder bed fusion process using high-speed X-ray imaging and diffraction

Laser powder bed fusion additive manufacturing is an emerging 3D printing technique for the fabrication of advanced metal components. Widespread adoption of it and similar additive technologies is hampered by poor understanding of laser-metal interactions under such extreme thermal regimes. Here, we elucidate the mechanism of pore formation and liquid-solid interface dynamics during typical laser powder bed fusion conditions using in situ X-ray imaging and multi-physics simulations. Pores are revealed to form during changes in laser scan velocity due to the rapid formation then collapse of deep keyhole depressions in the surface which traps inert shielding gas in the solidifying metal. We develop a universal mitigation strategy which eliminates this pore formation process and improves the geometric quality of melt tracks. Our results provide insight into the physics of laser-metal interaction and demonstrate the potential for science-based approaches to improve confidence in components produced by laser powder bed fusion. Laser-matter interactions during laser powder bed fusion additive manufacturing remain poorly understood. Here, the authors combine in situ X-ray imaging and finite element simulations to show how detrimental pores form under printing conditions and develop a strategy to suppress them.

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Dynamics of pore formation during laser powder bed fusion additive manufacturing.

Investigation into spatter behavior during selective laser melting of AISI 316L stainless steel powder

Soot formation in laminar counterflow flames

In laser and electron-beam welding, a deep cavity called a keyhole or beam hole is formed in the weld pool due to the intense recoil pressure of evaporation. The formation of the keyhole leads to a deep penetration weld with a high aspect ratio and this is the most advantageous feature of welding by high-energy-density beams. However, a hole drilled in a liquid is primarily unstable by its nature and the instability of the keyhole also causes the formation of porosity or cavities in the weld metal. In particular, the porosity formation is one of the serious problems in very high-power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The article will describe the real-time observation of keyhole and plume behaviors in the pulsed and continuous-wave laser welding by high-speed optical and x-ray transmission methods, the cavity formation process and its suppression measures.

Dynamics of keyhole and molten pool in laser welding

This paper describes the Laser Assisted Machining (LAM) that cuts and removes softened parts by locally heating the ceramic with laser. Silicon nitride ceramics can be machined with general machining tools as well, because YSiAlON, which was made up ceramics, is soften at about 1,000°C. In particular, the laser, which concentrates on highly dense energy, can locally heat materials and very effectively control the temperature of the heated part of specimen. Therefore, this paper intends to propose an efficient machining method of ceramic by deducing the machining governing factors of laser assisted machining and understanding its mechanism. While laser power is the machining factor that controls the temperature, the CBN cutting tool could cut the material more easily as the material gets deteriorated from the temperature increase by increasing the laser power, but excessive oxidation can negatively affect the quality of the material surface after machining. As the feed rate and cutting depth increase, the cutting force increases and tool lifespan decreases, but surface oxidation also decreases. In this experiment, the material can be cut to 3mm of cutting depth. And based on the results of the experiment, the laser assisted machining mechanism is clarified.

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Characteristics of laser assisted machining for silicon nitride ceramic according to machining parameters

