Ductility and malleability are plastic deformation behavior of materials. During CNC processing, these two parameters can directly influence the cutting edge morphology, burr control, risk of cracking, and final surface quality. Therefore, today we write this article to explain ductility and malleability meanings, their key differences and their influence on CNC machining.
What Is Ductility?
Ductility is the mechanical property of a material that allows it to undergo plastic deformation under tensile loads without losing its integrity. This property determines how much energy a material can absorb before breaking when stretched or bent.
Normally, the materials that have high ductility is called ductile materials, while the materials that have low ductility is called brittle materials.
How Ductility Is Measured
Ductility can be measured by two methods:
Tensile test
In tensile test, elongation and reduction of area are indicators for measuring the ductility of materials.
延伸率
Elongation is to check how much the material lengthens after breaking.
The calculating method is:
Elongation (%)=[(Lf – L0)/L0]*100%
Specific criteria for assessing the degree of ductility
The typical ductility range for ductile materials: (to determine low ductility or high ductility)
| Ductility Level | Elongation Range | Reduction of Area Range | Example Materials |
| High Ductility | > 30% | > 50% | Pure copper (45–60%), annealed austenitic stainless steel (40–60%), pure aluminum (50–55%) |
| Moderate Ductility | 15%–30% | 30%–50% | Mild steel (20–30%), 6061 aluminum alloy (12–18%) |
| Low Ductility | 5%–15% | 10%–30% | Quenched and tempered 4140 alloy steel, Ti-6Al-4V titanium alloy (10–14%) |
| Brittle Materials | < 5% | < 10% | Gray cast iron (0–2%), hardened high-carbon steel, ceramics |
This table can be an engineering reference for determining whether materials have high ductility or low ductility.
Reduction of Area
Reduction of area is to observe how much the fracture surface thins. And the calculating formula is:
Reduction of Area (%)= [(Af – A0)/A0]*100%
If the value of Reduction of Area is high, it means that the material underwent significant plastic deformation prior to fracture. The following table provides ranges for reference:
| Reduction of Area | 延展性 |
| < 10% | Brittle tendency |
| 10–30% | 中等 |
| > 30% | Good ductility |
| > 50% | Very ductile |
Why Ductility Matters in Manufacturing
During manufacturing, the ductility of materials determine the material’s resistance to cracking during the forming stage. High ductility materials enable deep drawing, pipe bending and thread rolling without annealing processes. In CNC machining, ductility can affect the chip continuity:
Ductile materials may produce long and continuous turning chips.
Brittle materials easily produce powdered or fragmented chips, even though they are easy to remove but prone to sudden fracture under vibration loads.
What Is Malleability
Malleability is the ability of a material to undergo plastic deformation without cracking under compressive loads. This property is directly relevant to the metal processing of flat sheets, casings, or three-dimensional complex shapes through processes such as forging, rolling, and stamping.
How Malleability Is Evaluated
Malleability is determined by evaluating a material’s ability to undergo plastic deformation without cracking under compressive force. It is usually evaluated through compression and forming processes.
Why Malleability Is Important in Metal Forming
Malleability can directly determine whether the materials can be manufactured into desired geometries through compression process. High-malleability materials allow cold forging or cold rolling under significant deformation, thereby reducing heating processes and reducing manufacturing costs. However, low-malleability requires thermal forming processes to enhance their plastic flowability so that their compressive deformation behavior can be improved.
Ductility vs Malleability: What Is the Difference?
The key difference between ductility and malleability is their types of stress and deformation. Ductility is a material’s ability to undergo plastic deformation under tensile stress, while malleability is the ability to undergo plastic deformation under compressive stress without cracking.
Ductility vs Malleability: Different Manufacturing Processes
Ductility and malleability can significantly affect the manufacturing methods used to machining materials. So, understanding ductility and malleability of materials can efficiently instruct the selections of the manufacturing processes.
Different Manufacturing Applications
High-ductility materials are suitable for manufacturing cables, springs, fasteners, and structural components. Instead, high-malleability materials are preferred for pressure vessel heads, automotive body panels, coins, and any components formed through forging or stamping processes.
