Laser Cutting Heat for Continuous Working

Laser Cutting Process

Professor Bekir Sami Yilbas , in Laser Heating Applications, 2012

4.4 Analysis for Heat Transfer to Liquid Metals with the Presence of Assisting Gas

In the laser cutting process an assisting gas is used either to protect the surface from high-temperature exothermic oxidation reactions, such as argon, or to produce an exothermic reaction, such as oxygen. A jet of gas, which produces an exothermic reaction, results in an increase in the rate of cutting; however, the accuracy and the fineness of the cut are partly affected by the addition of the gas stream for certain gas stream velocities. In an attempt to investigate the forces exerted by an inert gas jet on the thin molten layer, the equations of motion of the gas flow were solved previously [9,10]. In the analysis, the gas flow was assumed to be laminar and the chemical reaction contribution was disregarded. Therefore, an extension of the previous models became necessary. Dorrance [7], using a simple model reacting gas mixture flowing over a heated flat plate, was able to demonstrate that boundary layer equations applied up to the point where the reaction zone became attached to the flat plate. In the model proposed, the reaction zone was assumed to be attached to the liquid surface; reactions were considered to take place at the gas–liquid interface with the rest of the gas phase being frozen (the mass rate of change of species i per unit volume is constant). It has been demonstrated that the heat transfer from a boundary layer composed of reacting gas is independent of the location of the reaction zone within the boundary layer to a first order of approximation [9,10]. This approximation includes the assumption that the transport properties are independent of the boundary layer gas mixture, or at least their variation with the composition is of secondary importance compared with their variation with temperature. Since it is assumed that the chemical reactions are taking place at the liquid–gas interface and the rest of the gas phase is frozen, the composition of the gas mixture throughout the boundary layer will be determined by convection and diffusion of the products and reactants through the boundary layer, the gas species present in the external stream, and the gas species at the interface. The analysis related to the heat transfer into the liquid metal is presented below in the light of the previous study [9].

To derive equations for the heat transfer into liquid metal, which is experiencing chemical reactions at the surface (i.e., the gas–liquid interface), an equation can be written for the heat balance at the interface. Considering Figure 4.6, a heat balance at the gas–liquid interface gives

Figure 4.6. A schematic view of the liquid–gas interface.

(4.90) ${\dot{q}}_{\text{LJ}}-{(\rho V)}_{\text{g}}{[h_{\chi }(L)]}_{\text{g}}={\dot{q}}_{\text{g}}-{(\rho V)}_g{h}_{\text{g}}$

where χ denotes the chemical symbol for the material content undergoing a chemical reaction, (ρV) _g is the volume flow of vapor phase escaping from surface, ${\dot{q}}_{\text{g}}$ is heat transfer to the liquid metal surface due to conduction and diffusion $\left(-{\dot{q}}_{\text{g}}={\left(k\frac{\partial T}{\partial y}+\rho D_{12}\sum _i{h}_i\frac{\partial C_i}{\partial y}\right)}_{\text{g}}\right)$ , and ${\dot{q}}_{\text{LJ}}$ is the heat transfer from the gas boundary to the liquid metal in the presence of mass transfer occurring at the surface of the liquid. For material content χ

(4.91) ${[h_{\chi }(L)]}_{\text{g}}+L_v={[h_e(g)]}_{\text{g}}$

where h is the enthalpy and L_v   =   heat of vaporization of material content χ.

Combining Eqns (4.90) and (4.91) gives

(4.92) ${\dot{q}}_{\text{LJ}}={\dot{q}}_{\text{g}}-{(\rho V)}_{\text{g}}{h}_{\text{g}}+{(\rho V)}_{\text{g}}{[h_{\chi }(g)]}_{\text{g}-}{(\rho V)}_{\text{g}}{L}_v$

Introducing Eqn (4.91) into Eqn (4.92) results in

(4.93) ${\dot{q}}_{\text{LJ}}=C_H{\rho }_e{U}_e(I_e-I_{\text{g}})-\sum _i{h}_i^o[{(C_i)}_e-{(C_i)}_{\text{g}}]+\frac{G(\mathrm{\infty },\text{Pr})}{G(\mathrm{\infty },S)}\sum _i{(h_i)}_g[{(C_i)}_e-{(C_i)}_{\text{g}}]-MT1H_{\text{g}}+MT1{[h_{\chi }(\text{g})]}_{\text{g}}-MT1L_v$

where I is the total enthalpy $\left(h+\frac{U^2}{2}\right)$ , ρ_e is the free stream gas density, U_e is the free stream gas velocity, C_H is the heat transfer coefficient, C_i is the species mass fraction, MT1 is the mass transfer parameter, and L_v is the heat of vaporization. The heat transfer to the liquid metal surface $({\dot{q}}_{\text{g}})$ is [9]

