Acoustic sensing for laser machining processes

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1.0 Introduction

In past decades for about 20 years, the application of laser for wide industrial applications such as cutting, drilling, welding, marking and cladding has increased dramatically. In general laser is often assumed to be operating at peak performance when in reality it doesn't. Unlike laboratory lasers the industrial lasers are not designed with continuous monitoring. Only skilled technical operator can spot the problem quickly and go with corrective measures. To overcome the problems in conventional LBM operations and to optimize the process parameters of online monitoring system of LBM & to achieve the fine tuned precise operation, much attention is been shown on the research in this field.

2.0 Why online monitoring system for LBM ?

Generally the operators involved in LBM, often opt for non-electronic techniques such as burn papers, fluorescing plates or cards, or direct test burns, to evaluate the laser beam quality. While these methods are inexpensive, they do not reveal the subtleties that can affect end-use performance. Non-electronic techniques relay on the human's clarity in vision to evaluate the images. Since human eye is excellent at seeing many logarithmic levels of brightness, it can distinguish among only a few linear shades of gray, which is often where these subtleties are found. More over, non-electronic methods are static and they mostly do not reveal short-term fluctuations

The main factors for online monitoring of laser beam should be,

  • Simple in operation
  • Non- intrusive
  • Low cost
  • High speed and capable of operation in process

3.0 Supportive case studies for chosen topic

3.1 Case # 1

  • online monitoring of laser beam using acoustic sensing

Quality assurance is the hot topic in current years which involves in measurement of high speed laser beam parameters during machining process. Most of manufacturing research is been attracted by this increasing quality demand.

 In this subject case study acoustic sensing is been used to examine the laser drilling process (blind and through hole drilling). The drilling operation is performed on acrylic, aluminium oxide and steel. Also theoretical analysis of the relationship (for hole geometry and resonant frequency) was performed using Helmholtz resonator and wave equation models.

3.2 Application of laser Drilling

  • Cooling holes in turbine blades & combustors in jet aircraft engines
  • Manufacturing circuit boards and heat exchangers & many more

3.3 Overview of Laser Drilling

Laser drilling operation is the widely used & application oriented because of its unique features like high processing speeds, high dimensional accuracy, good surface quality, high degree of repeatability, exact tolerances and small dimensions can be machined & material flexibility, these factors also make the laser drilling economically viable. The thermal nature of laser drilling aims in producing holes in materials like ceramics, hardened metals & composites which is failed with conventional method. Highly accurate holes of dimensions ranging from 0.018mm to 1.3mm. With use of pulsed beam, high drilling rates can be achieved. By proper control of process variable allows rapid changes in hole diameters, hole shapes which eliminates the tool change. Drilling at high angle of incidence is also possible with laser drilling.

In general and principally the laser drilling is governed by following three factors;

  • Energy balance between the irradiating energy from the laser beam & conduction heat into the work piece.
  • Energy losses to the environment
  • Energy required for phase change in the work piece.

Material removal takes place by either melting or vaporization of the work piece to form the required hole. Additional energy losses can occur due to,

  • superheating  of molten layer above the melting point
  • partial absorption of laser energy due to plasma formation above the erosion front
  • reflection of beam energy from the work piece surface

3.4 Sensing

Generally sensors are used to monitor the process conditions and transform sensing variables such as acoustic or photon emissions into measured variable such as temperature, frequency or emissive energy. A typical sensing technique consists of sensor and sensor model. There is also another option to control the laser beam power using laser beam analyzer. The main drawback of beam control is that is do not capture any change in process parameters such as material properties, change in focal distance, plasma formation, molten layer effects & gas jet fluctuations. Better monitoring of LBM is by detection of photon emissions of various wavelengths from erosion front. The detected photon emission is then converted into measured variable through different methods such as creation erosion front temperature profile, visible light imaging of the erosion front and detection of the spark shower beneath the work piece & measurement of photon pressure on the resonator mirror. The typical control scheme of the laser machining system was shown below.

