How to achieve surface treatment of the internal cavity structure?

一, Technical principle: Surface modification through the combined effects of several physical fields
The main goal of surface treatment for internal cavity structures is to increase performance and optimize surface morphology via mechanical, chemical, or composite methods. There are three main groups of technical principles:
Mechanical removal type: uses the micro cutting effect of abrasive particles to get rid of layers of surface defects. Abrasive flow polishing method, for instance, uses semi-solid polymer abrasives that flow under pressure to polish complicated structures like cross holes and interior cavities evenly, resulting in a surface roughness of Ra0.1 μ m.
Chemical dissolution type: This sort of chemical dissolution uses the ideas of electrochemistry or chemical corrosion to selectively eliminate bumps from the surface. Electrolytic polishing technology controls the pace of anodic dissolution to make the surface micro geometric morphology smoother. It also makes a thick oxide film to make the surface more resistant to corrosion. The treatment of the interior cavity of 316L stainless steel can lower the roughness from Ra6 μm to Ra0.2 μm.
Composite reinforcement type: Making a functionally graded surface by using both physical deposition and chemical modification. For instance, PVD (Physical Vapor Deposition) technology puts TiN coating in the mold cavity. This coating is up to 2200HV hard and three times more resistant to wear. The rare earth infiltration technology adds elements like Ce and La during the nitriding process to make the infiltration layer 40% deeper, which greatly improves fatigue resistance.
二, Process implementation: exact answers for each situation
1. Deep hole inner cavity polishing: an innovative use of abrasive flow technology
Traditional polishing procedures don't work well on deep hole structures like the interior hollow of aircraft engine blades and automotive fuel injectors because they are hard to get to and don't work very well. The abrasive flow technology makes progress by using the following new ideas:
Medium optimization: A semi-solid abrasive mix of silicon carbide particles and polymer carriers is employed to make sure that it can cut and doesn't scratch the surface.
Channel design: By using computational fluid dynamics (CFD) to simulate and improve the tooling channel, we can make sure that the abrasive flow velocity in the 0.3mm micropores is more than 95% uniform.
Control of parameters: For example, while treating the inner cavity of a certain type of turbine blade, the roughness can be reduced from Ra3.2 μ m to Ra0.4 μ m after three cycles (5 minutes each). The pressure is 0.5MPa and the flow rate is 15mm/s.
2. For complex cavity deburring, use an electrochemical and mechanical composite approach.
When removing burrs from cross hole structures like transmission valve bodies and hydraulic valve blocks, you need to find a compromise between speed and quality. A company came up with the "electrochemical deburring+abrasive flow polishing" process:
Electrochemical stage: A 10% NaCl solution is used as the electrolyte, and a pulse power supply with a frequency of 10kHz and a duty cycle of 30% is utilized to remove 90% of the burrs at a current density of 0.5A/cm². The process takes no more than 2 minutes.
The grinding particle flow stage uses 800 mesh silicon carbide abrasive to polish for 2 minutes at a pressure of 0.3MPa. This removes electrochemical residues and leaves a surface quality of Ra0.2 μm.
3. Making the inside of the cavity resistant to corrosion: using both electrolytic polishing and coating technology
The inside of medical device implants, including prosthetic joints, must be both biocompatible and resistant to corrosion. One company uses the process of "electrolytic polishing + DLC (diamond-like carbon) coating":
Electrolytic polishing: By using a voltage of 15V and a current of 20A for 5 minutes in a phosphoric acid and sulfuric acid mixed electrolyte, the surface roughness of Ti6Al4V is decreased from Ra1.6 μm to Ra0.08 μm and a 100nm thick oxide coating is formed.
DLC coating: A 2 μ m thick DLC coating is applied using magnetron sputtering technique. The hardness approaches 20 GPa, the friction coefficient lowers to 0.05, and the corrosion resistance is increased by 10 times in a simulated bodily fluid environment.
三, Use in business: common examples in the high-end manufacturing sector
1. The field of aerospace
Selective laser melting (SLM) technology is used by GE Aviation to make fuel nozzles for LEAP engines. After being made, the internal flow channel is polished with abrasive flow to make the surface smoother (from Ra12 μ m to Ra0.8 μ m), make fuel flow more evenly (by 8%), and make the engine more fuel-efficient (by 1.5%).
2. In the business of making cars
Bosch has come up with a new way to clean and polish the high-pressure oil pump cavity of the common rail system. It uses both ultrasonic cleaning and electrolytic polishing.
Ultrasonic cleaning: To get rid of leftover cutting fluid from machining, clean for 10 minutes at a frequency of 40kHz and a power of 100W.
Electrolytic polishing: Use a phosphate-based electrolyte and 12V voltage for 3 minutes to make the 316L stainless steel cavity less rough (from Ra2.5 μ m to Ra0.4 μ m) and increase the duration it can withstand salt spray corrosion (from 500 hours to 2000 hours).
3. The field of medical devices
Johnson&Johnson DePuy Synthes makes acetabular cups using the "electrolytic polishing+micro arc oxidation" method.
Electrolytic polishing: Lower the surface roughness of the Ti6Al4V substrate from Ra3.2 μ m to Ra0.2 μ m and get rid of the unfused particles that were made during SLM molding.
Micro arc oxidation: A 20 μ m thick oxide coating with hydroxyapatite is made in a silicate electrolyte by applying 300V for 5 minutes. The implant's survival rate is 99.2%, and the strength of the bone bond is boosted by 40%.