(4.94) $-{\dot{q}}_{\text{g}}=C_H{\rho }_e{U}_e(I_e-I_{\text{g}})\left\{1+\frac{G(\mathrm{\infty },\text{Pr})}{G(\mathrm{\infty },S)-1}\times \sum _i{(C_i)}_e{h}_i^o\frac{1-{(Z_i)}_{\text{g}}}{I_e-I_{\text{g}}}+\frac{G(\mathrm{\infty },\text{Pr})}{G(\mathrm{\infty },S)}\times \sum _i{(C_i)}_e{(h_i)}_{\text{g}}\frac{1-{(Z_i)}_{\text{g}}}{I_e-I_{\text{g}}}\right\}$

Z_i is the reduced mass fraction $\left(Z_i=\frac{C_i}{{(C_i)}_e}\right)$ and

(4.95) $I_e-I_{\text{g}}={(I_f)}_e\left[1-{(g_f)}_{\text{g}}\right]+\sum _i{h}_i^o{(C_i)}_e\left[1-{(Z_i)}_{\text{g}}\right]$

and

$G(\eta \text{,}Z)={\int }_0^{\eta }\frac{Z}{C}\exp \left[-{\int }_{\mathrm{0}}^{{\eta }^{\prime }}\frac{Z}{C}fd{\eta }^{\prime }\right]d{\eta }^{\prime }\text{.}$

and MT1 is a mass transfer parameter defined by

(4.96) $MT1=\frac{{(\rho V)}_{\text{g}}}{{\rho }_e{U}_e{C}_H}$

Inserting total enthalpy $(I=I_f+\sum _i{C}_i{h}_i^0)$ in Eqn (4.93) gives

(4.97) ${\dot{q}}_{\text{LJ}}=C_H{\rho }_e{U}_e\left\{{(I_f)}_e-{(I_f)}_{\text{g}}+\frac{G(\mathrm{\infty },\text{Pr})}{G(\mathrm{\infty },S)}\times \left[\sum _i{(h_i)}_g\left[{(C_i)}_e-{(C_i)}_{\text{g}}\right]—MT2h_{\text{g}}+\frac{MT2}{h_{\chi }{(g)}_{\text{g}}}\right]-MT1L_{\text{v}}\right\}$

where MT2 in Eqn (4.97) is defined by $MT2=\frac{G(\mathrm{\infty },S)}{G(\mathrm{\infty },\text{Pr})}MT1$ .

Define another form called the chemical enthalpy potential h_c as

(4.98) $h_c=\sum _i{(h_i)}_g\left[{(C_i)}_e-{(C_i)}_{\text{g}}\right]-MT2\sum _i{(C_i)}_g{(h_i)}_{\text{g}}+MT2{[h_{\chi }(g)]}_{\text{g}}$

Since the enthalpy of a gas mixture is $h=\sum _i{C}_i{h}_i$ , then

(4.99) $h_c=\sum _{i=E}{(h_i)}_{\text{g}}\left[{(C_i)}_e-(1+MT2){(C_i)}_{\text{g}}\right]+h_{\chi }(g)\left[MT2-(1+MT2){(C_{\chi })}_{\text{g}}\right]$

where ${(C_{\chi })}_{\text{g}}=0$ , since all χ species leaving the liquid surface are confined to the boundary layer, according to the assumption made earlier.

Combining Eqns (4.97) and (4.99) gives

(4.100) ${\dot{q}}_{\text{LJ}}=C_H{\rho }_e{U}_e\left[{(I_f)}_e-{(I_f)}_{\text{g}}+\frac{G(\mathrm{\infty },\text{Pr})}{G(\mathrm{\infty },S)}{h}_c-MT1L_{\text{v}}\right]$

In Eqn (4.100), ${\dot{q}}_{\text{LJ}}$ is the heat transfer to the liquid metal in the presence of mass transfer occurring at the surface of the liquid. The term h_c represents the heat released or absorbed due to a chemical reaction among the gas species near the surface.

To determine the heat transfer rate, one has to find the mass fractions of various species at the surface and the edge of the boundary layer (which are used to determine h_c from Eqn (4.99)). It is also necessary to develop a method to calculate (ρV) _g as a function of the surface chemistry.