3.5 Acoustic Sensing

The another method of sensing is to measure the acoustic emission due to the interaction of a coaxial gas jet with erosion front while performing laser machining process. In this chosen case study, the acrylic machining with laser was performed. In the laser machining process the pressure waves generated by the flow of gas jet within a constrained volume create an acoustic emission with peak resonant frequency that is dependent on the erosion front size. So it is noted that by analyzing the frequency of

acoustic emission the characteristics of erosion front geometry can be determined. It is tested experimentally for three possible techniques which were discussed in chapter.

  • Hole depth detection
  • Beam break through detection
  • Monitoring surface quality

3.5.1 Concept

Laser beam is always used with co-axial or off-axis gas jet. The co-axial jet serves to protect the focussing lens from debris, to prevent plasma formation at erosion front & to provide an inert or reactive environment for laser machining, it depends on the type of gas used. The purpose of off-axis jet is to expel molten material from erosion front during laser machining of metals & ceramics. Due to resonance the acoustic emission is created while gas jet impinges on erosion front. The dynamic effects of the jet flow in erosion front region were described by 3D wave equation. Vibration of the gas within a constrained volume will result in pressure waves, which resonate at the characteristic frequency. The resonant frequency depends on cutting geometry, hence by analyzing the frequency of acoustic emission during laser machining the erosion front geometry can be determined.

The jet configuration in various laser machining operation was shown below;

3.5.2 Hole depth detection

In blind drilling process the hole depth depends on laser power, drilling time, material properties and gas jet parameters. Acoustic sensing is used to sample the resonant frequency from the interaction between the gas jet and hole and relates the frequency to the

hole depth. With this, the in-process estimation of the drilling depth and rate can be found out by integrating into depth regulation control scheme.

3.5.3 Beam break through detection

In this application the objective can be achieved by considering laser beam power and drilling time. This method is time consumable, it requires more time than time required for beam break and also wastage of beam energy through hole bottom could be seen. The acoustic sensing can be employed to detect onset of beam break through from shift in resonant frequency between blind and through hole at given depth. To be noted is that acoustic sensor can be used with integrated control scheme which varies the drilling time or laser power to stop the drilling process once beam break through occurs.

3.5.4 Monitoring of surface quality

In general laser machining is by material removal by means of melting. The re-solidified material gets accumulated at the hole wall and surface roughness gets increased, due to ejection of molten material. By acoustic sensing for increase in surface roughness the acoustic impendance of the hole gets increased with decrease in resonant frequency. Else if the hole depth was monitored with separate sensing technique then the monitoring of acoustic emission provides the estimate of surface roughness or hole taper. 

3.6 Experimental Construction & Monitoring Procedure

In subject experiment the coherent EFA-50 CO2 laser with maximum power of 625W. The entire system consists of computer - controlled work piece positioning system with two translational & one rotational degrees of freedom and acoustic sensing equipment. It has co-axial air jet of 20 psi used together with laser beam. A microphone with diameter of 1 inch and amplifier were installed to detect the acoustic emissions. The amplified signal was put into signal analyzer which transforms the acoustic emissions into frequency spectrum by Fast Fourier Transform. A frequency range between 1 kHz and 40 kHz was scanned with resolution of + 19.5 Hz to - 19.5 Hz. The sampling time for a single sweep of frequency range was 24 ms. Acoustic sensing experiments for through hole & blind drilling was performed on acrylic, aluminium oxide & 304 SS samples. The measured acoustic emissions were matched with depth measurements from optical microscopy to establish experimental relationships. Finally the experimental results were compared with theoretical estimates derived from wave equation analysis.

3.7 Results from case #1

3.7.1 Blind Hole

After measuring the acoustic emission the resonant peak exists in the frequency spectrum which is characteristic of hole geometry. For the acoustic measurement of laser drilling of aluminium oxide has resonant frequency surrounded by decreasing amplitude (between 12 kHz to 27 kHz). This region is caused by irregular surface on the hole wall due to presence of molten layer. Once the molten layer re-solidifies the rough surface was created at the wall. The resonant peak frequency is less for SS than compared to acrylic and aluminium oxide. Therefore the effect acoustic emission is more evident in laser drilling of steel.