In CNC machining, axial components require ductility to withstand shear stress induced by torque, whereas shell-type components need forgeability to facilitate secondary riveting or press riveting operations.
Metals With High Ductility and Malleability
Some materials can provide excellent malleability as well as high ductility. Let us introduce them for you.
铜
Copper exhibits both high ductility and malleability, which makes it one of the most formable metals. The elongation of pure copper in the annealed state can reach 45% to 60%, with a reduction of area exceeding 80%. Its high ductility allows it to be drawn into extremely fine metal wires, that is why copper is widely used for electric wire and cables. Meanwhile, copper with high malleability can be manufactured to be thin sheets, tube or other components by rolling.
However, it easy to produce long chips and burrs during CNC machining because of copper’s softness and high ductility,
Gold
Gold is one of the metals with the highest ductility and malleability. The elongation of the pure gold is about 45%. Due to the cost, gold is seldom used for ordinary CNC machining, but it is still necessary materials for electronics, aerospace and high-performance products.
铝
The elongation of pure aluminum is about 25%-45%. Pure aluminum and annealed aluminum exhibit higher ductility and good malleability. These features make aluminum suitable for rolling, extrusion, bending, and sheet metal processing.
In CNC machining, the high ductility of aluminum can easily cause continuous chips, which can be easy to adhere to the front edge of the cutting tool, forming a chip tumor. This can be controlled by applying high cutting speed and polishing the surface of cutting tools.
Mild Steel
Mild steel has medium to high ductility, and medium malleability. Its ductility can help mild steel avoid sudden fracture under tensile loads, and malleability enables it to be shaped under compressive force.
Compared with brittle materials, mild steel exhibits significant plastic deformation before fracture, making it suitable for bending, welding, rolling, and structural forming.
Mild steel is one of the most predictable materials for CNC machining because of it stable chip fracture behavior.
Materials With Low Ductility or Low Malleability
Materials with low ductility or low forgeability typically exhibit a body-centered cubic or hexagonal close-packed crystal structure, or contain hard and brittle second-phase particles. These characteristics restrict dislocation movement and induce early fracture.
Cast Iron
Cast Iron is typically a brittle material because its elongation is less than 1%., which meas it almost never occurs plastic deformation.
High Carbon Steel
High-carbon steel (carbon content exceeding 0.6%) typically has an elongation below 10% in its untreated state, and may drop below 2% after quenching treatment.
Ceramics
Ceramics materials are typical brittle materials. There is no plastic deformation. When it reaches the ultimate tensile strength limit, the catastrophic fracture occurs. Also, malleability doesn’t exist. In CNC machining, ceramics must be shaped through grinding or laser processing, but not traditional cutting techniques.
Why Ductility and Malleability Matter in CNC Machining
Ductility and malleability affect the cutting edge morphology, burr formation, and surface integrity during CNC machining.
Effect on Chip Formation
Ductile materials maintain continuity even after undergoing significant plastic deformation in the shear zone, forming elongated or spiral-shaped chips. These chips tend to wound around the workpiece or tool, causing automatic tool change failures or surface scratches.
However, low-ductility materials is easy to discharge chips but exhibits significant fluctuations in cutting force, which can easily lead to vibration.
The solution:
- Use chip breaker geometry
- Use high feed rate
- Use high-pressure coolant
Effect on Burr Formation
Ductile materials is prone to produce large burr because they can withstand significant plastic bending before fracture. And malleability influences the formation of entry burrs.
Solution:
- Increase the tool negative chamfer
- Use a climb climb milling strategy
- Apply burr pre-breaking grooves
Influence on Surface Finish
Ductile materials are prone to forming built-up edge during machining, which can leave micro-pits and scratches on the processed surface, resulting the increase of surface roughness Ra value.
During practical production, the solution is improving cutting speed to improve the cutting temperature and then decrease the yield stress of the materials. By applying this method, the formation of built-up edge can be inhibited, and the surface finish can be better.
Ductility vs Malleability of Common Metals
The differences in ductility and malleability among various metal materials stem from variations in their internal atomic structures and plastic deformation capabilities.The following presents a paired comparison from an engineering application perspective.