To calculate (ρV) _g the following equation can be considered [9]:

(4.101) ${(\rho V)}_g=-{\rho }_e{U}_e{\mu }_e\left[2{(2\overline{s})}^{1/2}{f}^{\prime }\frac{\partial \eta }{\partial s}+\frac{f}{{(2\overline{s})}^{1/2}}\right]$

At the surface (liquid)

(4.102) $f^{\prime }(0)=0$

Therefore,

(4.103) ${(\rho V)}_g=-{\rho }_e{U}_e{\mu }_e\frac{f(0)}{{(2\overline{s})}^{1/2}}$

where

(4.104) $f(0)=-{\left(\frac{C_i}{S}\right)}_g$

since

(4.105) $C_i=\frac{{\rho }_i}{\rho }$

where ρ is the density of the total mixture. Once the pressure at the edge of the boundary layer and the interface temperature are known, species mass fractions C_i can be calculated. Consequently, f(0) and hence (ρV) _g can be determined.

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Laser-based machining – an advanced manufacturing technique for precision cutting

Mohamed Sobih , in Advanced Machining and Finishing, 2021

1.3.1.1 Spot size

Laser cutting process is highly affected by the laser beam spot size as it determines the power density (irradiance) in the cutting zone and the width of the produced kerf. As the spot size decreases, the power density increases (at constant power), and consequently, the used cutting speed can be increased resulting in a reduction in the kerf width.

The minimum spot size is located at the focal plane and is called focal spot. Mainly three factors affect the size of the focal spot; these are the quality of the incoming beam, diffraction, and the incoming beam diameter. The quality of the beam is quantified by the beam divergence in directly influencing the focal spot size. Consequently, small divergence angle beam is focused to a small focal spot while high divergence angle beam is focused to focal size with a large diameter, [2]. Eq. (12.1) can be used to calculate the laser spot size.

(12.1) ${\omega }_o=\frac{4M^{2}f\lambda }{\pi \omega },\left[2\right],$

where ω _ο is the diameter of the laser focal spot, M ² is the quality factor of the laser beam, f is the focal length, λ is the wavelength, and ω is the unfocused beam diameter.

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Analytical methods in laser cutting

Bekir Sami Yilbas , in The Laser Cutting Process, 2018

3.1 Introduction

The laser cutting process is involved with multiphysical phenomena including initial solid heating, melting and evaporation toward keyhole formation, cutting initiation, kerf formation, and dross ejection. The efforts toward introducing analytical approaches that capture the engineering and multiphysics aspects of the laser cutting process need to be appreciated. Although a considerable number of assumptions are usually introduced to describe the process in a mathematical frame, the resulting equations describing the process are, in most cases, useful and predict results close to the experimental data. In general, the laser cutting process is associated with high-temperature phenomena taking place under the high-pressure assisting gas environment. When modeling the laser cutting process, treatment of the assisting gas needs to be incorporated such that the effects of cooling and the flow resistance on the cut formation should be considered. Because the assisting gas has a high velocity, it acts like an impinging jet onto the surface while forming a stagnation zone and radial flow in the cut section. In addition, in some regions of radial flow, the behavior of the flow can be expressed as laminar, and for other regions, it behaves turbulent. Therefore, flow resistance and cooling effects should be examined in the cases when the flow behavior is laminar as well as turbulent. On the other hand, oxidation reactions taking place during the cutting process at high temperatures also influences the physical processes taking place in the cutting section. This is because the high temperature oxidation reactions are exothermic and provide excessive heat while modifying the physical ambient of the cutting section. In some situations, the molten flow instability in the cut section results in the formation of striation and dross attachments in the kerf sites. The high-temperature oxidation reactions contribute to the molten flow instability and the geometric features of the striation patterns being formed. In addition, oxygen diffusion in the molten material modifies the cooling rates and the fracture toughness of the recast layer at the kerf surfaces. Therefore, in analytical approaches toward formulating the multiphysics processes taking place during laser cutting, care must be taken to tackle the problem as close to realistic as possible. In this chapter, analytical approaches for the laser cutting process, the effect of assisting gas on the physical processes, oxygen diffusion, and entropy and efficiency analyses are introduced.