Resonant frequency decreases with increasing hole depth for acrylic, aluminium, SS samples. From the experiment for acrylic shows that hole depth greater than 2mm, since laser drilled holes have smaller diameter than mechanically drilled holes (05.mm for laser vs. 1.4mm mechanically drilled). For aluminium and steel the resonant frequency over estimates the experiment results. This discrepancy is due to effect of molten layer on surface roughness at erosion front. The molten material is removed by expulsion by gas jet, the ineffective gas jet results in increased surface roughness at hole wall. Due to this increased surface roughness at hole wall which in turn increases the acoustic impedance for gas jet flow through the hole, finally it increases the acoustic damping and reduces the resonant frequency of the system.

There is also another discrepancy which is due to hole taper resulting from laser drilling. The theoretical assumed hole was horizontal erosion front and rectangular cross section with vertical hole walls & flat bottom. The actual drilled hole cross section shows tapered hole with sharp hole at bottom. Due to this creation it results in transverse flow and jet mixing inside the groove. The transverse flow results in greater expansion of cross sectional area of gas jet inside the hole and lower the resonant frequency compared with analytical estimation.

3.7.2 Through hole drilling

In through hole drilling the resonant frequency is higher for acrylic, aluminium & steel than the blind hole drilling at same depth. The resonant frequency decreases as the depth of the blind hole increases, beam break through occurs once the hole depth reaches the work piece thickness causing upward shift in resonant frequency. In through hole drilling the magnitude of resonance was less than that of blind drilling.

 Similar to blind drilling, for all the three samples the resonant frequency shows an asymptotic decrease with increase in hole depth. The measured frequency was also higher than blind drilling (5kHz for depth under 2mm to 10kHz for depth above 8mm). The theoretical analysis shows better results without damping for acrylic and it over estimates about 230% for aluminium oxide and steel. Same as in blind drilling the presence of molten material cause surface roughness and decrease the resonant frequency of the acoustic emission

3.8 Advantages of Acoustic sensing technique for LBM

With acoustic sensing technique for laser drilling the advantages of this technique was discussed below.

3.8.1 Measurement speed

There is sensor delay for acoustic sensing is about 10ms to 30 ms which depends on the frequency interval sampled. By implementation of fully developed dedicated hardware selective sampling (smaller frequency) & fast Fourier transform is possible. Because of this time delay between successive measurements can be reduced to below 1ms. Usually sensor models requires more time for numerical iterations, so it can be avoided.

3.8.2 Ability for area measurement

We know that acoustic sensor detects the emission over erosion front, the change in erosion front geometry results in change in acoustic emission. In photo diode based sensing technique some local variations may not be detected by the sensor since it is based on point measurement.

3.8.3 Sensor positioning flexibility

The unique feature of acoustic sensor is, it is omni-directional, the resonant frequency can be detected by sensor located at a distance from erosion front at any orientation. By this feature it can detect top-side measurement of laser blind drilling which was not possible with some image based techniques.

3.8.4 Sensor cost

The cost of acoustic sensor is cheap compared to image processing equipment or infrared vision system.

3.9 Other applications of acoustic sensor

The acoustic sensing can be used for regulation of hole depth and detection of beam break through. In depth regulation acoustic sensor can provide the feedback of hole depth estimates while drilling operation.

In beam break through the drilling time is preset in most of the industrial applications. However the drilling time may be longer than the actual time required to produce a through hole. In the additional drilling time the laser beam do not remove material from the work piece and laser energy is wasted. In order to increase material removal rate and energy efficiency of the laser drilling, proper sensing and control scheme is required and a shift frequency can be detected between a blind hole and a through hole at equivalent depth and with help of acoustic sensor the frequency shift can be monitored and beam break through can be detected. The sensor model is used to determine the nominal frequency and the controller the compares the nominal value with measured value, once the measured value is reached the process can be terminated or stopped by disengaging the power supply.

