In the production of Aluminum
pistons for high load motors, a
ring support made of gray cast
iron is casted. The bonding between this ring support and the
rest of the piston material is subjected to a nondestructive ultrasonic test.
There are two methods of testing:
- Through-transmission testing with groove probes (Fig. 1 left).
- Testing with straight beam probes using angular scanning and surface waves (Fig.1 right).
Today both methods are used but give different results. Testing with groove probes produce indications about the quality of the bond within the groove whilst the second method (angular scanning) is better for detection of bonding flaws which propagate from the surface. In addition to a bonding test between the upper flank and the lower flank, a straight beam probe tests the bonding at the back of the ring support using the pulse reflection method.
Due to the fact that the production rate for pistons (appr. 500 pieces per hour) is high, a demand was soon made for automatic testing.
An automatic testing machine was developed which was integrated into the production line,making automatic operation possible.
The testing machine operates with groove probes, but can also operate with straight beam probes using angular scanning. These two methods can also be applied simultaneously.
The testing machine (Fig. 2) comprises the following components:
Fig.2: The testing machine.
Test mechanism with input and output conveyors Cabinet for equipment and evaluation, with a 8 channel ultrasonic testing system, evaluation computer (PC compatible) as well as the control. The test sequence is as follows: The pistons are fed to the test station via a cycled conveyor and lowered into a tank with a double grip system (Fig. 3). After reaching the test position (Fig. 4) the groove probes are swung in, the pistons are then rotated, and the test starts.
Fig.3: Conveyor
and a tank with a double grip system (21k)
Fig.4:Test position (13k)
During one rotation of the piston, 1 000 measurement pulses are triggered. This means for every 0.36 degrees there will be a pulse.
Each test result is transferred to the computer and stored. After this, an evaluation is carried out according to pre adjusted evaluation criteria:
- Length and number of the individual flaws
- Length of the flaw summation
- Overlapping of bonding flaws between the upper flank and the lower flank
Depending on the test results, the piston is then classified into one of two groups, ACCEPTor REJECT. The computer acquires the number of pistons which have been tested, separated into ACCEPT and REJECT, and, if required, produces a displayed report. The circumference of the piston with a display of the flaw areas can be printed. After the ultrasonic test, which takes approximately 8 seconds per piston, sorting is made and the piston is transported further.