Surface Requirements of Aluminum Castings
Automotive manufacturers widely utilize aluminum castings as the most successful way to create aluminum parts. However, this process can involve inorganic contaminants on the surface which interfere with potential bonding, sealing, or coating. BTG Labs conducted a surface analysis test for an automotive company by measuring contact angle with the Surface Analyst™ to determine the condition and cleanliness level of their aluminum castings.
Aluminum castings can sometimes entail silicone mold release or conversion coating which spells disaster for bonding, sealing, or coating. So the surface preparation processes of these castings plays a pivotal role in the success of any adhesion. For this analysis, BTG took surface measurements of the company’s machined engine front engine cover sealing surfaces in 10 different locations approximately 1.5″ apart. BTG measured in three different states: as received; after multiple isopropanol wipes; and after multiple hexane wipes. They also measured after treatment with 220-grit aluminum abrasive paper.
Knowing if the Surface Treatment Process Actually Works
The as received surface produced inconsistent contact angle variations. The isopropanol and hexane wipes both produced more consistent but high contact angles. The isopropanol produced an average contact angle of 80° and the hexane produced an average contact angle of 85°. These high contact angles are consistent with silicones or flurocarbons. Thus, the wipes removed the upper layers of contaminants to reveal a surface consistent with silicones or flurocarbons often found in conversion coating and mold release.
It wasn’t until a treatment with the 220-grit aluminum abrasive paper that the contaminants–organic and inorganic–were properly removed to reveal a contact angle of 11°. This demonstrates the elusiveness of contaminants which can be both organic and inorganic and layered. The wipes removed the organic contaminants only to reveal the more threatening inorganic contaminants which sometimes occur as a result of machining. Without the Surface Analyst, these contaminants may have remained undisclosed. Surface processes differ in their style and requirements; these tests emphasize the importance of precise surface monitoring via the Surface Analyst.