Recast Formation Research Articles

Theoretical investigation of multipulse femtosecond laser processing on silicon carbide: ablation, shielding effect, and recast formation

In this study, we propose a transient multi-physics coupling model for ultrafast laser ablation based on the material point method (MPM). By employing the continuum assumption for material, heat transfer equation and particle phase change, as well as spatial discretization of the model, we achieve simulations of various coupled physical phenomena such as material temperature, phase change, stress, and recast layer formation. In terms of time, we simulate laser processing processes with up to 1000 pulses. The model is validated by comparing with experimental results on ablation morphology, recast layer formation from melted particles, and surface oxidation. The proposed model accurately captures the multi-physics aspects of ultrafast laser ablation processes in SiC ceramics. The experimental validation confirms the model’s reliability and offers valuable insights into the underlying physical phenomena.

Study of the surface integrity and mechanical properties of turbine blade fir trees manufactured in Inconel 939 using laser powder bed fusion

In this study, Inconel 939 turbine blade fir tree samples were fabricated by the additive manufacturing method using a direct metal laser sintering (DMLS) machine. This critical gas turbine connection part requires high geometric accuracy and excellent mechanical properties. The fir tree profiles of turbine blades were machined using creep feed grinding (CFG) and wire electrical discharge machining (EDM) methods. The effects of CFG and wire EDM on the recast formation, heat affected zone (HAZ), geometrical accuracy, microhardness, and tensile strength were compared. The experimental results revealed that the CFG process was superior in recast formation, HAZ, and microhardness, whereas better geometrical accuracy and ultimate tensile strength values were measured for the wire EDM method. The ultimate tensile strengths of turbine blades shaped using wire EDM were higher than the CFG method and measured 769 MPa and 748 MPa, respectively. Profile deviation and recast formation were the most influential parameters on the ultimate tensile strength of the fir tree geometry. The specimens were fractured from the pressure faces of the fir tree root during the tensile tests. Microscopic voids, cracks, and chloride impurities that would facilitate the propagation of fatigue cracks were observed on the fractured surface.

SURFACE ABLATION OF 52100 BEARING STEEL USING FEMTOSECOND LASER IRRADIATION

Laser ablation is a rapid-material removal technique with potential application in remanufacturing of bearings to clean damaged surfaces. This study reports the ablation morphology and change in surface composition of 52100 bearing steel irradiated with a femtosecond laser. A smooth ablated surface was achieved by operating the laser at fluence just above the ablation threshold, but higher fluences resulted in roughened surfaces with recast formation. Such coarse morphology is believed to result from higher energy deposition and surface melting. Minimal oxidation occurred at lower fluences. Periodic ripple patterns formed in the ablated area at low fluence; their morphology and possible formation mechanism is discussed.

Laser Micromachining of Titanium Alloy in Water with Different Temperatures

Underwater laser machining process has been employed as an alternative process to ablate materials with minimum thermal damage. Though many studies provide comprehensive investigations to enable the understanding of laser-water-material interactions during the laser ablation process in water, the effect of water temperature on the ablation performance has not been revealed yet. To cope with this challenge, this paper presents the roles of water temperature on cut dimensions in the underwater laser micromachining of titanium alloy (Ti-6Al-4V). The effects of laser power, traverse speed and number of laser passes were also examined in this study, where groove width and depth were measured and analyzed. The experimental results showed that a deep cut can be produced by using slow traverse speed with multiple-pass technique. However, using too high laser power can cause a shallow cut due to the large formation of recast in the laser-ablated area. According to the findings of this study, the laser energy density of about 750 J/mm2can provide the deepest cut among the other conditions examined in this study.

