Mohammad Sheikh

Numerical simulation of laser machining of carbon-fibre-reinforced composites:

Abstract The growing use of carbon-fibre-reinforced polymer (CFRP) composites as high-performance lightweight materials in aerospace and automotive industries demands efficient and low-cost machining technologies. The use of laser machining for cutting and drilling composites is attractive owing to its high speed, flexibility, and ease of automation. However, the anisotropic material properties of composites, and issues related to the heat-affected zone (HAZ), charring, and potential delamination during laser processing, are major obstacles in its industrial applications. In order to improve the quality and dimensional accuracy of CFRP laser machining, it is important to understand the mechanism of the transient thermal behaviour and its effect on material removal. Based on the ‘element death’ technique of the finite element (FE) method, a three-dimensional model for simulating the transient temperature field and subsequent material removal has been developed, for the first time, on a heterogeneous fibre—matrix mesh. In addition to the transient temperature field, the model also predicts the dimensions of the HAZ during the laser machining process. Experimental results obtained with same process variables using a 355 nm DPSS Nd:YVO4 laser were used to validate the model. Based on the investigation, the mechanism of material removal in laser composite machining is proposed. The results suggest that the employed FE approach can be used to simulate pulsed laser cutting of fibre-reinforced polymer composites.

Characterization of machining of AISI 1045 steel over a wide range of cutting speeds. Part 1: Investigation of contact phenomena

Abstract Friction conditions at the tool-chip interface are one of the most important inputs for modelling and simulation of the machining process. However, the nature of the tool-chip contact is often assumed in developing finite element models, thereby seriously affecting their reliability. In this paper, results of an investigation into the tool-chip contact interface using uncoated tungsten-based cemented carbide tools in dry high-speed turning of AISI 1045 steel are presented. The tests were conducted at cutting speeds ranging between 198 and 879m/min with a feed rate of 0.1mm/rev and a constant depth of cut of 2.5 mm. The effects of cutting speed on tool rake face contact length, contact area, friction, element mapping, and surface roughness are studied and discussed. It is shown that the quantitative methods, used here to characterize the tool-chip contact region, can provide valuable data for accurate and reliable modelling of the metal machining process over a wide range of cutting speeds.

An investigation of the tool¿chip contact length and wear in high-speed turning of EN19 steel

Abstract In this paper, existing models for the tool-chip contact length are reviewed with regards to high-speed machining theory. Results of an investigation into the tool-chip contact length and tool wear of uncoated tungsten-based cemented carbide tools for dry high-speed turning of EN19 alloy steel are presented. The tests were conducted at cutting speeds ranging between 200 and 1200m/min with feed rates of 0.14 and 0.2 mm/rev and a constant depth of cut of 0.1 mm. From measurements, the effect of cutting speed on contact length and tool life has been determined and several important relationships established. It was found that the contact length changes according to the contact phenomena in the tool-chip interface zone, which is predominantly affected by the cutting speed. Moreover, the influence of the cutting speed on the contact length changes significantly from conventional to high-speed cutting environments. The study concludes that existing models are quantitatively inadequate for predicting tool-chip contact lengths in high speed turning.

Characterization of machining of AISI 1045 steel over a wide range of cutting speeds. Part 2: Evaluation of flow stress models and interface friction distribution schemes

Abstract To ensure that the simulation of the orthogonal metal-cutting process yields accurate results, the material and frictional behaviours during simulation have to be defined accurately. Flow stress models are used extensively in the simulations of deformation processes occurring at high strains, strain rates, and temperatures. In this work, the Johnson-Cook, Maekawa et al., Oxley, El-Magd et al., and Zerilli-Armstrong flow stress models are evaluated. AISI 1045 steel is used as the workpiece material because it is well characterized. First, the predictive capability of these flow stress models is compared with the published experimental data at high strain rates and the modelling errors are quantified. Different friction conditions along the tool rake face are also discussed. Then the friction conditions based on results of scanning electron microscopy-energy-dispersive X-ray analysis from Part 1 are implemented together with other friction models. The material flow stress models and friction conditions are assessed using an updated Lagrangian finite element code simulating continuous chip formation over a range of cutting speeds. The assessment of these models is carried out for their accuracy in predicting the cutting force and shear angle with those obtained experimentally in order to draw conclusions regarding their comparative performance.

Microstructural finite-element modelling of a ceramic matrix composite to predict experimental measurements of its macro thermal properties

A new experimental test rig has been used to measure the thermal diffusivity of a fibre-reinforced ceramic matrix composite material (C/C-SiC) using laser pulse heating under conditions of both one-dimensional and three-dimensional heat transfer. To predict these measurements, a microstructural geometric unit cell model of the material has been developed from optical micrographs and which incorporates fibre tows, matrix and interface materials. With the selection of a suitable finite-element mesh to model each material phase and the prescription of appropriate boundary conditions, this model has then been analysed for conditions of steady-state and transient heat conduction. In this way macro-thermal properties have been calculated from the micro-thermal properties of each phase of the composite. The micro-thermal properties deduced from the analyses for these thermal conditions are in close mutual agreement, and within the bounds of numerical error. These results are presented in this paper alongside the experimental measurements. It is concluded that with careful geometric modelling and sensible property data selection, the micro-thermal properties obtained from the unit cell model can accurately predict the experimentally measured macro-thermal properties.