It is generally acknowledged in any steels or alloys that porosity is liable to be formed in a keyhole type of deeply penetrated fusion zones made with a high power laser. Therefore, this investigation was carried out with the objectives of elucidating the formation mechanism of porosity and developing preventive procedure of pores in high power laser welding.The formation behavior of keyhole, bubbles and porosity was observed during butt-joint or bead-on-plate CO2 laser welding of stainless steel by microfocused X-ray transmission insitu imaging system with high speed video camera. The influence of various laser welding conditions on porosity formation was investigated. As a result, it was observed that many bubbles were predominantly frequently formed at the bottom of a molten pool from the tip part of a deep keyhole in consequence of its dynamic motion due to intense evaporation, and some bubbles were formed from the middle part of the keyhole in the case of focal point under the plate surface or high power laser irradiation. It was also seen that most of the bubbles were soon captured or trapped into pores by the solidifying solid-liquid interface during floating up in a stainless steel. SEM observation result of a fractured surface demonstrated traces that liquid was penetrated and solidified inside pores, probably because evaporated materials trapped in the bubble solidified and/or the temperature dropped to render their inside pressure lower. According to Q mass-spectroscopic analysis of gas content of porosity, it was revealed that He shielding gas and H2 gas were included inside a pore near the bottom of the weld fusion zone. It was observed to be feasible to produce a sound full-penetration weld bead without porosity in steel plates of 10 mm thickness, since no bubbles were formed from the bottom part of the fully penetrated keyhole. Bubbles, porosity or pores were confirmed to be reduced by utilizing N2 shielding gas and forward keyhole-inclination welding in He shielding gas even in the case of partially penetrated weld.It is generally acknowledged in any steels or alloys that porosity is liable to be formed in a keyhole type of deeply penetrated fusion zones made with a high power laser. Therefore, this investigation was carried out with the objectives of elucidating the formation mechanism of porosity and developing preventive procedure of pores in high power laser welding.The formation behavior of keyhole, bubbles and porosity was observed during butt-joint or bead-on-plate CO2 laser welding of stainless steel by microfocused X-ray transmission insitu imaging system with high speed video camera. The influence of various laser welding conditions on porosity formation was investigated. As a result, it was observed that many bubbles were predominantly frequently formed at the bottom of a molten pool from the tip part of a deep keyhole in consequence of its dynamic motion due to intense evaporation, and some bubbles were formed from the middle part of the keyhole in the case of focal point under the plate surface or high ...

Formation mechanism and reduction method of porosity in laser welding of stainless steel

Industrial ceramics is one of the new materials that have advantages like high strength characteristics, wear-resistance, chemical stability and corrosion-resistance. In general, the production of high strength ceramic requires finishing processes because of the sintering processes, but the machining of ceramic is difficult due to its high strength and brittleness. If such disadvantages are complemented, the engineering ceramics will probably be applied in more diverse fields. Therefore, as a way to overcome such weaknesses, laser assisted machining that locally heats, softens and cuts materials was studied intensely by using laser beams as a heat source. In this paper, material changes by difference of a laser power and the structural changes of the material surface when it was heated by laser beam were observed. When the temperature increased, the Si and N which are comprised the silicon nitride decomposed and turned to gas, eventually erupting outside or forming porosity inside to induce micro-cracks. In addition, as YSiAlON, which surrounded β-Si3N4, softened at 1,000°C or above, cutting was enable due to plastic deformation. In particular, the surface of the specimen oxidized at a high temperature formed an amorphous silicate, and subsequently experienced deterioration of hardness compared to silicon nitride.

https://link.springer.com/content/pdf/10.1007%2Fs12541-014-0466-y.pdf

Microstructural Variations and Machining Characteristics of Silicon Nitride Ceramics from Increasing the Temperature in Laser Assisted Machining

Experimental and numerical analyses of laminar diffusion flames were performed to identify the effect of fuel mixing on soot formation in a counterflow burner. In this experiment, the volume fraction, number density, and particle size of soot were investigated using light extinction/scattering systems. The experimental results showed that the synergistic effect of an ethylene-propane flame is appreciable. Numerical simulations showed that the benzene (C6H6) concentration in mixture flames was higher than in ethylene-base flames because of the increase in the concentration of propargyl radicals. Methyl radicals were found to play an important role in the formation of propargyl, and the recombination of propargyl with benzene was found to lead to an increase in the number density for cases exhibiting synergistic effects. These results imply that methyl radicals play an important role in soot formation, particularly with regard to the number density.

Jong-Do Kim

Papers

Dynamics of keyhole and molten pool in laser welding

Characteristics of laser assisted machining for silicon nitride ceramic according to machining parameters

Formation mechanism and reduction method of porosity in laser welding of stainless steel

Microstructural Variations and Machining Characteristics of Silicon Nitride Ceramics from Increasing the Temperature in Laser Assisted Machining

Experimental and numerical investigation of fuel mixing effects on soot structures in counterflow diffusion flames