Experiment and analytical model of laser milling process in soluble oil

The liquid-assisted laser machining process is a promising method to cut materials with minimum thermal damage caused by laser, and water is typically used in the process due to its high thermal conductivity, nontoxicity, and relatively low price. However, water can intrinsically oxidize ferrous metals and in turn deteriorate the workpiece through corrosion during laser ablation in water. This study has for the first time proposed laser ablation in soluble oil to effectively cut the ferrous metals by using laser in a high cooling rate and low potentiality of corrosion to the metals. A nanosecond pulse laser was used to scan over the AISI H13 steel sheet to create a square cavity, while the workpiece surface was covered by a thin and flowing soluble oil film throughout the laser milling process. The effects of laser scan overlap, traverse speed, and liquid flow rate on cavity dimensions and milled surface morphology were experimentally examined. The results revealed that a clean and uniform cavity with a smooth machined surface can be attained by using 70% scan overlap, 6 mm/s traverse speed, and 3.9 cm3/s soluble oil flow rate. Furthermore, analytical models based on heat transfer equations were formulated to predict the cavity profile and cooling of molten droplets in flowing liquid. The predicted profile was found to correspond well to the experiment, and the calculated temperature of cut particles can endorse the experimental findings on debris deposition and recast formation. The implications of this study could bring a new technological approach for damage-free fabrication and fine-scale manufacturing.

Review on laser drilling I. Fundamentals, modeling, and simulation

High peak power lasers have been used for years for ablating matter. The most relevant application of this process is laser marking. Marking meets the demands of applications although the quality of ablation has potential to be further improved. However, the qualitative results of the ablation process especially for highly efficient removal of matter in the liquid phase like drilling have not met the standards of alternative processes—the latter is only the case in niches. On the other hand, the ablation by ultrafast lasers in the pulse regime of ps or below, which might meet the quality demands in terms of geometric precision, was too slow for economically feasible application because of the lack of average power. In fact, both process domains have been developed substantially and thus lead to a technological level which make them ready for industrial innovations. In a series of three articles on laser drilling—from fundamentals to application technology—the results of more than a decade of research and development are summarized with the purpose of displaying the bright application future of this laser process. This present part I deals with fundamentals, modeling, and simulation of laser drilling. Part II covers processing techniques, whereas part III is dedicated to systems and application technology. Fundamentals, modeling, and simulation: Theoretical analysis of the process from fs- to μs-pulses involves three inputs: numerical simulation, relevant analytic modeling, and as an important input for understanding, process analysis. The reduction of the models guided by experimental input leads to descriptions and knowledge of the process, which allows for strategic improvement of the applicability. As a consequence, process strategies can be derived, meeting the challenges of the application related to shape and accuracy of the surface free of recast as well as the economical demand for high speed processing. The domains of “cold ablation,” “hot ablation,” and “melt expulsion” are differentiated. Especially, the formation of recast up to closure of the drill is quantified. Tailoring the process parameters toward the individual application according to the know-how reached by the state of the art modeling and simulation leads to sound innovations and shorter innovation cycles.

Underwater femtosecond laser micromachining of thin nitinol tubes for medical coronary stent manufacture

Microprofiling of medical coronary stents has been dominated by the use of Nd:YAG lasers with pulse lengths in the range of a few milliseconds, and material removal is based on the melt ejection with a high-pressure gas. As a result, recast and heat-affected zones are produced, and various post-processing procedures are required to remove these defects. This paper reports a new approach of machining stents in submerged conditions using a 100-fs pulsed laser. A comparison is given of dry and underwater femtosecond laser micromachining techniques of nickel–titanium alloy (nitinol) typically used as the material for coronary stents. The characteristics of laser interactions with the material have been studied. A femtosecond Ti:sapphire laser system (wavelength of 800 nm, pulse duration of 100 fs, repetition rate of 1 kHz) was used to perform the cutting process. It is observed that machining under a thin water film resulted in no presence of heat-affected zone, debris, spatter or recast with fine-cut surface quality. At the optimum parameters, the results obtained with dry cutting showed nearly the same cut surface quality as with cutting under water. However, debris and recast formation still appeared on the dry cut, which is based on material vaporization. Physical processes involved during the cutting process in a thin water film, i.e. bubble formation and shock waves, are discussed.

Hydrodynamics of material removal by melt expulsion: Perspectives of laser cutting and drilling

