From the Secret Formula Vault
The “Solution” for Cleaning the “Super Metal” Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Demand for its use in aerospace and automotive applications has seen a dramatic rise over the past decade, as its high strength and light weight make it very desirable for structural and suspension components. Additionally, its low CTE (coefficient of thermal expansion) of approximately 5 ppm/°F makes it the material of choice for joining composite panels, which also have quite low CTE — making aluminum completely unsuitable for this application (CTE of 13ppm/°F).
Titanium comes with its own set of challenges. It is expensive, difficult to machine, and to the surprise of many test engineers, has a surface to which it is very difficult to bond a strain gage. So, what makes titanium such a challenge for strain gage bonding?
- Titanium grows surface oxide very quickly.
- The porous nature of the material allows it to readily absorb machine oils and other contaminates.
Surface-abrading titanium will remove the oxide but will just drive the other contaminates deeper into the metal. These contaminates migrate out over time, thus compromising the strain gage bond or causing it to completely fail. Halogens should never be used for cleaning titanium, as they can lead to hydrogen embrittlement.
Heating titanium is required to drive the contamination to the surface so that it may be removed by subsequent cleaning steps. It typically takes several repetitions of heating and cleaning for the surface to be sufficiently clean for gage bonding, after which grit blasting or hand sanding can be performed to achieve the required surface roughness. Conventional surface preparation techniques for strain gage bonding using isopropyl alcohol, conditioner, and neutralizer will often suffice, depending upon the degree of contamination; however, in this author’s experience, conventional cleaning techniques are successful in about 75 % of cases.
The “Solution” Solution
Sometimes the solution to a challenge is using the proper “solution.” In this case, it may be as simple as looking under your kitchen sink or making a trip to the local supermarket. What is this secret solution? Lemon Joy dishwashing liquid; a simple solution consisting of one ounce of dishwashing detergent per one pint of water will do the trick.
Heat the titanium part to 350 °F for thirty minutes, cool to room temperature, clean with the detergent solution, rinse with water, and repeat the heating and cleaning process four times. After the fourth cleaning cycle, abrade the surface, perform a final cleaning with isopropyl alcohol, and then bond the strain gages.
Remember that titanium grows surface oxidation very quickly. Ideally, the gages should be bonded 10 to 20 minutes after the last cleaning step.
“P.S. — The above process is applicable to foil strain gages, but there is no guarantee for strain-gauges.”
To learn more about Surface Preparation for Strain Gage Bonding, please read Micro-Measurements Instruction Bulletin B-129.
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