Cavitation is common in interaction between a moving fluid and a gas to form a negative pressure (emptied hole) in fluid. When a negative pressure bubble encounters a solid surface during movement or flow, bubble annihilation releases a huge burst of energy so that solid surface forms a concave hole. This phenomenon is common in blades used in seawater. Surface of blades forms negative pressure bubbles due to interaction of water and swirling air. It annihilates and forms cavitation pits on the surface of blades, which is commonly referred to as "pits".
Cavitation corrosion also occurs during production of aluminum alloy high-pressure die-casting. For die-casting aluminum alloy appearance parts products, such as communication products, sometimes surface of mold forms a recessed pit (also known as "pitting") after production of hundreds or thousands of products. This is also caused by cavitation. Corresponding pits will be produced on the surface of aluminum alloy parts. Macro shape is shown in Figure 1.

 

Fig. 1 "cavitation" leads to bulging spots on the surface of aluminum alloy die castings

It should be noted that in addition to cause of "cavitation" formed on the surface of aluminum alloy die castings, there are other reasons, such as incomplete grinding of mold surface after electrical discharge machining, or mold surface is too bright, which makes release agent unable to form a good protective film on the surface of mold, resulting in spot-like dissolution and mold sticking on the surface of mold. However, pitting caused by "cavitation" on mold surface is morphologically different from this type of defect, sufficient experience is required to make a correct judgment.

Aluminum alloy die castings is a high-speed injection of liquid aluminum alloy into entire cavity of mold from punch to obtain desired product. During flow of aluminum liquid, it needs to pass through runner to reach gate and then spray into mold cavity. Runner of mold is necessary path for molten aluminum to flow. Design principle is that cross-sectional area of runner should be in a contracted state, so that during flow of molten aluminum, air in runner can be discharged to front end of molten aluminum.
Main cause of cavitation is violation of above design principles in mold design. On the path of runner through which aluminum alloy liquid flows, cross-sectional area of runner has an expansion phenomenon. When molten aluminum flows through expanding inner runner, there will be volume expansion and a sudden drop in pressure, resulting in formation of negative pressure (or emptied) holes in molten aluminum. Cavity collapses inside runner and mold cavity during injection pressurization. When burst, an internal blast is formed, which can generate huge energy. During production, continuous internal blasting causes fatigue deformation of surface of metal mold or runner, forming pits visible to naked eye. Corresponding pits are formed on the surface of casting product. As shown in Figure 1 and Figure 2.

 

Fig. 2 Cavitation causes protruding "pits" in casting and runner

Figure 3. Scanning electron micrograph of local cavitation produced on mold surface after 300 products were produced. It can be seen that when surface of mold is destroyed by negative pressure bubbles in aluminum alloy liquid, surface of metal mold is deformed due to repeated inward blasting. Energy released when negative pressure bubble is annihilated destroys surface of mold, and a concave hole is formed on the surface of mold. Figure 4 is severe damage to mold surface caused by severe cavitation erosion of mold after continuous production.

 

Figure 3 Mold surface after cavitation under an electron microscope

 

Figure 4 Photographs of surface of mold severely damaged due to cavitation after long-term use

In design of mold runner, it is necessary to observe principle that cross-sectional area of runner from cake needs to be reduced, cross-sectional area of main runner needs to be larger than sum of cross-sectional areas of two or more branch runners when runner has branches. Specific example: In runner design shown in Figure 5, main runner of A is divided into two runners of B-E. Cross-sectional area of A is 825 mm2, cross-sectional area of B is 567 mm2, cross-sectional area of E is 750 mm 2. At this time, total cross-sectional area of two branch runners is 1,317 mm2, and cross-sectional area of A as main runner is 850 mm2. Result is AB + E so that cavitation can be avoided.

 

Figure 5 Schematic diagram of bad runner design

Can surface treatment of mold material solve "pits" formed by cavitation on mold surface?
Figure 6 is a side view metallographic photograph of H13 mold material after PVD coating treatment. It can be seen that cavitation can still form "pitting" holes on mold surface. Negative pressure bubbles in aluminum alloys are destroyed when die-casting. Energy released is enough to penetrate high hardness coating and form holes in mold surface.

 

Figure 6 Cavitation holes (X500) on mold surface after PVD surface coating treatment
As is known to all, hardness of die casting mold of H13 material is generally 46-48HRC (≈470HV), and after PVD (TiCN) coating treatment, its microhardness is about 2200HV. Although mold surface hardness is improved, it still does not help to solve cavitation problem.

Cavitation is also a common quality problem in aluminum alloy die casting process. To overcome such problems requires good consideration and calculation of entire runner system during design of mold to ensure that entire runner from cake to gate is in a compressed state. Method of improving surface hardness of mold by surface treatment cannot solve problem of cavitation erosion in die casting.