The strength, hardness, elasticity, formability, ductility, tensile strength, elongation, toughness, electrical conductivity, and their relationships in the processing of materials.

In modern mechanical manufacturing and precision machining, material properties directly determine the machining process, service life, and reliability of the final product. Machining is not merely about cutting and shaping materials; it is a process of matching and synergistically integrating material properties with machining techniques. Common machined materials include carbon steel, stainless steel, aluminum alloys, copper alloys, titanium alloys, and various engineering plastics. The strength, hardness, elasticity, formability, ductility, tensile strength, elongation, toughness, and electrical conductivity of machined materials collectively constitute their material properties.

1.Material Strength

Strength is a material's ability to resist damage under external forces and is one of the most crucial mechanical properties of machined materials. Strength typically includes yield strength, tensile strength, compressive strength, and shear strength. For load-bearing parts, such as shafts, supports, and structural components, the material must possess sufficient strength to prevent plastic deformation or fracture under working loads. High-strength materials often allow for smaller part cross-sectional dimensions and lightweight designs, but they can also increase machining difficulty, such as accelerated tool wear and increased cutting forces.

2.Hardness

Hardness is a material's resistance to localized plastic deformation or surface wear, and it is of great importance in machining. Common hardness indicators include Brinell hardness (HB), Rockwell hardness (HRC), and Vickers hardness (HV). Higher hardness means greater wear resistance, but also greater difficulty in machining. For example, hardened steel or high-hardness alloy steel typically requires carbide or coated tools and lower cutting speeds during machining. Properly controlling material hardness helps achieve a balance between machinability and performance.

3.Elasticity

Elasticity is a material's ability to return to its original shape after deformation under external force. Its main evaluation indicator is the elastic modulus. Elastic properties directly affect machining accuracy and assembly results. For example, materials with low elastic modulus are prone to elastic deformation during clamping and cutting, leading to dimensional deviations. Therefore, in high-precision parts machining, it is necessary to fully consider the material's elastic characteristics and reduce the impact of elastic deformation by optimizing clamping methods and machining parameters.

4.Formability

Formability refers to a material's ability to achieve the desired shape through processes such as casting, forging, stamping, and extrusion. Good formability can reduce manufacturing costs and improve production efficiency. For example, aluminum alloys and low-carbon steel typically have good plasticity and formability, making them suitable for mass production of complex structural parts. Materials with poor formability, such as high-alloy steel or certain high-temperature alloys, often require more complex processes and higher manufacturing costs.

5.Ductility

Ductility is a material's ability to undergo plastic deformation before fracture, usually measured by elongation or reduction of area. Materials with good ductility can absorb energy through plastic deformation under stress, making them less prone to brittle fracture. In machining, good ductility facilitates chip formation and removal, reducing tool breakage and surface defects. Low-carbon steel, aluminum alloys, and copper alloys typically have good ductility, while cast iron and high-hardness quenched steel have relatively poor ductility.

6.Tensile Strength

Tensile strength is the maximum stress a material can withstand in a tensile test and is an important indicator of a material's load-bearing capacity. For parts subjected to tensile loads, such as tie rods, bolts, and connectors, tensile strength is particularly critical. High tensile strength materials can improve the structural safety factor, but often come with increased cutting difficulty and reduced machining efficiency during processing. Therefore, in practical engineering, the relationship between tensile strength and machinability needs to be considered comprehensively.

7.Ductility

Ductility is the ability of a material to be stretched or flattened without breaking under tension or compression. It is usually closely related to ductility, but emphasizes the continuity of the material during plastic deformation. Materials with good ductility are more suitable for cold working processes, such as cold drawing, cold rolling, and cold stamping. In machining, good ductility helps to obtain higher surface quality and reduce defects such as cracks and tears.

8.Toughness

Toughness is the ability of a material to absorb energy and resist fracture under impact or dynamic loads, and is a comprehensive reflection of strength and ductility. High-toughness materials perform more reliably under complex working conditions, and are especially suitable for parts subjected to impact loads or alternating stresses, such as gears, shafts, and engineering machinery components. In machining, materials with high toughness are generally less prone to brittle fracture, but may experience chip entanglement and higher machining resistance, requiring optimization through tool geometry and cutting methods.

9.Electrical Conductivity

Electrical conductivity is a material's ability to conduct electric current, usually expressed as conductivity or resistivity. While not a critical indicator for all machined parts, it is crucial in electrical, electronic, and new energy fields. Copper and its alloys have excellent electrical conductivity and are widely used in conductive parts and electrical connectors; aluminum alloys offer a balance of conductivity and lightweight. When machining highly conductive materials, attention must be paid to issues such as tool adhesion and heat concentration during cutting.

10.Relationship between Strength and Plasticity

Strength refers to the maximum force a material can withstand. Plasticity refers to the percentage of deformation a material can undergo. For example, a steel bar that can withstand a maximum force of 100 MPa has a strength of 100 MPa. If it deforms by 20% under a force of 100 MPa before breaking, its plasticity is 20%. In industry, a typical application requiring high strength and high plasticity is automotive structural components. On the one hand, we want them to withstand greater forces; on the other hand, we want them to deform significantly during a collision to absorb energy and protect passengers. For example, we might want automotive structural components to withstand 2000 MPa of pressure while deforming up to 60% without fracturing. (Absorbed energy = force on the structural component x degree of deformation). This is essentially toughness.

Generally, strength and plasticity in materials are mutually exclusive; they are like two sides of the same coin: increased strength usually leads to decreased plasticity. Research shows that plastic deformation in metallic materials is typically achieved through dislocation slip. During work hardening, metals undergo plastic deformation, grain slip occurs, and dislocation entanglement occurs, causing grain elongation, fragmentation, and fibrosis, preventing further deformation and ultimately leading to failure and fracture.

11.Elasticity and plasticity are relative.

Elasticity is simple: the deformation can be completely recovered after the external force is removed. Plasticity refers to plastic deformation of a material; after the external force is removed, the deformation cannot be completely recovered, leaving residual plastic deformation. For example, the elongation rate is used to evaluate the plasticity of steel. After a steel specimen breaks, the elastic deformation recovers, but the residual deformation is plastic. Therefore, elongation rate can be used to evaluate the plastic deformation capacity of steel.

12.Ductility and brittleness are relative.

Ductility refers to the ability of a structure or component to continue bearing load until it reaches its ultimate bearing capacity after reaching what we consider a failure state. Brittleness refers to the failure of a structure or component without any warning signs. Therefore, brittle failure is more severe than ductile failure.

13.Stiffness, ductility, and plasticity

First, all three are concepts that measure the degree of deformation. Stiffness is the load/displacement value in the elastic stage, which is EI, measuring the degree of hardness or softness. Ductility and plasticity are deformations in the inelastic stage. The ductility coefficient can be quantitatively calculated, while plasticity is a qualitative concept.

14.Toughness = Strength + Plasticity

Toughness is the energy a material absorbs during the process from resisting external force to fracture, including the energy consumed in both elastic and plastic deformation stages. Increasing both strength and plasticity has the greatest effect on improving toughness, but increasing strength and plasticity are contradictory.

Toughness refers to the energy absorbed by a material during the process from being subjected to force to fracture. The greater the energy absorbed during fracture, the better the toughness. Energy consumption means that work must be done on the material from outside, indicating the need for force and displacement (deformation). The ability to withstand stress is characterized by strength, and the ability to deform is characterized by plasticity. Therefore, materials with good toughness also have good plasticity. The difference between toughness and plasticity lies in whether the applied stress is considered when the material deforms.