The main driving force for the application of additive manufacturing in the aerospace industry is weight reduction, and operators are willing to pay for the reduction in operating costs brought about by improved fuel economy. The initial doubts about the density, porosity and grain structure of additively manufactured parts are being overcome, and the technology has also been increasingly used to produce certified components.
However, there is still an unsolved challenge in additive manufacturing technology-surface finish. In theory, additive manufacturing can create complex shapes completely freely, but in the actual production process, the requirements for surface finish often need to constrain the design. If surface processing is required, the restrictions on the structure are the same as in the past; if shot peening is used, restrictions on thin and slender structures are required. In addition, surface quality problems can also lead to stress concentration, which in turn leads to cracks, which greatly reduces the fatigue resistance and fracture toughness of the part, which brings risks to the use of the part.
Although improving the quality of metal powder, optimizing the forming direction and process parameters can improve the surface quality of additive manufacturing parts to a certain extent, it still cannot completely solve the problem of rough surface of additive manufacturing parts. Therefore, post-processing of additive manufacturing parts is necessary, and the current main post-processing methods include finishing and mechanical processing.
The finishing method is mainly manual polishing, sand blasting or CNC grinding and polishing. The quality of manual polishing depends heavily on the operator's experience level, poor repeatability and consistency, high labor and time costs, and the dust generated during the polishing process is harmful to human health. Sandblasting and CNC grinding and polishing have poor accessibility to parts with complex inner surfaces and porous structures, and are generally used for cleaning and polishing the outer surfaces of parts and removing oxide layers.
For aerospace complex structural parts with high surface quality requirements (0.8μm <Ra <1.6μm, no surface loose residues and high performance requirements), the finishing process after additive manufacturing is facing huge challenges. In addition to the above methods, there are adaptive grinding, laser polishing, chemical and electrochemical polishing, and abrasive flow processing.
Comparison of Finishing Methods for Additive Manufacturing Complex Structural Parts
Adaptive grinding is to determine the shape of the contact surface of grinding and polishing by adjusting the shape of the contact head, so that the grinding and polishing process realizes the processing process of using the contact surface as the grinding and polishing working surface, and improves the efficiency of the grinding and polishing process. Foreign researchers have used an adaptive grinding method with a spherical flexible grinding head to polish the free surface of the titanium alloy manufactured by additive manufacturing. The rough layer and the fine polishing have removed the defect layer on the surface of the additive manufacturing, and the final surface roughness Ra has reached Below 10nm.
Principle of adaptive grinding
Laser polishing is a new type of polishing method that uses high-energy laser beam to remelt the surface materials of parts to reduce the surface roughness. At present, the surface roughness Ra of the parts after laser polishing is 2 ~ 3μm. Because laser polishing equipment is relatively expensive, it has not been widely used in actual engineering.
Laser polishing processing principle
Chemical and electrochemical polishing is based on the selective dissolution of the uneven surface of the sample surface by the chemical etching effect of chemical reagents to eliminate wear marks and erosion and leveling, and the surface of the hollow structure or array structure parts of small additive manufacturing is loose and easy to fall off. The removal effect of the nodular layer is remarkable. The combination of chemical polishing and electrochemical polishing is used to polish the porous structure bracket. The surface roughness Ra of the polished bracket can be reduced from 6 to 12 μm before polishing to 0.2 to 1 μm. Chemical polishing mainly removes the metal spheres adhered to the surface quickly Powder, electrochemical polishing further reduces the surface roughness on this basis.
Electro-polished medical implants
Abrasive particle flow processing technology is to use a flowable semi-solid viscoelastic abrasive medium to reciprocate through the processed area under a certain pressure, and use the micro-cutting effect of abrasive particles in the abrasive medium to remove the microscopic rough peaks on the surface of the workpiece. In order to achieve the purpose of processing these areas. This technology is suitable for polishing, deburring, and rounding of parts with complex internal cavities, freeform surfaces, and irregular shapes, and can process almost all metal materials. The high processing accessibility of abrasive flow processing technology provides a new way to overcome the problem of surface finishing of complex parts manufactured by aviation additive manufacturing.
Effect of abrasive flow polishing on complex structural parts by additive manufacturing
Surface roughness of abrasive flow polishing of complex structural parts by additive manufacturing
Powder bed laser melting technology can achieve the best surface quality in all metal additive manufacturing processes. In addition to the above-mentioned finishing methods, sometimes the key positions of parts need to be machined. These two post-processing methods are widely used in molds.
There are many visible weld beads on the surface of parts manufactured by directional energy deposition technology, and the width of the corrugation reaches a few millimeters, then the required dimensional accuracy and surface finish must be achieved by mechanical processing.
Parts made by directional energy deposition technology must be machined before they can be used
At present, the post-processing technology of parts is becoming the bottleneck of the development of additive manufacturing. The traditional manual post-processing technology is easy to cause parts damage or different sizes. Through the comparative analysis of the current finishing methods of additive manufacturing parts, we can see that most of the research work is still at the stage of improving the surface roughness. In order to meet the accuracy requirements of the parts after finishing, additive manufacturing of complex structural parts Polishing technology should develop in the direction of precision and control.