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Thermal analysis of the laser cutting process

Bekir Sami Yilbas , in The Laser Cutting Process, 2018

2.1 Introduction

The laser cutting process is initiated with the absorption of the laser beam by the substrate material through which heat is generated within the absorption depth of the irradiated surface. Although surface reflection plays an important role on the peak intensity absorbed by the irradiated surface, the influence of surface reflection reduces as the surface temperature increases [ 1]. Because the absorption depth is shallow in the surface vicinity, heat generation becomes significantly high during high-intensity laser irradiation. In this case, a melting temperature is reached rapidly and surface evaporation takes place almost immediately after the melting process. The evaporated front detaches from the surface while the melted zone recesses towards the substrate material, forming a cavity during the heating process. As the heating progresses, the cavity formed becomes deeper, causing the formation of a key-hole prior to cutting along the workpiece movement. The vapor pressure in the cutting section diminishes once the key-hole is formed. Hence, the vapor and liquid phases of the substrate material remain in the cutting section causing burrs and dross attachments around the cut edges. In addition, the presence of oxygen causes the initiation of high-temperature oxidation reactions at the melt/vapor surface. This results in additional heat generation in the cutting region. However, the irregular behavior of the oxidation reactions causes oscillatory behavior of heat input in the cutting section. Consequently, irregular thermal erosion causes large variation in the kerf width size while reducing the cut section quality. In order to avoid irregular behavior of the high temperature exothermic oxidation reactions, an assisting gas is used in the cutting section. The assisting gas is usually inert, such as argon, helium, nitrogen, etc., and prevents or minimizes the oxidation reactions taking place in the cutting section. On the other hand, the assisting gas causes convection cooling in the cutting section while enhancing the temperature gradients in the cutting section vicinity. This, in turn, enhances the thermal strain and stress levels causing an adverse effect on the cutting section quality. Therefore, the velocity of the assisting gas and its pressure needs to be regulated to minimize the adverse effect on the cutting quality. The mathematical analysis pertinent to absorption, temperature, and stress fields during the key-hole formation, and the assisting gas effect, is presented.

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Concluding remarks

Bekir Sami Yilbas , in The Laser Cutting Process, 2018

6.6 Laser cutting of wedges in steel

In the laser cutting process, laser parameters such as laser output power and pulsating frequency, cutting speed, assisting gas pressure, and focal distance determine the end-product quality. The sideways burning along the cutting paths results in poor cutting quality; particularly wedge cuts forming in sheet metal. In order to improve the cutting quality for such situations, the laser parameters should be readjusted to minimize the molten flow toward the wedge surface during the laser cutting process. The findings of the research study reveal that cutting quality is affected significantly by the laser output power intensity, particularly in a wedge cutting situation. In the case of a low power setting, molten metal flow on the top surface of the workpiece is observed. This reduces the cutting quality. On the other hand, for wedge cutting situations, the dross ejection from the bottom of the kerf causes thermal erosion of the cutting edges at the bottom surface of the workpiece. This also reduces the cutting quality. The dross height varies with low-power intensity while the variation of out of flatness is smaller than its counterpart corresponding to the dross height. It is observed from the neural network output that the normal pattern of striation is dominant over the other patterns. This suggests that molten flow induced instability occurs randomly along the cut edges. The cutting quality improves for a specific laser output power setting.

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Laser Machining and Surface Treatment

A.Z. Sahin , ... T. Ayar , in Comprehensive Materials Processing, 2014

Abstract

Thermal analysis of laser cutting process is carried out and the first and the second law efficiencies of the cutting process are formulated. Thermal efficiencies are predicted for various laser scanning speeds and laser output power levels. The experiment is conducted to examine the resulting cutting sections. The F-test is conducted to assess the end product (cutting section) quality and, later, the thermal efficiencies are related to the end product quality. It is found that increasing laser output power lowers the first and the second law efficiencies of cutting, which is more pronounced with reducing the laser scanning speeds. The end product quality improves for low laser output power levels and high laser scanning speeds as similar to the thermal efficiencies.

