Rahul Patwa

Micromachining with tailored nanosecond pulses

Fiber lasers in MOPA configuration are a very flexible tool for micromachining applications since they allow to independently adjust the pulse parameters such as pulse duration, repetition rates and pulse energy while maintaining a constant beam quality. The developed fiber laser provides an average power of 11 W and maximum pulse energy of 0.5 mJ for a wide range of pulse parameters at diffraction limited beam quality. Its pulse duration and repetition rate are continuously adjustable from 10 ns to cw and from 10kHz to 1MHz respectively. Ablation experiments were carried out on stainless steel, nickel and silicon with the goal of optimizing removal rates or surface finish using nanosecond pulses of different parameters. Maximum removal rates are achieved on all three materials using relatively similar pulse parameters. For silicon, pulse duration of 320ns at 100kHz and 45mJ resulted in optimum removal. In single shot experiments on silicon a significant influence of the pulse duration was found with a distinct optimum for removal rate and surface finish. The optimum intensity at the work piece is in the range of 35MW/cm2 to 70MW/cm2. Lower values are below the ablation threshold, while the plasma shielding effect limits considerable increases in removal rates for intensities exceeding 70MW/cm 2.

Parameter optimization for high speed remote laser cutting of electrodes for lithium-ion batteries

To reduce carbon emission, transportation sector has adapted lithium-ion battery-based hybridization of gasoline and diesel engines due to its efficiency, the availability of technologies, and nation-wide infrastructures. To cut prismatic and cylindrical electrodes for lithium-ion batteries, die cutting and rotary knife slitting have been used. Both techniques have disadvantages such as tool wear, process instability, inconsistency of cut quality, and redesign of mechanical cutting processes due to various battery sizes. High speed remote laser cutting overcomes these disadvantages with characteristics such as contact-free process, high energy concentration, low noise level, fast processing speed, very narrow heat affected zone, applicability to nearly all materials, and flexibility of laser power. Optimization of key parameters, or power and scanning speed, has been presented for laser cutting of electrodes for lithium-ion batteries. An acceptable clearance width is observed. The line energy is defined a...

Finite Element Thermal/Mechanical Analysis of Transmission Laser Microjoining of Titanium and Polyimide

Detailed analysis of a residual stress profile due to laser microjoining of two dissimilar biocompatible materials, polyimide (PI) and titanium (Ti), is vital for the long-term application of bio-implants. In this work, a comprehensive three-dimensional (3D) transient model for sequentially coupled thermal/mechanical analysis of transmission laser (laser beam with wavelength of 1100 nm and diameter of 0.2 mm) microjoining of two dissimilar materials has been developed by using the finite element code ABAQUS, along with a moving Gaussian laser heat source. First the model has been used to optimize the laser parameters like laser traveling speed and power to obtain good bonding (burnout temperature of PI>maximum temperature of PI achieved during heating> melting temperature of PI) and a good combination has been found to be 100 mm/min and 3.14 W for a joint-length of 6.5 mm as supported by the experiment. The developed computational model has been observed to generate a bonding zone that is similar in width (0.33 mm) to the bond width of the Ti/PI joint observed experimentally by an optical microscope. The maximum temperatures measured at three locations by thermocouples have also been found to be similar to those observed computationally. After these verifications, the residual stress profile of the laser microjoint (100 mm/min and 3.14 W) has been calculated using the developed model with the system cooling down to room temperature. The residual stress profiles on the PI surface have shown low value near the centerline of the laser travel, increased to higher values at about 165 μm from the centerline symmetrically at both sides, and to the contrary, have shown higher values near the centerline on the Ti surface. Maximum residual stresses on both the Ti and PI surfaces are obtained at the end of laser travel, and are in the orders of the yield stresses of the respective materials. It has been explained that the patterned accumulation of residual stresses is due to the thermal expansion and contraction mismatches between the dissimilar materials at the opposite sides of the bond along with the melting and softening of PI during the joining process.

Finite Element Thermal Analysis for Microscale Laser Joining of Nanoscale Coatings of Titanium on Glass/Polyimide System

Finite element thermal analysis and comparison with experiments of microscale laser joining of biocompatible materials, polyimide (PI) and nanoscale coating of titanium (Ti) on glass (Gl), is vital for the long-term application of bio-implants and important for the applications of nanoscale solid coatings. In this study, a comprehensive three dimensional (3D) transient simulation for thermal analysis of transmission laser micro-joining of dissimilar materials has been performed by using the finite element (FE) code ABAQUS, along with a moving Gaussian laser heat source. The laser beam (wavelength of 1100 nm and diameter of 0.2 mm), moving at an optimized velocity (100 mm/min), passes through the transparent PI, gets absorbed by the absorbing Ti, and eventually melts the PI to form the bond. The laser bonded joint area is 6.5 mm long on three different Ti coating thicknesses of 400, 200 and 50 nms on Gl surface. Non-uniform mixed meshes have been used and optimized to formulate the 3D FE model and ensure very refined meshing around the bond area. During the microscale laser heating finite element modeling shows widths of PI surface experiencing temperatures above the glass transition temperature are similar to the widths of bonds observed in experiments for coating thicknesses of 400 and 200 nms of Ti on Gl. However, for the case of 50 nm coating bond width using finite element analysis cannot produce and is lower than the bond width observed experimentally.© 2009 ASME

