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Smoke Testing Aluminum Welding Wire for Surface Contamination
What Contributes to Weld Porosity?
Weld porosity results from the entrapment of hydrogen gas. This gas entrapment results in lower weld strength and ductility by reducing the crosssectional area of sound metal and by acting as stress risers which cause premature failure. Several variables can produce gas porosity, one of which is the surface condition of the aluminum filler wire. The qualities relating to the surface characteristics of the filler wire include:
· The removal of surface oxides (hydrated oxides).
· The absence of any water or water vapor.
· The removal of hydrogen-containing compounds(hydrocarbons).
Of these three surface conditions, the most common cause of weld porosity is the presence of hydrocarbons. Examples of these compounds include residual wire drawing lubricants, mill dirt or even fingerprints. One relatively quick and inexpensive method of testing aluminum welding wire for freedom from residual hydrocarbons is by means of a “Smoke Test".
What Is A Smoke Test?
The “Smoke Test" is a qualitative test performed by heating a sample of wire using an electrical resistance heating machine. While conducting the test, the wire is visually examined for the presence of smoke, caused by the burning of any surface contamination. Minute amounts of contamination, even a fingerprint, will result in smoke. The schematic shows a typical smoke tester machine(1) . Just about any commercial welding power source will suffice. The weld cables are connected to two hobby vises. The wire sample completes the circuit. A light with a dark viewing background is recommended to aid in observing any smoke as the test is performed. Care must be taken in selecting and placing the sample in the vise grips so that the wire does not come in contact with any contamination, including human hands.
CAUTION: Do not touch the wire after testing since it becomes extremely hot.
The schematic also details typical amperage settings based upon the alloy and diameter of the sample to be tested. The amperage is chosen to control the melt rate of the sample and allow adequate time to detect the presence of any smoke. The amperage should be sufficient to melt the sample in 3 to 5 seconds.
SUGGESTED AMPERAGE SETTINGS
BY ALLOY SERIES
Sizes 2XXX 4XXX 5XXX
.030 45 40 40
.035 50 50 50
.040 60 60 60
.047 90 90 70
.062 140 120 120
.094 225 225 225
What Can I Interpret From the Smoke Test?
A direct correlation exists between smoke test results and weld porosity. Zero smoke should indicate minimal weld porosity. A small amount of smoke will indicate some evidence of weld porosity generated by contamination. A large amount of smoke will indicate severe contamination and the filler wire should be further examined before continuing production welding.
Design, technical reference charts and other information presented herein have been researched for accuracy, however, are givenwithout expressed or implied warranty, guarantee or responsibility of any kind on the part of AlcoTec.
The "Twist Test" can be conducted on spooled or straight length wire to show the presence of surface imperfections. It can also be done on electrode which has passed through the welding equipment. A sample taken after going through the welding gun may show abrasion from drive rolls, guides, or the contact tip. The abrasions create fines, which in turn plug the liner and contact tip, and create burnbacks.
To check the wire for surface abrasions, take a 10-inch sample and put a 90° bend 2 inches from each end. Twist one end 5 or 6 times afull 360°.
The surface of the twisted sample should stay smooth. Check for lines by dragging a finger nail down the length of the twisted portion. Metallic fines (aluminum dust) may also be present if the wire surface has been disturbed. AlcoTec routinely conducts twist tests on wire to ensure surface smoothness and freedom from surface imperfections.