Optical Surface Roughness Measurements Research Articles

Material Characterization of Hybrid Components Manufactured by Laser-Based Directed Energy Deposition on Sheet Metal Substrates

Laser-based directed energy deposition (DED-LB/M) allows the application of a wear-resistant metal coating to the surface of a sheet metal substrate. Subsequent deep drawing of the part enables high material efficiency, significantly shorter production times, and lower unit costs compared to, for example, solely machined production of the entire component. At the same time, energy-intensive global heat treatment strategies can be avoided. For the numerical analysis of such hybrid process chains, both the sheet metal substrate and the additively applied coating are usually characterized individually. However, the low thickness of the coating in combination with a relatively high welding depth, which is required for a good bond to the sheet metal substrate during subsequent forming, lead to a strong gradient of the mechanical properties as well as complex mechanical interactions in the bonding zone. Therefore, appropriate characterization methods are required. In this work, an investigation of different influencing factors, like the rolling, hatch and tensile direction, is carried out with the aid of tensile tests using hybrid specimens. In this way, interactions between the influencing factors are identified. As substrate, 3.5 mm thick 16MnCr5 blanks are used, with an approximately 0.68 mm thick coating of Bainidur® AM. In addition, an optical surface roughness measurement and metallographic analysis of the tensile specimen’s edge area after laser cutting is performed.

Improvement of Part Accuracy by Combination of Pulsed Wave (PW) and Continuous Wave (CW) Laser Powder Bed Fusion

In this paper, a processing strategy is investigated to increase the geometric accuracy of parts fabricated by laser powder bed fusion (LPBF). Pulsed wave emission (pw) is used for contour exposure in combination with a continuous wave (cw) emission for bulk volume exposure. With the goal of discrete solidification of adjacent melt pools, process parameters of laser power PL, scanning speed vs, and relative pulse overlap ∆xo are developed for contour exposure. Samples with variable contour angles are fabricated to investigate the effect of the pw contour exposure on excessive melting in critical areas of the part prone to overheating. Geometric accuracy and surface roughness are evaluated using SEM images of surface topography and optical surface roughness measurements, respectively. It is observed that excessive melting can be suppressed by using pw contour exposure and usage of modified process parameters. Due to the discretized energy input in pw emission mode, smaller melt pools with lower melt pool fluctuation and powder erosion are produced. The maximum applicable scanning speed is limited by the solidification time of the melt pool and is significantly lower compared to conventional cw contour exposure parameters. Therefore, a combination of cw volume exposure with high process productivity and pw contour exposure for high geometric accuracy is beneficial to limit productivity losses and increase accuracy in part building.

Prediction Modeling of Surface Roughness Using Capacitive Sensing Technique in Machining Process

The surface roughness measurement of machined specimen is an unproductive time in industries. The contact profile measurement takes more time for its measurements than other measurements. The non-contact optical surface roughness measurement techniques are faster with considerable accuracy. However, it requires the knowledge about surface orientation. In this paper, surfaces obtained from the turning, grinding, and vertical milling were modeled based on the surface data measured using contact type profilometer. The sensor and surface model has been developed using real sensor with 0.5-mm sensing diameter and surface data in ANSYS Maxwell. The roughness values measured using capacitive sensor ( ${\text{R}}_{c}$ ) and stylus profilometer ( ${\text{R}}_{a}$ ) have been compared to find correlation coefficient at the standoff distance of 20, 25, and 50 micron. The capacitive sensor with standoff distance of $50~\mu \text{m}$ gives highest correlation coefficient for surface obtain from turning, grinding, and vertical milling. The proposed method can provide better non-contact faster online measurement.

Comparison of envelope detection techniques in coherence scanning interferometry.

The aim of this work is to make a comparison of the most current signal processing techniques used to analyze the fringe signal in coherence scanning interferometry (CSI), a major technique for optical surface roughness measurements. We focus here on classical AM-FM signal-processing algorithms such as the Hilbert transform (HT), the five-sample adaptive (FSA), and the continuous wavelet transform (CWT). We have recently also introduced a new family of compact and robust algorithms using the Teager-Kaiser energy operator (TKEO). We propose an improved version of TKEO using a combination of different techniques of pre-filtering and demodulation processing to remove the noise and offset component and to retrieve the fringe envelope to either determine the surface height information or to separate adjacent transparent layers. In particular, as a pre-filtering approach, we have focused on empirical mode decomposition in combination with the Savitzky-Golay filter. An added Gaussian post-filtering is helpful for a precise peak extraction. The experimental results show that TKEO performs better than CWT in terms of computation time and provides a better surface extraction than HT and FSA. Results have been obtained on synthetic and real data taken from a layer of resin on a silicon substrate.

Determination of cutting parameters in end milling operation based on the optical surface roughness measurement

In this study, a milling system based on the in-line surface roughness measurement during machining process is developed using Artificial Neural Network (ANN) technique. In the proposed system, optimum feed rate and cutting speed are determined by ANN so as to provide the desired surface roughness, which is an important criterion for high quality surface. For this purpose, firstly an algorithm determining the operating principle of the system is developed. According to this algorithm, the optimum cutting parameters are predicted for end milling (finishing) operation by measuring semi-finish machining surface roughness via an optical sensor and then end milling operation is performed with the cutting parameters determined by the system. In the experimental part of this study, surface quality is observed for the milling process before and after the intervention of the system and the results is compared. The experimental results show that the system can be integrated with the modern machining systems in order to obtain the desired surface quality levels.