With the introduction of fiber-guided radiation at 1 μ wavelength emitting in the milti-kW range at better beam quality than CO2-lasers the most established application in laser processing, namely laser fusion cutting, came back into the industrial and scientific focus. Laser sources with extraordinary optical and economical properties — disk and fiber lasers — in a stormy way enter the market of cutting machines so far reserved for the 10 μ radiation source and led to a volatile situation. The new laser sources can already address a market-relevant class of applications, namely, fusion cutting of steel up to a sheet thickness of 2 mm with pronounced advantages in productivity. However, there is a significant lack of cut quality for larger sheet thickness. The main reason for the drawback and its physical background are given. With the availability of cutting machines with 1 μ fiber-guided radiation the race for the worldwide market regarding the larger sheet thickness is opened and the priority issues to improve the cut quality are related to the three levels: wavelength, beam delivery and the application stage of the machine. The stability model called QuCut is presented which for the first time allows to analyze stability of cutting with fiber-guided radiation. Experimental ripple patterns and ripple spectra resolved with respect to the cutting depth are well reproduced by the new stability model.A number of different experimental methods towards an improved understanding of the dynamics in laser drilling are developed, however, there are gaps related to in-situ observation which is obscured by the hole walls. There are four novel experimental methods resolving the dynamics from a μms-down to a ns-time scale having a spatial resolution with respect to transient drilling depth on the μm scale. As result, the different mechanisms contributing to recast formation and dynamical features of drilling are revealed in more detail. In particular, the action of double pulses and its changes depending on the evolving drill are investigated.

Statistical analysis of recast formation in laser drilled acute blind holes in CMSX-4 nickel superalloy

The present study investigates the effects of four controllable variables on the metallurgy and drilling velocity of laser percussion drilled acute (30° to surface) blind holes in 2 mm thick flat pieces of nickel superalloy—CMSX-4 material. Design of experiment and statistical modelling were used in the experimental study to understand parameter interactions. Pulse energy, pulse width, pulse frequency and gas pressure were chosen as the independent process variables. The response surface method was used to develop the models for each of the responses: physical characteristics of wall and bottom recast layer and drilling velocity (function of material removal rate). A central composite design was chosen as it offered the most economical number of experiments for the required information. The significant process factors in each model were identified based on the analysis of variance and the models were checked by complete residual analysis. It was found that high pulse energy and short pulse width gave the maximum drilling velocity and lowest recast layer thickness at the hole bottom, whereas low pulse energy and short pulse width minimised the wall recast layer thickness.

An optimal process of femtosecond laser cutting of NiTi shape memory alloy for fabrication of miniature devices

The mechanical properties of NiTi shape memory alloy (SMA) components are sensitive to thermal influence during laser machining. To make the femtosecond laser cutting of NiTi material meet the strict fabrication requirements for miniature SMA devices with high precision, complex patterns and minimal heat affected zone (HAZ) along with high throughput, we report an optimal process of sideways-movement path planning in this article. Femtosecond laser processing of NiTi SMA using the fundamental wavelength of 775 nm from a Ti:sapphire laser along with its second and third harmonic irradiations were systematically investigated. We observed that the main impact of ultrashort laser pulse induced air breakdown on materials processing was beam widening. The laser beam at fundamental wavelength suffered less widening than its harmonic wavelengths. Femtosecond laser machining of metals is still basically a thermal mechanism. High ablation rates at higher laser fluences causes significant recast formation, while lower fluences resulted in better cutting quality at the expense of efficiency. The optimal process involving the method of sideways-movement path planning enables recast-free high-precision features at higher laser fluences with better throughput.

A Model for Laser Hole Drilling in Metals

A direct computer simulation technique is developed to analyze quantitatively the influence of the fluid flow and heat transfer in the transient development of a laser drilled hole in a turbine airfoil material, where the material removal is effected by vaporization and melt ejection. The coupled conduction heat transfer in the solid and the advection-diffusion heat transfer in the liquid metal, the fluid dynamics of melt expulsion and the tracking of solid–liquid and liquid–vapor interfaces have been mathematically modeled for the 2D axisymmetric case. The donor–acceptor cell method using the volume of fluid approach is used to solve the complex problem and a versatile numerical code has been developed. It takes into account all thermophysical properties including latent heat of vaporization, gravity, and surface tension driving forces. The novelty of this model is to treat the melted pool surface as a deformable free surface. The impressed pressure and temperature on the melt surface is provided by an 1D gas dynamics model whose vaporization kinetics are also discussed. The model is used to simulate drilling for a number of spatially and temporally varying laser intensity profiles. It is found that resolidification of melt (recast formation) occurred throughout the pulse interval and had significant effect on the developing hole geometry, while the effect of vaporization material removal on the hole geometry is found to be small. Comparison of the simulated results indicates the material removed per joule of energy absorbed appears to be inversely proportional to the square root of the peak beam intensity and the drilling rate appears to be proportional to the square root of the surface pressure.