An evaluation of heat partition in the high-speed turning of AISI/SAE 4140 steel with uncoated and TiN-coated tools

Abstract In manufacturing by machining, thermal loads on cutting tools can have a major influence on tool wear and hence process cost, especially at higher cutting speeds. An investigation has been undertaken to determine heat partition into the cutting tool for high-speed machining of AISI/SAE 4140 high-strength alloy steel with uncoated and TiN-coated tools. The cutting tests have been performed at cutting speeds ranging between 100 and 880 m/min with a feed rate of 0.1 mm/rev and a constant depth of cut of 2.5 mm. Cutting temperatures are measured experimentally using an infrared thermal imaging camera. The sticking and sliding regions are investigated from an examination of the tool—chip contact region using a scanning electron microscope (SEM). In addition, non-uniform heat intensity is modelled according to the contact phenomena. In this work, evaluation of the fraction of heat flowing into the cutting tool is carried out by iteratively reducing the available heat flux until the finite element method (FEM) temperatures are simultaneously matched at multiple points with the experimentally measured temperatures. This paper elucidates on the differences in thermal shielding for uncoated and TiN-coated tools. It is found that heat partition into the cutting tool decreases from a fraction of 0.41 to 0.17 for conventional cutting speeds and increases from 0.19 to 0.24 for high-speed machining when using uncoated carbide cutting tools. On the other hand, with TiN-coated tools, heat partition varies from 0.35 down to 0.095 for the whole range of cutting speeds. These results clearly show that the use of TiN-coated tools generally reduces heat partition into the cutting tool, but does so more significantly in high-speed machining (HSM) as compared with conventional machining speeds. The driver behind this study on heat partition in machining with TiN coatings is the design of coatings with enhanced thermal shielding properties.

Numerical analysis of the effects of non-conventional laser beam geometries during laser melting of metallic materials

Laser melting is an important industrial activity encountered in a variety of laser manufacturing processes, e.g. selective laser melting, welding, brazing, soldering, glazing, surface alloying, cladding etc. The majority of these processes are carried out by using either circular or rectangular beams. At present, the melt pool characteristics such as melt pool geometry, thermal gradients and cooling rate are controlled by the variation of laser power, spot size or scanning speed. However, the variations in these parameters are often limited by other processing conditions. Although different laser beam modes and intensity distributions have been studied to improve the process, no other laser beam geometries have been investigated. The effect of laser beam geometry on the laser melting process has received very little attention. This paper presents an investigation of the effects of different beam geometries including circular, rectangular and diamond shapes on laser melting of metallic materials. The finite volume method has been used to simulate the transient effects of a moving beam for laser melting of mild steel (EN-43A) taking into account Marangoni and buoyancy convection. The temperature distribution, melt pool geometry, fluid flow velocities and heating/cooling rates have been calculated. Some of the results have been compared with the experimental data.

An analysis of the effect of laser beam geometry on laser transformation hardening

The effect of transformation hardening depends upon both heating and cooling rates. It is desirable to have a slow heating rate and a rapid cooling rate to achieve full transformation. To date laser transformation hardening has been carried out using circular or rectangular beams which result in rapid heating and cooling. Although the use of different beam intensity distributions within the circular or rectangular laser beams has been studied to improve the process, no other beam geometries have been investigated so far for transformation hardening. This paper presents an investigation into the effects of different laser beam geometries in transformation hardening. Finite element modeling technique has been used to simulate the steady state and transient effects of moving beams in transformation hardening of EN 43A steel. The results are compared with experimental data. The work shows that neither of the two commonly used beams, circular and rectangular, are optimum beam shapes for transformation hardening. The homogenization temperature exceeds the melting point for these beam shapes for the usual laser scanning speeds and power density. Triangular beam geometry has been shown to produce the best thermal history to achieve better transformation and highest hardness due to slower heating without sacrificing the processing rate and hardening depths.

Laser glass cutting techniques—A review

With the advancement of glass technology in recent times, glass has become one of the most important engineering materials in architectural, medical, automotive, flat panel display, and electronics applications. Desired shape and size of the glass can only be achieved through accurate and precise cutting technique. Laser technology has an advantage over traditional cutting processes for glass due to good quality, surface finish, and high speed of operation. This paper provides a review of all the laser glass cutting techniques discovered in recent work and forms a comparison framework, in particular, their limitations and their current status which would facilitate prospective research and future development.

The Effect of Laser Beam Geometry on Cut Path Deviation in Diode Laser Chip-Free Cutting of Glass

In laser cleaving of brittle materials using the controlled fracture technique, thermal stresses are used to induce a single crack and the material is separated along the cutting path by extending the crack. One of the problems in laser cutting of glass with the controlled fracture technique is the cut deviation at the leading and the trailing edges of the glass sheet. This work is about minimizing this deviation through an optimization process, which includes laser beam geometry. It has been established that the thermal stresses generated during laser scanning are strongly dependent upon laser beam geometry. Experimental techniques are used to quantify cut deviation for soda-lime glass sheets under a set of conditions while finite element modeling is used to optimize the process and reduce (or eliminate) cut deviation. The experimental results of the effect of different laser beam geometries on cut path deviation have been presented in this study, along with the finite element modeling of the cutting process to simulate the transient effects of the moving beam and predict thermal fields and stress distribution. These predictions are compared with the experimental data. In comparison to other beam geometries, the triangular-forward beam at the leading edge and triangular-reverse and circular beam geometry at the trailing edge produces lower tensile stresses (sigma(xx)) and hence minimizes the cut path deviation. The work also shows that beam divergence inside the glass plays a significant role in changing the cut path deviation at the bottom leading and trailing edges of the glass.