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Laser dicing of silicon and electronics substrates

H.Y.Z. Heng , ... Z.K. Wang , in Advances in Laser Materials Processing, 2010

5.6.3 355   nm DPSS UV laser cutting of FR4 and BT/ epoxy based PCB substrates

The PCB substrates used in the laser cutting process were FR4 and BT/ epoxy-based PCB substrates. The sample thickness ranges from 0.1 to 0.5  μm. A Coherent avia X 10   W q-switched diode pumped solid state (DPSS) ultraviolet (UV) laser system was used for the experiments. The laser wavelength was 355   nm. The laser pulse frequency ranged from 10   kHz to 100   kHz. The pulse duration of the laser beam was 20 to 35   ns depending on the laser pulse frequency used. The output beam profile was Gaussian shape. The spatial mode is TeM₀₀ (M ²<   1.3). The beam divergence is less than 0.3 mrad. The laser beam with a diameter of 3.5   mm (@ 1/e ²) was introduced to the PCB substrate using a galvanometric scanner with an f-theta flat field lens achieving a spot size of 25   μm (1/e ²). As discussed in Section 5.6.2, in order to achieve good quality cutting in terms of charring and HAZ, an O₂ side jet was used in the cutting process. The morphology and cutting quality of the laser cut PCB substrates were analyzed using optical microscope and SEM.

Figure 5.18 shows the optical images of a 0.3   mm thick FR4 substrate laser cut at different scanning speeds with different pass numbers. In all cases, the cumulative speed was the same to be at 100   mm/s. Here, the cumulative speed was defined as the scanning speed divided by the pass number. As shown in Fig. 5.18(a), there is significant melting, charring and HAZ along the cutting line. However, with an increase in the scanning speed but maintaining the same cumulative cutting speed of 100   mm/s, the charring and HAZ were reduced gradually as shown in Fig. 5.18 (b)–(e). The kerf width was also reduced from 43 to 26   μm.

5.18. Effects of scanning speed on HAZ and charring. (a) 100   mm/s for 1 pass, (b) 200   mm/s for 2 passes, (c) 500   mm/s for 5 passes, (d) 1000   mm/s for 10 passes and (e) 2000   mm/s for 20 passes.

It is known that high speed multi-pass cutting has the potential to minimize thermal input to the substrate by spatially separating each pulse on the surface. ⁹⁹ It can also improve energy coupling by displacing each pulse to avoid impinging on 'defocusing-plasma' generated by the previous pulses. As a result, the lateral heat conduction is minimized for the high speed multi-pulse cutting so that the HAZ, charring and kerf width are reduced with increasing scanning speed.

In order to understand how the charring occurred during the laser cutting process, we also studied the effects of interval time during scanning on the HAZ and charring. The cutting was done under two conditions, one was at 1000   mm/s for 100 continuous passes, another was at 1000   mm/s for 100 passes with 2 seconds pause at every 10 passes. Both cases had the same cumulative cutting speed of 10   mm/s. As shown in Fig. 5.19, more HAZ and charring are observed for continuous non-stop scanning than intermittent scanning. It is known that the FR4 substrate has a low glass transition temperature of 130–170   °C. When the UV laser beam irradiates the FR4 substrate, the laser photons are absorbed and cause the FR4 surface temperature to increase, causing melting and vaporization of the FR4 material. If the scanning was conducted in non-stop continuous mode, the excessive heat cannot be dissipated away effectively due to the low thermal diffusivity of the epoxy resin and the heat effect of subsequent scanning is overlapped with previous scanning. As a result, these accumulated excessive heat effects cause the sample temperature to increase and then cause more melting and burning, HAZ and charring as shown in Fig. 5.19 (a). When the cutting was done in an intermittent way where the cutting was carried out at 1000   mm/s for 100 passes with a 2   s pause between every 10 passes, the excessive heat could be dissipated away during the pause time before next scanning. Consequently, the HAZ and charring was reduced significantly as shown in Fig. 5.19 (b). The results show that there is a certain amount of heat generated during laser cutting of FR4 substrate so that a certain amount of cooling time is needed before subsequent laser passes is scanned during high speed multi-pass laser cutting process. When a big sheet of FR4 substrate is cut, the time for each pass is usually more than 2   s, so the intermittent stop is not needed during practical PCB cutting process. High speed multi-pass cutting was demonstrated to be a viable method to cut FR4 PCB substrate with high quality.

5.19. Effect of interval time between scanning of the line on HAZ and charring. (a) continuous non-stop scanning for 100 passes, and (b) scanning for 100 passes with 2 seconds pause at every 10 passes.