Copper vs Aluminum
The elongation of pure copper is higher than that of pure aluminum. However, as for the malleability, aluminum is prior to copper. The yield strength of aluminum is approximately 1/3 that of copper.
In CNC machining, copper exhibits stronger chip adhesion and tend to form built-up edge. Aluminum produces continuous chips, while it can be easier to break off by using high-pressure coolant.
Stainless Steel vs Carbon Steel
Stainless steel, especially austenitic stainless steel exhibits extremely high ductility, higher than that of carbon steel. However, the malleability of carbon steel is superior to austenitic stainless steel.
From a CNC machining perspective, the cutting force of stainless steel is approximately 1.5 times that of carbon steel, and its chips are less prone to fracture.
How to Choose Materials Based on Ductility and Malleability
During the mechanical design phase, material selection must be based on the part’s manufacturing process and service conditions, balancing ductility and forgeability while making a comprehensive decision that considers strength, cost, and machinability.
For CNC Machined Parts
For CNC parts machined through material removal, materials with moderate ductility are preferred, with elongation controlled between 15% and 25%. This range helps chips break naturally into C shapes or short spirals, while preventing flash at the cutting edge caused by excessive brittleness.
In practice, 6061 aluminum, 1215 steel, and C3600 brass are the ideal choices.
Pure copper or annealed austenitic stainless steel, which have elongation exceeding 40%, should be avoided unless you use a high pressure cooling system and specialized chip breaking tooling.
For thin walled components or workpieces with many fine features, choose materials with higher ductility. These materials absorb vibration energy better and help prevent wall penetration caused by fluctuations in cutting forces
For Sheet Metal Parts
For sheet forming components, forgeability should be the primary selection criterion, particularly for bending, embossing, and deep drawing processes. The ratio of the material’s ultimate bending radius to its plate thickness must be less than the specified manufacturing requirement, as this ratio directly reflects the quality of forgeability.
For plates thicker than 3 mm, prioritize high forgeability materials such as medium-and low-carbon steels treated with spheroidizing annealing or 5052 aluminum alloy.
Ductility serves as a secondary criterion in such applications, primarily influencing the tensile limit of flange edges. When components contain tensile holes or bulging features with diameters less than five times the plate thickness, the material elongation must exceed 20% to prevent cracking.
For Forged Components
Forging parts require materials to exhibit excellent forgeability in both hot and cold states, enabling the metal to fill the mold cavity completely without folding.
- For hot forging processes, alloy steels such as 4140 and 4340 demonstrate superior forgeability at temperatures between 1000°C and 1200°C, with elongation temporarily exceeding 50%.
- For cold forging processes, pure aluminum, low-carbon steel, and brass are preferred materials because they can withstand high compressive strain at room temperature without cracking.
Ductility serves as a secondary criterion in forging, primarily influencing the flash removal operation; materials with high ductility are more likely to maintain crack-free edge quality after flash removal.
结论
Both ductility and malleability are plastic deformation capacity of materials, but they differ in their stress and manufacturing methods. Ductility reflects a material’s ability to deform under tensile stress, while malleability reflects its formability under compressive stress. These variations directly impact processes such as CNC machining, sheet metal forming, rolling, and forging.
In practical manufacturing, sheet metal components prioritize forgeability, while CNC machining focuses on balancing ductility and chip control. Understanding the differences between these two aspects helps optimize material selection, machining processes, and the final part performance.
常见问题
Is Ductility the Same as Malleability?
No. Extensibility refers to a material’s ability to resist tensile fracture, quantified by elongation or reduction of area. Malleability denotes a material’s resistance to compressive failure, assessed through the degree of compressive deformation.
Which Metals Are Most Ductile?
Pure gold, pure silver, and pure copper are the three most ductile materials among all metals. The elongation rates are approximately 45% for gold, 50% to 55% for silver, and 45% to 60% for copper. Annealed low-carbon steel and austenitic stainless steel also belong to high-ductility materials, with elongation rates ranging from 30% to 60%.