Ultrasonic inspection of rough surface aluminium die castings

The condition of the surface through which a sound beam enters a material is an important factor in ultrasonic non-destructive testing(1). Detection of discontinuities such as cracks, voids and porosities in castings is greatly affected by the extent of roughness. Increased roughness reduces the transmitted energy of the sound beam and this, in turn, reduces the amplitude of the received signal, leading to difficultly in measuring the size of the discontinuity(2). The measurement of back-wall echo amplitude provides a better understanding of the effects of material characteristics on signal attenuation. Appreciation of the problem has led to a large number of studies investigating various aspects of material characteristics on ultrasonic test signals(3-9). From the literature, it is evident that the results obtained were based on theoretical or model-based studies(9-18). Thavasimuthu et al(9) investigated the effect of front surface roughness on the ultrasonic signal amplitude in samples with various discontinuities using ultrasonic contact testing. They have concluded that the increase in surface roughness results in loss of ultrasonic signal amplitude and selection of equipment and probes are dependent on surface roughness and defects to be inspected. A similar study was carried out by Bilgen(10) in his doctoral research work on the effect of surface roughness on ultrasonic immersion testing in detail using a theoretical model. According to Bilgen(10), only the coherent part of the ultrasonic wave field participates in focusing and it dominates the backscattered signal. Hence, in the focal region, the signal-to-noise ratio is relatively unaffected by surface roughness, as the signal and noise are altered in the same manner. However, for a surface roughness above 25 μm, the coherent part of the ultrasonic beam becomes negligible even in the focal area and a high loss in the signal-to-noise ratio results(10). Focusing changes the backscattered signal and consequently attention must be given to select a suitable ultrasonic focus probe in order to accommodate the surface roughness variation. Rose et al(12) used an ultrasonic NDT method to identify gas porosity defects in aluminium alloy castings. They also investigated the effects of surface roughness on ultrasonic signals from the castings. Their study concentrated on quantitative assessment of gas porosity defects in die casting aluminium materials of plate-like geometries. Similarly, Adler et al(13) investigated porosity defects in aluminium cast materials, and used volumetric analysis to identify gas porosity defects. They studied the effects of backscatter in their work on the ultrasonic inspection of aluminium cast materials. Their theoretical analysis of the attenuation ratio indicated that it was independent of frequency or surface roughness up to 40 μm root mean square value (ie, equal to 36 μm Ra). An extensive literature search has indicated that an ultrasonic inspection system has not been successfully developed to inspect aluminium die castings with surface roughness above 50 μm for sub-surface defects. The lack of previous research for this particular application may be due to the nature of castings and the sensitivity of ultrasound. In this particular research, it was important to inspect the casting in the as-cast state (with surface roughness between 50 μm and 150 μm) because further processing (ie, machining) weakens the justification for a non-destructive testing application. Hence, there is a need to determine the limitations of ultrasonic inspection of castings with surface roughness values greater than 50 μm. The initial intention was to obtain ultrasonic signals with the maximum possible amplitude from defects within the rough surface areas of the castings.