Figure 5.20 shows the effects of an O₂ assist side jet on charring and HAZ. It can be seen that with O₂ assist gas, the HAZ and charring was reduced considerably. Just as stated in Section 5.6.2, when the laser beam irradiates on a PCB substrate, the laser induced decomposition of polymer material resulted in 51% of the polymer weight being converted to gaseous products consisting mostly of CO (67%), HCN (15%), C₂H₂ (12%), and some (<   5%) CO₂ and the remaining major solid product was "glassy" carbon]. ^{97 , 98 , 100} It is also known that small species containing only a few carbon atoms which are directly formed in the ablation process, react with each other to form carbon soot or react with the O₂ or O based radicals to form CO₂ in competing reactions. ⁹⁸ This suggests that smaller carbon species formed during laser ablation are more rapidly oxidized to CO₂ with O2 assist gas. So, it is expected that O2 assist gas can effectively scavenge the carbon debris. Also the O2 gas helps to blow debris away from the ablation site. Furthermore, the assisting O2 gas helps to dissipate the heat and cool the substrate during the cutting process to give less HAZ and charring.

5.20. Effects of assisting O₂ gas on HAZ and charring, (a) no assisting gas, and (b) with assisting O₂ gas.

Figure 5.21 shows the images of laser cut FR4 samples at different repetition rates. In the graph, the corresponding average power and pulse energy were also indicated. As shown in Fig. 5.21, with increasing repetition rate, the HAZ and charring were reduced and almost no HAZ and charring were observed at 80   kHz. As shown in Fig. 5.21, with an increase in the repetition rate, the average power increased up to a maximum of 60   kHz, and then decreased with further increasing the repetition rate whereas the corresponding pulse energy decreased gradually. In addition, the laser pulse duration increased with the increase of the repetition rate. As a result, the laser intensity of the laser beam decreased with an increase in the repetition rate. So, the laser beam had a higher laser intensity at lower repetition rate Pulse energy (μJ) so as to induce more HAZ and charring whereas the laser beam at higher repetition rate had a lower laser intensity so as to produce less HAZ and charring as shown in Fig. 5.21. The laser cutting was also done at different repetition rates by keeping the average power constant. As shown in Fig. 5.22, the HAZ and charring decreased with an increase in the repetition rate. It is believed that laser cutting of FR4 PCB substrate at higher repetition rate can achieve high quality cutting in terms of HAZ and charring as long as the laser beam intensity is high enough to ablate the reinforced glass fibre.

5.21. Effects of repetition rate on the cutting quality in terms of HAZ and charring.

5.22. Effects of repetition rate on the heat-affected zone.

Figure 5.23 shows the laser cut 0.3   mm thick FR4 substrate at optimized conditions. The cutting speed can be achieved at 60   mm/s for 0.1   mm thick, 40   mm/s for 0.2   mm thick and 20   mm/s for 0.3   mm thick. It can be seen from Fig. 5.24 that the cutting quality is good, with no debris, almost no charring, and minimum thermal damage. The result demonstrated the suitability of UV laser to cut thin PCB substrates with heat-sensitive embedded components.

5.23. Picture of UV laser cut FR4 substrate. (a) entrance view; (b) exit view.

5.24. Schematic structure of laminar Cu/BT-epoxy/Cu PCB substrate.

The optimized laser cutting process was also applied to cut a laminar Cu-BT/epoxy-Cu PCB substrate. BT/epoxy has enhanced thermal, mechanical and electrical properties over standard epoxy systems such as high glass transition temperature (180   °C), low coefficient of thermal expansion and excellent electrical insulation in high humidity and high temperatures. Figure 5.24 shows the schematic structure of the laminar Cu-BT/epoxy-Cu PCB substrate. The copper foil is 15   μm thick and the central BT/epoxy is 100   μm thick. Figure 5.25 shows the SEM images of UV laser cut Cu/BT-epoxy/ Cu PCB substrate. It can be seen that the cutting edge is quite clean, with no burr, no distortion, minimum HAZ and the contour is quite straight. Figure 5.26 shows the cross-sectional view of the laser cut BT/epoxy-based PCB substrate. It is apparent that the cutting quality is very good. There is no evidence of delamination and deformation, no epoxy recession, no fibre pulling out and the cutting surface is clean and uniform.

5.25. SEM images of laser cut BT-epoxy-based PCB substrate. (a) entrance view, and (b) exit view.

5.26. Cross-sectional image of laser cut BT-epoxy based PCB substrate. (a) overview, and (b) enlarged view.

Finally, based on the achieved optimum cutting conditions, high quality laser cutting of the real multi-layered PCB substrates was demonstrated. Figure 5.27 shows the results of the laser cut multi-layered FR4 PCB substrates. It can be clearly seen that the cutting quality is very good. Almost no charring, no debris was observed. From Fig. 5.27(c), the cutting surface is very clean and consistent, with no delamination, no fibre pulling out, and no epoxy recession. Also, it was found as shown in Fig. 5.27 (d, e) that the cutting edge is sharp and uniform.