3.10 Inferences

From the subject case study it is noted that the acoustic emission technique appeared to be an effective sensing technique for in-process estimate of hole geometry (hole depth). Due to presence of molten material at erosion front the surface roughness of the wall gets increased and in turn increases the impedance of the gas jet inside the hole and reduces the both magnitude and frequency of the resonant peak. 

4.0 Case Study #2

Inductive depth sensing & controlling method and system for laser drilling

In this subject case study the control of depth of hole and accurate depth sensing at cheaper method was focussed.

4.1 Experimental

The magnetic field pick up coil is placed adjacent to front surface of the work piece. The laser beam is then applied to work piece on the point were hole is to be drilled. The laser beam causing a magnetic field in ionized plasma of particles escaping from the surface. The magnetic field is sensed by coil and depth signal is generated which represents the depth of hole being drilled. This depth signal is dependent on sensing of the magnetic field. The coil can be extended around laser beam in a stream of ionized plasma of particles. Two or more magnetic field pick up coils can be used for sensing the magnetic field. The depth of hole can be determined by comparing the depth signal with desired depth. Once the desired depth is reached the process is automatically halted by use of comparator or microprocessor. This method is suitable for materials like metal, ceramics or any other material which will generate ionized plasma of particles when struck by a laser beam.

5.0 Case Study #3

Laser welding quality monitoring with optic fiber system

In this case, the laser beam is focussed through optic fiber and collection lens. The thermal plume and plasma radiation signals are transported back to a single element photodiode detector through the collection lens, optical fiber and band pass filter. The photodiode detected signals are digitized and analyzed in PC based oscilloscope. In order to select the right band pass filter a halogen lamp on welding plate is used to obtain the spectra of the light reflected back from the plate through the mirrors and optical fiber. After analyzing the spectra by spectrometer the wavelength absorbed can be determined. At centre of the travelling distance the laser beam was focussed well on the surface. In this subject case the inference derived was the penetration depth was maximum when welding signal is similar to DC signal but the radiation signal is minimum. Hence the welding quality is good when radiation signal is like DC signal.

6.0 Proposal for improvements in online monitoring of LBM (Alternates)

(Alternate solution for challenges in chose cases) 

  • Use of high speed camera
  • Combination of optic and acoustic sensing methods
  • Closed - loop control scheme for better sensing ability
  • Use of neural networks and fuzzy logic for increased accuracy
  • Better illumination on work place

Use of camera and photodiodes for quality assurance purposes with less time delay

High speed camera can be used to capture the process parameters for demanding quality aspects. The camera was placed below the cutting table to get free from spark dust and molten material. Due to this camera arrangement the spark cone pattern observation is possible. Other elements and experimental construction can be understood from below figure.

7.0 Recent Advancements / Solution from Manufacturer

7.1 Factors Affecting Beam Quality

Global Manufacturers shows continuous research in developing online monitoring system for LBM to track the machining performance and other parameters to focus on continuous improvements or further advancements and developments. Generally, industrial lasers performance multimode operation in which deriving the beam quality is essential. Since laser manufacturing has bounced to higher performance levels, but the beam quality decides the range of difference between acceptable and unacceptable parts

7.2 Beam Quality Evaluation Technique

Sophisticated instrumentation to monitor the laser beam machining process parameters is difficult to incorporate since it has to withstand aggressive shop environments.

Electronic beam profiling is one of the methods for evaluating beam quality. The process involves measuring the spatial intensity of the beam and displays the images in two or three dimensions. However, this process does not measure the fundamental property of good beam quality. The two images shown in Fig, depicts the beam profile which appears good and does not necessarily mean that the beam focusability also is good.

7.3 Online Beam Focusability Technique

Beam focusability, in fact, is the one parameter that unambiguously determines the amount of useful energy delivered to the work surface.

