The optical geometry characterization of wrought hot components can help to quantify material distortion effects during air-cooling. The component's shrinkage behavior is affected by inhomogeneous heat dissipation due to the object's complex geometry and - in case of hybrid materials - differing thermal expansion coefficients. As optical triangulation techniques rely on the rectilinear expansion of light, the hot component's heat input into the surrounding medium air influences the reachable accuracy of optical geometry measurements due to an inhomogeneous refractive index field around the hot component. In previous work, the authors identified low pressure measurements in air as a possible approach to reduce the magnitude and expansion of the inhomogeneous refractive index field above cylindrical high-temperature objects and thereby allow precise geometry acquisition. We now present experimental data of the 2D refractive index field above a hot cylinder in different pressure scenarios using the well-known background oriented schlieren (BOS) method in order to illustrate the decrease in refractive index variations dependent on the pressure state. For this purpose, a ceramic rod is placed in a vacuum chamber and heated up to temperatures of about 1000°C. Using a monochromatic camera, a wavelet background and an optical ow algorithm, the developing 2D refractive index field for a low pressure scenario is compared to ambient pressure conditions. The experimental data illustrates a reduction in the convective heat flow above the hot heating rod at lower pressure values and therefore a homogenization of the density-coupled refractive index in air, validating former simulation results.
We present a fringe projection system to measure glowing hot hybrid components in between production processes. For this a high power green light projector, based on TI DLP technology, is used to create the highest possible contrast between fringes on the red glowing specimen. It has a resolution of 1140 x 912 pixels with a maximum frame rate of 120 images per second for fast measurement. We use a green bandpass filter (525 nm) on the camera lens to block unwanted incoming radiation from the specimen caused by self-emission. Commercial measurement standards are not calibrated for temperatures other than 20° C, so they cannot be used to validate measurement data at the required temperatures of up to 1000°C since thermal expansion invalidates the geometry specification from the calibration data sheet. In our first development we use a uniformly heated pipe made of stainless steel as a dummy specimen to examine the measured geometry data. A pyrometer measures the temperature of the pipe so the expansion can be easily calculated using the thermal expansion coefficient. Different impact and triangulation angles are investigated to identify the effects of hot ambient air on the measurement. The impact of the induced refractive index gradient is examined to check the need for pre-processing steps in the measurement routine.
In the manufacturing process of Tailored Forming components, the inline inspection of the joining zone directly after each single process step can yield advantages - such as early error detection and real-time process control. Since measuring times need to be synchronized with the production chain, there is no time to cool down the components in between two hot forming processes. On the one hand, the chosen measurement technique needs to be non-tactile due to the heat of the measurement object. On the other hand, the object's areal surface texture needs to be captured rapidly to realize a fast inline inspection. These requirements are only matched by optical 3d measurement systems. Additional challenges arise due to the high temperature of the Tailored Forming components: the ambient air is heated up and the air's temperature increase results in an inhomogeneous refractive index field surrounding the hot workpiece, effecting the light's path emitted by the illumination unit of the optical sensor. We present a simple measurement setup based on the laser light section method to visualize the measurement accuracy loss induced by the convectional heat flow from a hot cylindrical measurement object. To attain a direct validation of the measurement results, the measurements are performed with and with reduced influence of the inhomogeneous refractive index field induced by the hot object.