5.27. Images of laser cut multi-layered PCB substrate (a) entrance, (b) exit, (c) cutting surface, (d) and (e) SEM images of the cut.

In summary, 355   nm UV DPSS laser was demonstrated to be a promising tool to cut PCB substrates with high quality. The effects of selected laser processing parameters on the cutting quality of thin PCB substrates in terms of HAZ and charring were investigated. Multi-pass laser cutting with high cutting speed was shown to give good quality cutting of FR4 or BT-epoxy based PCB substrates. It was also found that a certain amount of interval time between scans, assisting O₂ gas, and laser pulse repetition rate had significant effects on the cutting quality.

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Laser cutting quality assessment and numerical methods for modeling of cutting

Bekir Sami Yilbas , in The Laser Cutting Process, 2018

4.6 Entropy analysis of the laser cutting process

In this study, laser cutting of thick sections is considered and modeling of the laser cutting process is introduced. Thermal efficiency of the cutting process and the specific energy required for the cutting are formulated. In the analysis, a lump parameter approach is introduced to formulate energy gain by the substrate material during the laser cutting process. The analysis and findings are presented in line with the previous study [ 18].

The specific energy required for a laser cutting of a metallic substrate is formulated using a lump parameter technique. In this case, the rate of energy balance for cutting of sheet metal is considered in the formulations. The cutting situation and geometric view of the cut section is shown in Fig. 4.31.

Fig. 4.31. A laser heating situation and coordinate system.

The rate of energy required for cutting of a section can be written as:

(4.50) ${\stackrel{.}{E}}_r={\stackrel{.}{E}}_l+\stackrel{.}{m}\left[\mathrm{\Delta }u+L_m+\beta L_{en}\right]$

where ${\stackrel{.}{E}}_l$ , $\stackrel{.}{m}$ , Δu, L_m , β, L_ev are the rate of total losses during the laser cutting, the rate of total mass of material removed during laser cutting, internal energy gain of the substrate material removed, latent heat of melting, fraction of mass evaporated, and latent heat of evaporation, respectively.

The total material removed during laser cutting, after assuming constant density of the substrate material, is:

(4.51) $\stackrel{.}{m}=\rho \frac{d\forall }{dt}=\rho \frac{d\left(w\cdot L\cdot Th\right)}{dt}=w\cdot Th\frac{\mathit{dL}}{dt}$

where $\forall$ is the volume of the laser cut hole, ρ is the density of the substrate material, w and L are the width and length of the cut section, and Th is the thickness of the sheet metal. The term dL/dt represents the cutting velocity, which is the laser scanning velocity during the cutting process.

In addition, the rate of total loss includes the rate of conduction and convection losses during the cutting; that is:

(4.52) ${\stackrel{.}{E}}_{\mathit{loss}}={\stackrel{.}{E}}_{\mathit{conduction}}+{\stackrel{.}{E}}_{\mathit{convection}}$

where ${\stackrel{.}{E}}_{\mathit{conduction}}$ and ${\stackrel{.}{E}}_{\mathit{convection}}$ are the rate of conduction and convection losses.

The rate of conduction loss from the cutting wall site can be written as:

(4.53) ${\stackrel{.}{E}}_{\mathit{conduction}}=\frac{d}{dt}\left[{\displaystyle {\int }_0^{Th}\left({\displaystyle {\int }_0^{50w}\rho \cdot C_{p}T\cdot L\cdot \mathit{dy}}\right)}dx\right]$

where 40w represents the distance away from the solution domain in the y-direction (radial direction).

It is assumed that the natural convection occurs at the top and bottom surfaces of the cut section. The heat transfer coefficients across both surfaces are the same. In this case, the convection losses can be written as:

(4.54) ${\stackrel{.}{E}}_{\mathit{convection}}=A_s\left[h_1\left(T_s-T_o\right)+h_2\left(T_s-T_o\right)\right]=w\cdot L\cdot \left(T_s-T_o\right)\left(h_1+h_2\right)$

where A_s is the surface area of the cut section, h ₁ and h ₂ are the heat transfer coefficient at top and bottom surfaces of the cut section.

The specific energy required for the cutting is determined from:

(4.55) $E_{\mathit{spec}}=\frac{1}{\forall }{\displaystyle {\int }_o^{t_c}{\stackrel{.}{E}}_r}dt$

where t_c is the time taken to cut the edges in the sheet metal.

