AMP_06_September_2021

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 1 5 0 H eat treating certain metals can lead to the formation of abnormal crystals called dendrites. For example, when an ingot is drawn through a die to produce a bar with a specific diameter, the heat treatment conditions may be changed, causing a difference in recrystallization between the center and the peripheral area of the bar. The result is that the grain size of the crystals in the center remains large and becomes solidified. These dendrites can cause appear- ance flaws during surface treatment (one of the final steps in themanufacturing parts processes) or result in fatigue cracks when stress is concentrated (Fig. 1). DIFFERENTIAL INTERFERENCE MICROSCOPY A technique for inspecting the surface of these heat-treated parts is differential interference microsco- py. The DSX1000 digital microscope comes with fully mo- torized and encoded brightfield, darkfield, oblique, MIX (brightfield and darkfield), polarized light, and, of par- ticular relevance, true Nomarski differential interference contrast (DIC). Combining these techniques allows for significantly different visualization of the sample. While brightfield light reflects directly from the sample back to the camera to maximize resolution, modifying the bright- field light using polarizer, analyzer, and Nomarski DIC prism, it is possible to visualize submicron differences in surface topography by utilizing constructive and destruc- tive interference of light. Controlling these with motoriza- tion allows repeatability of the optical shear on the Nomar- ski prism for repeatable inspection results. The DSX1000 digital microscopes have a standard ex- tended focal image (EFI) function. EFI enables the user to utilize the fine focus adjustment to combine many imag- es at different Z-levels to create a single, combined image that is completely in focus. With EFI, users can easily focus on any area, even for a sample whose entire surface cannot be focused because of fine surface irregularities caused by etching. In addition, the microscope’s high dynamic range makes it easy to optimize contrast, simplifying particle profilemeasurements. These features enable users to eval- uate the presence and size of dendrites in aluminum alloy parts. The DSX1000 microscope comes equipped with most of the functions necessary for efficient metal struc- ture inspection, so users can select the best image gener- ated using different techniques. CAPABILITIES Using DIC enables the observation of fine surface changes in crystals, surface finishes, and thin films that otherwise wouldn’t be visualized. Conventional micro- scopes and other less sophisticated digital microscopes on the market use manual components to achieve DIC, meaning there is no way to reliably repeat an inspection to visualize specific flaws. Another advantage of DIC is that it can make surface variations visible that would otherwise require the use of a secondary piece of equipment, such as a laser confocal mi- croscope or interferometer, at a fraction of the cost or com- plexity (Figs. 2 and 3). However, it’s important to remem- ber that while this technique can be used to measure large features for approximate dimensional inspection, DIC is a visualization technique that fundamentally changes the appearance of the sample, making fine detailed measure- ment unreliable. OPERATION DIC is an optical technique, so assuming the part will fit under the microscope, the only interaction with it is white light. However, in the case of heat treated alumi- nums, there are two approaches: the first is to cut, pol- ish, and etch the surface to visualize grain structure and dendrites, so naturally this would classify as a destructive technique. However, the second relates to surface finish, DETECTING FLAWS IN ALUMINUM-ALLOY PARTS A microscope using differential interference contrast inspects grains and inclusions on heat treated aluminum parts to help predict the performance of the metal. 9 T CHNICAL SPOTLIGHT Fig. 1 — The effect of dendrites on the appearance of a treated surface. 12