A New Insight on Phased Array Ultrasound Inspection in MIG/MAG Welding

Abstract

Weldment inspection is a critical process in the metal industry. It is first conducted visually, then manually and finally using instrumental techniques such as ultrasound. We made one hundred metal inert/active gas (MIG/MAG) weldments on plates of naval steel S275JR+N with no defects, and inducing pores, slag intrusion and cracks. With the objective of the three-dimensional reconstruction of the welding defects, phased array ultrasound inspections were carried out. Error-free weldment probes were used to provide the noise level. The results can be summarized as follows. (i) The top view obtained from the phased array provided no conclusive information about the welding defects. The values of the echo amplitudes were about 70 mV for pores and cracks, and greater than 150 mV for slag intrusion, all of which showed great variability. (ii) The sectional data did not lie at the same depths and they needed to be interpolated. (iii) The interpolated sectional views, or C-scans, allowed the computation of top views at any depth, as well as the three-dimensional reconstruction of the defects. (iv) The use of the simplest tool, consisting of the frequency histogram and its statistical moments, was sufficient to classify the defects. The mean echo amplitudes were 33 mV for pores, 72.16 mV for slag intrusion and 43.19 mV for cracks, with standard deviations of 8.84 mV, 24.64 mV and 12.39 mV, respectively. These findings represent the first step in the automatic classification of welding defects.

Keywords: MIG/MAG welding; digital images; phased array; ultrasound inspection.

Figure 1

(a) Schematic cross-section of the welding probe; (b) a finished probe, ready for inspection at the workshop of the School of Naval and Ocean Engineering facilities in the University of Cádiz.

Figure 2

Geometry of the inspection process (see text for explanation).

Figure 3

Top view for three cases of probes (G) and the cases of pores (F1), slag intrusion (F2) and cracks (F3). The x–axis is the distance from the left side and along the joint in millimeters and the y–axis is the transversal distance from the lower limit of the plate. Distances are in millimeters and echoes in millivolts.

Figure 4

C–scan of the F3–13 probe (crack) from slice 139 to 154, every two millimeters. Echo amplitude is expressed in millivolts and distances in millimeters.

Figure 5

Horizontal top views of the F3–13 probe for the depths of 3.475 mm to 4.35 mm every 0.15 mm.

Figure 6

Three-dimensional reconstruction of (a) a pore (F1–11), (b) slag intrusion (F2–02) and (c) crack (F3–13). The three-dimensional reconstructions are on the left and the corresponding histograms are on the right.