Thermal efficiency of the laser drilling process is the ratio of the rate of energy required for through cutting over the power used for the cutting process. In this case, thermal efficiency becomes:

(4.56) $\eta =\frac{{\stackrel{.}{E}}_{req}}{P}$

where P is the laser power on the workpiece surface during the cutting process. The solutions of Eqs. (4.50)–(4.56) are obtained by using Mathematica software. The data used in the solution are given in Table 4.5.

Table 4.5. Properties used in the simulations

Source of variation	Value	Units
Fraction of evaporation contribution (β)	0.1	–
Latent heat of melting of workpiece	247,112	J/kg
Thermal conductivity of workpiece	63	W/mK
Specific heat capacity of workpiece	460	J/kgK
Thermal diffusivity of workpiece	1.6134   ×   10−   5	m2/s
Melting temperature of workpiece	1700	°K

Fig. 4.32 shows thermal efficiency with laser output power for different cutting velocities. Thermal efficiency increases with reduced laser power. This is more pronounced for high cutting velocities. In this case, power required for cutting reduces with increased velocity, which is more pronounced for high-cutting velocities. This is because the energy required for cutting per unit time reduces with increased cutting speed. Moreover, as the laser power increases, thermal efficiency remains the same, provided that thermal efficiency attains high values with increased cutting velocity. In the case of the effect of cutting speed on thermal efficiency, increasing cutting speed improves thermal efficiency significantly because the energy required per unit time increases so that the ratio of energy required per unit time for cutting to laser power intensity increases. It should be noted that although increasing cutting speed while keeping the laser power constant enhances thermal efficiency, the cutting process may not be completed due to insufficient laser power for complete cutting.

Fig. 4.32. Thermal efficiency of laser cutting with laser power for different scanning speeds [18].

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The challenges ahead for laser macro, micro, and nano manufacturing

L. Li , in Advances in Laser Materials Processing, 2010

2.1 Introduction

Laser materials processing has been practised for over four decades and is playing an important role in the modern manufacturing industry and the economy. While laser cutting, welding, marking and drilling processes have reached maturity with wide industrial acceptance, new developments in recent years in additive manufacturing and micro/nano fabrication have enabled new capabilities that lasers can bring to the manufacturing industry. With the availability of high brightness lasers such as fibre and disk lasers as well as ultra-fast lasers such as femto- and pico-second lasers, new beam material interaction phenomena appear. Research in new technology development, optimisation, modelling/simulation, and understanding the basic science (beam/ material interactions, material characteristics and new material properties generated by lasers) involved in laser processing plays a critical role in advancing laser materials processing science and technology. This chapter gives the author's view (based on 25 years of R&D in laser processing) on the challenges and future prospects of laser materials processing research.

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Surface modification of Ti–Ni alloys for biomedical applications

M.F. MAITZ , in Shape Memory Alloys for Biomedical Applications, 2009

8.2.1 Blasting techniques

Slurry basting is a method for degreasing and for removing the drawing texture from the inner diameter of tubings or for deburring vascular stents after the laser cutting process. ¹⁴ A dispersion of abrasive media, such as silica, alumina, carborundum, carbides or diamond is circulated through the tube and forms a smooth finish. The method is highly abrasive and causes a measurable loss of material. In contrast to the mainly tangential flow at slurry blasting, microblasting uses mainly orthogonal impinging particles. The method is an attenuated modification of sand blasting with fine abrasive media and lower flow rates. It is mainly applied to remove oxides and produces a matt-finished, finely textured surface. The rough surface after sand blasting is frequently preferred for improved osseointegration of dental or orthopaedic implants. The mechanical abrasive techniques are followed by cleaning procedures in ultrasound, typically with water and/or organic solvents to remove abrasive particles, organic contamination and debris. The surface quickly oxidizes in air, forming an oxide layer 2–3 nm thick. ¹⁵

It was found that blasting techniques frequently remove the natural, pre-passivated oxide film only incompletely. Shabalovskaya et al. found that at least one flake of thick oxide layer remained per centimetre of sand blasted wire. ¹⁶ Particles of the abrasive media remain impinged in the surface and cannot be removed by ultrasound cleaning. Lubricants are captured within cracks or beneath the loosened oxide flakes. Such impurities of the surface lead to enhanced corrosion of the material with pitting and release of toxic Ni ions. Moreover, the corrosion and passivation behaviour is highly unpredictable and irregular. ^{16, 17}

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