What are the requirements for material selection in precision parts processing?
I.Physical Property Requirements
Hardness
Machinability:
For precision machining, material hardness should be moderate. Excessively hard materials, such as certain high-hardness alloy steels, accelerate tool wear and increase machining difficulty, often requiring specialized tools and processes. For example, mold steels with hardness above HRC 60 are challenging to mill with standard carbide tools, necessitating cubic boron nitride (CBN) or ceramic tools. Conversely, overly soft materials like pure aluminum are prone to deformation during machining, affecting part accuracy. Solutions include high-speed cutting with controlled cutting forces.
Functional Performance:
Precision parts must withstand operational stresses, requiring optimal hardness for wear resistance and deformation resistance. For instance, precision gears in mechanical transmissions need a surface hardness of HRC 45–60 to prevent excessive wear and tooth deformation, ensuring long-term transmission accuracy.
Strength and Toughness
Fracture Resistance:
Precision parts endure machining forces (e.g., cutting, clamping) and operational loads. Materials must balance strength and toughness to prevent fractures. For example, aerospace components like jet engine blades (often made of titanium alloys) must resist centrifugal and aerodynamic forces to avoid catastrophic failure.
Machining Quality Assurance:
Excessive toughness can cause issues like built-up edge (BUE), degrading surface finish. For instance, high-toughness stainless steels may produce stringy chips during turning, increasing surface roughness. Solutions include chip-breaking tools or adjusted cutting parameters.
Thermal Expansion Coefficient
Machining Precision Control:
Thermal deformation from cutting heat must be minimized. Materials with low thermal expansion coefficients (e.g., quartz for optical lenses) ensure minimal dimensional changes during grinding, achieving sub-micron accuracy.
Operational Stability:
For parts in variable-temperature environments (e.g., precision instruments), low thermal expansion ensures dimensional stability. Example: Invar alloy is used in metrology devices for its near-zero thermal expansion.
II.Chemical Property Requirements
Corrosion Resistance
Machining Process Protection:
Materials must resist corrosive media during processes like electrochemical machining. Stainless steels perform well in saline coolants, reducing rust risks.
Service Environment Adaptability:
Parts in corrosive settings (e.g., chemical valves) require alloys like Hastelloy (Ni-Cr-Mo), which resists strong acids/alkalis, maintaining precision functionality.
Oxidation Resistance
Post-Machining Quality Retention:
Poor oxidation resistance leads to surface degradation. Example: Copper parts oxidize to verdigris, necessitating protective coatings to preserve accuracy.
High-Temperature Applications:
Parts in extreme heat (e.g., jet engines) need oxidation-resistant materials like nickel-based superalloys, which form protective oxide layers to prevent degradation.
III.Material Purity and Homogeneity Requirements
Purity
Performance Impact:
Impurities critically affect performance. Semiconductor-grade silicon (99.9999% pure) avoids metal contaminants that alter electrical properties in chips.
Machining Defect Prevention:
Inclusions (e.g., sulfides/oxides in steel) cause stress concentrations, leading to cracks or surface defects during machining.
Homogeneity
Consistent Physical Properties:
Uniform composition/structure ensures even performance. Example: Homogeneous mold steels provide uniform hardness and wear resistance, extending mold life.
Machining Process Compatibility:
Uniform materials enable stable processing. In EDM (electrical discharge machining), consistent conductivity ensures even discharges and high precision. Non-uniform materials result in uneven surface finishes.