Welding head for ‘self guided’ laser welding

Precise positioning of the laser beam on the work piece is crucial for high quality laser welds; e.g. for butt welding the focal point of the laser beam with respect to the joint must be maintained within an accuracy better than 20µm - 150µm, depending on the focused beam radius. These stringent accuracy requirements call for high precision robots, a repeatable work piece profile and precise clamping. To compensate for insufficient repeatability of work piece or clamping, seam-tracking devices are used. A sensor measures the joint position and computes a correction vector to follow the actual joint trajectory. The deviation is compensated either by robot trajectory adjustment or by an additional tracking axis. Disadvantages of this approach are complex installation of the devices due to interfacing with the robot control, the need of teaching and calibrating the sensor and principle based accuracy restriction that limit the usability in more complex 2d contours and with low accuracy robots.We recently introduced a more flexible and precise approach that utilizes an advanced camera-based sensor that is capable of measuring seam position, relative displacement between work piece and sensor and melt pool of the process with one single device. This paper describes a realized ‘self guided’ welding head, which uses this approach in combination with an integrated high power scanner. The result is a welding head that follows a curved or linear butt weld with high precision and independent of the actual robot trajectory; without the need of calibration, robot interfacing and alignment.Precise positioning of the laser beam on the work piece is crucial for high quality laser welds; e.g. for butt welding the focal point of the laser beam with respect to the joint must be maintained within an accuracy better than 20µm - 150µm, depending on the focused beam radius. These stringent accuracy requirements call for high precision robots, a repeatable work piece profile and precise clamping. To compensate for insufficient repeatability of work piece or clamping, seam-tracking devices are used. A sensor measures the joint position and computes a correction vector to follow the actual joint trajectory. The deviation is compensated either by robot trajectory adjustment or by an additional tracking axis. Disadvantages of this approach are complex installation of the devices due to interfacing with the robot control, the need of teaching and calibrating the sensor and principle based accuracy restriction that limit the usability in more complex 2d contours and with low accuracy robots.We recently int...

Multi-beam laser additive manufacturing

Today, Laser Additive Manufacturing (LAM) is typically performed using a single beam with power up to multiple-kilowatts. The associated high heat input and limited process control hampers tight manufacturing tolerances and the applicable material spectrum. This paper highlights the development of Multi-beam LAM technology to address the shortfalls of today’s technology and to broaden the applicability to many industries. Multi-beam LAM deploys several low power beams, each precisely controllable with a minimum heat input thus providing the capability to tailor the applied energy to the specific needs of the application. The single beams either work in parallel to scale productivity without sacrificing precision or in close proximity creating desired heat profiles. This new approach is scalable in productivity through multiplication and is expected to allow deposition of difficult to coat materials through tailored heat profiles. Advances are expected in near net shape manufacturing of complex structures with fine features and high dimensional accuracy.A compact prototype processing head for Multi-beam LAM was designed and built to investigate the capability of the new technology. The head incorporates latest high-brightness diode laser technology and a compact powder nozzle design. Two laser beams are being emitted, a stationary beam with fixed position on the work piece and a movable beam that can be positioned relative to the stationary beam. A very effective solution with high spatial resolution and fast actuation was developed for steering the movable laser beam. The movable beam cannot only be set to a fixed position but it can also be scanned at high frequencies. The power of both beams is individually controlled. Ongoing process investigations and future MB-LAM target specific applications for vehicles, jet engines and medical devices serving the automotive, aerospace, medical and defence sector. Initial results are being presented.Today, Laser Additive Manufacturing (LAM) is typically performed using a single beam with power up to multiple-kilowatts. The associated high heat input and limited process control hampers tight manufacturing tolerances and the applicable material spectrum. This paper highlights the development of Multi-beam LAM technology to address the shortfalls of today’s technology and to broaden the applicability to many industries. Multi-beam LAM deploys several low power beams, each precisely controllable with a minimum heat input thus providing the capability to tailor the applied energy to the specific needs of the application. The single beams either work in parallel to scale productivity without sacrificing precision or in close proximity creating desired heat profiles. This new approach is scalable in productivity through multiplication and is expected to allow deposition of difficult to coat materials through tailored heat profiles. Advances are expected in near net shape manufacturing of complex structures ...