Non contact inspection of the fatigue damage state of carbon fiber reinforced polymer by optical surface roughness measurements

This work presents the evaluation of a new non-contact technique to assess the fatigue damage state of CFRP structures by measuring surface roughness parameters. Surface roughness and stiffness degradation have been measured in CFRP coupons cycled with constant amplitude loads, and a Pearson׳s correlation of 0.79 was obtained between both variables. Results suggest that changes on the surface roughness measured in strategic zones of components made of the evaluated CFRP, could be indicative of the level of damage due to fatigue loads. This methodology could be useful for other FRP due to similarities in the fatigue damage process.

Rapid optical measurement of surface roughness of polycrystalline thin films

Excimer laser crystallization is the most commonly employed technology for fabricating low temperature polycrystalline silicon (LTPS) thin films. Investigations on the surface roughness of polycrystalline silicon (poly-Si) thin films become an important issue because the surface roughness of poly-Si film is widely believed to be related to its electrical characteristics. A simple optical measurement system for rapid surface roughness measurement of poly-Si thin films is developed in this study. It is found that y=−1.4533 x+0.6932 is a trend equation for predicting the surface roughness of poly-Si thin films. The x and y represent the peak power density and the surface roughness of poly-Si thin films, respectively. The incident angle of 60° is a good candidate for measuring the surface roughness of poly-Si thin films. The surface roughness of poly-Si thin films ( y) can be directly determined from the peak power density ( x). The maximum measurement error rate of the optical measurement system developed is less than 2.1%. The savings in measurement time of the surface roughness of poly-Si thin films is up to 83%.

Rapid optical measurement of surface roughness of BiFeO3 films for nonvolatile memory application

An optical inspection system for rapid surface roughness measurement of BiFeO3 (BFO) thin films is developed. It is found that y = −121.45 x + 212.81 is a trend equation for characterizing the surface roughness of BFO thin films. The incident angle of 60◦ is a good candidate for measuring the surface roughness of BFO thin films. The maximum measurement error rate of the optical inspection system developed is less than 2.6%. The savings in inspection time of the surface roughness of BiFeO3 thin films is up to 90%.

Surface roughness measurement using an optical system

In recent years there have been a number of research studies carried out into optical surface roughness measurement, since the optical method has several advantages over the mechanical measuring system. In the present study, an optical method relying on the reflected beam intensity profile is introduced. An He–Ne laser beam is used to scan the surface and a fibre optic probe is employed to collect the reflected beam from the workpiece surface. The reflected beam intensity distribution is approximated by a Gaussian function. Since the data collection is sampled every 1 ms, a considerable number of profiles corresponding to each point at the workpiece surface are collected. However, for a known surface, there exists a unique average surface roughness value. Therefore, the data collected for each profile should be combined to produce a profile that represents the average surface roughness value for that particular surface. Consequently, the reflected beam intensity profile is processed using two different methods, which are: (i) averaging the data points in the intensity profile before introducing the curve-fitting technique; and (ii) introducing curve fitting to obtain the constants for the Gaussian function for each intensity profile, then averaging the constants over the number of profiles. The surface roughness profiles and average surface roughness value (Ra) is measured initially using a Bendix surface proficoder. The relationship between Ra values and the standard deviation (S.D.) of Gaussian function (SDGF) is developed. The study is extended to include uncertainty analysis and standard estimate of errors in the measurement system. It is found that the first method gives an improved standard estimate of error. Moreover, a linear relationship exists between the Ra values and the SDGF of the reflected beam intensity, the greater are the SDGF values the greater is the surface roughness. However, the measurement is limited to a certain range of Ra values; in this case, the accuracy of the measurement drops considerably as the Ra value increases beyond 1 μm.

Error Reduction Method for Optical Surface Roughness Measurement System.

Circular S-polarized light is proposed to reduce error in an optical surface roughness measurement system. Theoretical analysis of the focused circular S-polarized light shows that as the circular aperture ratio increases, 1) the spot size decreases, 2) the focal depth increases, and 3) the first-order defracted light increases. The measurement accuracy using the circular S-polarized light is experimentally evaluated. Circular S-polarized light is separated from randomly polarized light using a calcite crystal with birefringence. A nanometer-order step surface is then fabricated by sputtering carbon graphite on a glass plate. The step heights are measured mechanically with a stylus, and are used as standard measurements. The same step heights are also measured optically using randomly polarized light and circular S-polarized light. Comparison of the latter two measurements with the standard measurements shows that measurement error is reduced more with circular S-polarized light than with randomly polarized light.

Practical and theoretical aspects of applied image analysis

It is demonstrated how special industrial measurement tasks can be solved by Image Analysis Systems. Software for Vision Systems is discussed. Especially the Image Analysis Software Library developed at the Fraunhofer Institute for Manufacturing, Engineering and Automation (IPA) is presented and compared with other libraries. Some examples are given concerning scale invariant pattern recognition as well as the description of structures by their fractal dimension. The concept of fractal dimension is applied to optical surface roughness measurements. First results are very surprising and it is suggested to increase the efforts to introduce this concept in wider fields of measurement.

High-Speed Surface Roughness Measurement

A prototype apparatus is described that performs a real-time optical measurement of surface roughness in the 2 to 20 microinch AA range, typical of high-quality grinding, honing, and lapping. Ground metal manufactured parts, with surface roughness in the stated range, are illuminated with a 3.39 micron helium-neon laser. The scattering pattern from the surface is scanned across a linear array of lead selenide detectors. Analog and digital electronics control the scanning and measure the intensity incident on each detector. The electronics computes both reflected (specular) and total incident intensity. These two values are used to compute the surface roughness. The system completes a scan and the electronics computes surface roughness in 20 milliseconds. The fast measurement time allows a large area to be scanned. The results demonstrate the feasibility of using the apparatus for online inspection.