Will post-processing damage the internal structure?

一, Technical principle: The main problem with post-processing machine processing
The main purpose of post-processing is to improve the surface quality, dimensional accuracy, or mechanical qualities of parts by cutting, polishing, heat treating, and other methods. The processed objects are usually parts that were made by procedures like additive manufacturing (AM), casting, or forging. The internal structure of these parts may contain the following features:
Microscopic flaws, such porosity, lack of a fusion zone (LOF) in parts made using additive manufacturing, or shrinking porosity and cracks in cast parts.
Residual stress is tension that builds up inside an object due to a change in temperature or phase. This can cause the object to bend or shatter after it has been processed.
Gradient materials and non-uniform grain structure are examples of uneven organization that might change how materials are removed during processing.
Interventions in post-processing may modify these internal structures by mechanical pressures, thermal impacts, or chemical reactions, resulting in performance degradation or increased failure risks.
二, The effect and case study of typical procedures
1. Mechanical cutting: letting go of stress and activating defects
When a tool and a part come into direct contact during mechanical cutting (such milling and turning), material is removed. This can produce the following changes in the part's internal structure:
Redistribution of residual stress: Cutting forces can affect the part's surface stress state and potentially cause internal microcracks to form. An aircraft company, for instance, observed that the residual stress of titanium alloy blades made by additive manufacturing went from -150MPa to +80MPa after milling. This cut their fatigue life by 30%.
Defect propagation: Cutting vibration can cause small holes or areas of incomplete fusion inside the material to grow into big fissures. Studies indicate that post-rough milling, the porosity of aluminum alloy components produced using laser powder bed melting (LPBF) escalates from 0.5% to 1.2%, while the fracture toughness diminishes by 25%.
Answer:
Use ultra-precision machining (like single-point diamond turning) to lower the cutting force. Do heat treatment (like stress relief annealing) before cutting to equalize internal stress. Optimize the tool path to stay away from locations where vibrations tend to build up.
2. Heat treatment: changes in the organization and stability of the dimensions
Changing the phase state of materials through heat treatment (such quenching, tempering, and hot isostatic pressing) might improve performance, but it can also cause:
Deformation produced by phase transformation: The volume increase that happens during martensitic transformation can cause the pieces to change shape. After carburizing and quenching, the tooth profile error of a specific vehicle gear, for instance, rose from ± 0.02mm to ± 0.05mm.
Thermally induced porosity (TIP): After hot isostatic pressing (HIP), inert gas pores may grow again in parts that were made using additives. Studies indicate that post-HIP, if the annealing duration of Ti-6Al-4V alloy surpasses 4 hours, porosity may rise by 0.3%.
Answer:
Using graded quenching or isothermal quenching to keep an eye on the pace of phase change;
To stop TIP, fine-tune the HIP process parameters (such temperature, pressure, and time).
Stress is discharged through the process of "rough machining → heat treatment → precision machining," which combines heat treatment and machining.
3. Strengthening the surface: residual compressive stress and fatigue performance
techniques that reinforce surfaces, such shot peening and rolling, add residual compressive stress, which increases fatigue life. However, these techniques may also cause:
Damage to the surface: Too much shot peening might cause microcracks or surface grain refinement. For instance, following shot peening, the surface roughness of a specific aircraft engine shaft went from Ra1.6 μ m to Ra0.4 μ m, while the depth of the fatigue fracture source went up by 0.1mm.
Stress gradient imbalance: When the residual compressive stress layer and the matrix stress don't match up, it might cause delamination. Studies indicate that aluminum alloy components subjected to laser shock peening (LSP) are susceptible to microcracking at the interface when the residual compressive stress depth surpasses 0.5mm.
Answer:
Control the intensity of shot peening (for example, by measuring the coverage of an Almen test piece); use composite strengthening procedures (for example, shot peening and rolling) to balance stress gradients; and use numerical simulation to find the best process parameters.
三, Risk management: from designing the procedure to keeping an eye on it online
The industry needs to set up a thorough process control system to limit the damage that post-processing does to the internal structure.
During the process design stage, choose a mix of post-processing processes that fits the parts' material, structure, and performance needs. For instance, HIP+electrolytic polishing is better than direct mechanical polishing for items made with additive manufacturing.
Use finite element analysis (FEA) to figure out how stress will spread and how things will change shape when they are being machined. A certain company used simulation to improve milling settings, which cut the machining deformation of titanium alloy parts from 0.15mm to 0.03mm.
Stage of execution for processing:
Using smart monitoring tools like acoustic emission and cutting force sensors to give real-time input on how the machining is going. For instance, a certain machine tool maker invented the "adaptive cutting system," which can change the feed rate on the fly to avoid too much vibration.
Use closed-loop control and change process parameters depending on data from online detection. If an aircraft firm employs a laser interferometer to measure how rough a surface is and then automatically adjusts the pressure of the polishing.
Stage of quality inspection:
Use non-destructive testing (NDT) methods like X-ray computed tomography and ultrasonic testing to find problems inside the object. Studies reveal that industrial CT can find pores that are 0.02mm wide with 98% accuracy.
Set up a chain of processing testing data and use machine learning to guess how long a part will last. For instance, a given business can use past data to train a model that can anticipate the probability of gear fatigue failure six months in advance.