Investigation of different laser cutting strategies for sizing of Li-ion battery electrodes

Lithium-ion batteries are currently considered to be the most promising advanced battery technology for electric vehicles that require high energy capacity. A lot of research and development activities have been focused on their development to achieve efficient mass production capabilities and to successfully commercialize the technology. This paper discusses the laser cutting process for coated anode/cathode and uncoated copper/aluminum tabs for both cylindrical and prismatic Li-ion cell designs. A number of different cutting strategies have been investigated using IR (fiber/disc) and UV laser sources in pulsed/cw configurations. An in-depth development study has been performed to understand the effect of different processing parameters on the maximum cutting speed, cut edge quality and overall energy efficiency of the process. Results show that excellent cut quality can be achieved using optimal processing parameters with cutting speeds ranging from several m/min up to 300 m/min depending on the processing requirements and the corresponding cutting approach.Lithium-ion batteries are currently considered to be the most promising advanced battery technology for electric vehicles that require high energy capacity. A lot of research and development activities have been focused on their development to achieve efficient mass production capabilities and to successfully commercialize the technology. This paper discusses the laser cutting process for coated anode/cathode and uncoated copper/aluminum tabs for both cylindrical and prismatic Li-ion cell designs. A number of different cutting strategies have been investigated using IR (fiber/disc) and UV laser sources in pulsed/cw configurations. An in-depth development study has been performed to understand the effect of different processing parameters on the maximum cutting speed, cut edge quality and overall energy efficiency of the process. Results show that excellent cut quality can be achieved using optimal processing parameters with cutting speeds ranging from several m/min up to 300 m/min depending on the process...

Laser drilling up to 15,000 holes/sec in silicon wafer for PV solar cells

One approach to realize a back contact solar cell design is to ‘wrap’ the front contacts to the backside of the cell [1]. This results in significantly reduced shadowing losses, possibility of simplified module assembly process and reduced resistance losses in the module; a combination of measures, which are ultimately expected to lower the cost per watt of PV modules. A large number of micro-vias must be drilled in a silicon wafer to connect the front and rear contacts. Laser drilling was investigated using a pulsed disk laser which provided independent adjustment of pulse width, repetition rate and laser power. To achieve very high drilling rates, synchronization of the laser pulses with the two-axis galvanometer scanner was established using a FPGA controller. A design of experiments (DOE) was developed and executed to understand the key process drivers that impact the average hole size, hole taper angle, drilling rate and hole quality. Laser drilling tests were performed on wafers with different thicknesses between 120 μm and 190 μm. The primary process parameters included the average laser power, pulse length and pulse repetition rate. The impact of different laser spot sizes (34 μm and 80 μm) on the drilling results was compared. The results show that average hole sizes between 30 – 100 μm can be varied by changing processing parameters such as laser power, pulse length, repetition rate and spot size. In addition, this study shows the effect of such parameters on the hole taper angle, hole quality and drilling rate. Using optimized settings, 15,000 holes per second are achieved for a 120 μm thick wafer with an average hole diameter of 40μm.

Finite element thermal/mechanical analysis for microscale laser joining of ultrathin coatings of titanium on glass/polyimide system

Finite element (FE) thermal/mechanical analysis of microscale laser joining of biocompatible materials, polyimide (PI) and nanoscale coating of titanium (Ti) on glass (Gl), is important for the long-term application of bio-implants and the applications of nanoscale solid coatings. In this study, a comprehensive three-dimensional (3D) transient simulation for thermal/mechanical analysis of transmission laser microjoining of dissimilar materials has been performed using the FE code ABAQUS, along with a moving Gaussian laser heat source. The laser beam (wavelength of 1100 nm and diameter of 0.2 mm), moving at an optimized velocity (100 mm/min), passes through the transparent PI, gets absorbed by the absorbing Ti, and eventually melts the PI to form the bond. The laser bonded joint area is 6.5 mm long on three different Ti coating thicknesses of 400, 200, and 50 nm on Gl surface. Non-uniform mixed meshes have been used and optimized to formulate the 3D FE model and ensure very refined meshing around the bond area. During the microscale laser heating, FE modelling shows that the widths of PI surface experiencing temperatures above the glass transition temperature are similar to the widths of bonds observed in experiments for coating thicknesses of 400 and 200 nm of Ti on Gl. However, for the case of 50 nm coating, bond width using FE analysis cannot produce and is lower than the bond width observed experimentally. After these verifications, the residual stress profile of the laser microjoint (200 nm of Ti on Gl) has been calculated using the developed model with the system cooling down to room temperature. The transverse (to the laser travel direction) stress profiles on Ti surface have shown high tensile stress near the centre-line of laser travel, decreased to lower values from the centre-line symmetrically at both sides, and to the contrary, have shown compressive stresses near the centre-line on Gl surface. Maximum von-Mises residual stresses on PI and Gl surfaces are obtained at the start of laser travel, and on Ti surface they occur in between the ends. It has been explained that the patterned accumulation of residual stresses is due to the thermal expansion and contraction mismatches between the dissimilar materials at the opposite sides of the bond along with the melting and softening of PI during the joining process.

Characterization of Laser Processed Sub-Millimeter Lap Joints Between Copper and Aluminum Under Tensile Load

This paper presents the results of laser joined copper-aluminum lap shear samples without filler materials using an IPG 500W SM fiber laser. The length of the processed laser joint was about 20 mm and the width was about 200 μm. Laser-joined samples were tested under tensile loading to determine joint strengths. In addition, finite element analysis (FEA) was conducted to understand the stress distribution within the bond area under such loading. The FEA model provides a full-field stress distribution in and around the joint that cause eventual failure. We are still working on the topic, and more data will be published soon.© 2010 ASME