The recent advances in online monitoring instrumentation allow the online beam focusability technique to be either retrofitter to an existing laser or engineered into a new laser. The whole system consist of beam - splitting device, sealed tubes, turning mirrors & analyzer / monitor module (fig 2). These are permanently mounted accessories can be mounted anywhere on laser or its mounting plate. The instrumentation is suitable for both continuous -wave (CW) and pulsed Nd:YAG lasers. This instrument samples small amount (0.4%) of the energy leaving the laser and calculates the beam focusability then the instrument computes beam focusability factor by measuring the beam width in X & Y directions and applying curve parameters. Finally the results are displayed on a screen, such that operator can monitor the real time performance of the laser (fig 3). In the view of real time, this display offers new insights to the true performance of the laser

7.4 Capabilities and Limitations

Capabilities

If laser operating parameters drift outside the acceptable range then the entire process has to get halted until the reason behind the laser's problem is identified and rectified.

In multiple laser installations, the data acquired from one laser shall be used to track the performance of the other laser, with these all laser produce the same performance when set to the same operating parameters.

Limitations:

  • The analysis is performed at the laser's outlet, so it cannot determine any malfunctions in the delivery or focusing optics.
  • Retrofitting requires that the laser be taken out of service while the beam-splitting optics is installed.
  • This instrumentation is costly to install than non-electronic equipment.

8.0 Discussion from all three 3 cases - Efficiency, Effectiveness & accuracy

From case#1

  • For laser drilling of acrylic materials results with 18% deviation from the theoretical analysis for blind drilling and for aluminium oxide and steel the theortical estimates 300% than actual, in case of blind drilling
  • Similarly the theoretical estimates shows 20% of optical measured value of hole depth for acrylic in case of through hole drilling and for aluminium oxide the theortical value over estimates by 230% than actual.
  • In general, for laser drilling of acrylic like materials usually have theortical results of 25% within experimental results.
  • The discrepancy and deviations in theortical and actual values was due to effect molten metal at erosion front (increased surface roughness at hole wall) and hole taper
  • With use of proper control scheme the entire system can be down to halt position once the operation is completed.

From case#2

The use of magnetic field in laser drilling was discussed and hole depth regulation methodologies were derived with various control schemes. The method seems accepting the industrial requirements, such as the amount of material vaporized can be determined indirectly. The proper control over the process was possible and both manual drilling of individual holes and automated drilling with microprocessor and robotic movements is possible with the subject control scheme. Similar to case#1 the entire process can be halted by microprocessor once the operation was completed. Over all the accuracy and effectiveness seems increased than case#1, unless without the experiment results from case#2 the exact accuracy, effectiveness and efficiency cannot be quoted.

From case#3

Here the radiation signals are more stable & smaller, the welding depth & frequency fluctuation are much stronger when welding is shallow. Since this system uses optic fiber with the proper illumination & band pass filter selection is major criteria. Also welding pool is deeper the radiation signals were not reflected back properly and for better results the welded samples has to be cut down for examination. Use of high speed camera was one of most promising technique to enhance the system performance. Therefore FFT analysis of radiation signals for quantitative analysis these fluctuations are required. Proper closed - loop control scheme is required to improve the overall performance in terms of efficiency, accuracy & effectiveness for obtaining proper laser welding quality.

References

[1] P. Sheng, G. Chryssolouris, "Investigation of acoustic sensing for laser machining processes, Part:1 laser drilling", M.I.T, Univeristy of California, USA, October 20, 1993

[2] Rudolph A.A.Koegl, Richard A.Hogle, Susan D. Bauer, "Inductive depth sensing and controlling method and system for laser drilling", General Electric Company, NY, USA, October 22, 1991 (US Patent)

[3] Teresa Sibillano, Antonio Ancona, Vincenzo Berardi & Pietro Mario Lugara, "A real time spectroscopic sensor for monitoring laser welding processes"CNR-INFM Regional Laboratory, Italy, May 7, 2009.

[4] Dohyoung Kim, Jin-Tae Kim, "Laser welding quality monitoring with an optical fiber system", Chosun University, KOREA, March 25, 2003.

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