Ductility is the ability of a material to deform plastically under tension without breaking. Malleability is the ability to deform plastically under compression without breaking.
Ductility and malleability are two terms often used as synonyms. In reality they describe two distinct behaviours of the same phenomenon: the ability of a metal to deform plastically without breaking. The difference concerns the direction of the deformation, and understanding it isn't a simple theoretical exercise: it has direct consequences on the choice of material, on machinability and on the behaviour of the component in service.
This guide explains what ductility and malleability are, what the differences are, how they are measured, which metals are more ductile and malleable and why these values are important in the design of CNC-machined components. The values reported refer to the materials available for online CNC machining at Weerg and to the 3D printing materials in our range.
Ductility is the ability of a material to deform plastically under tensile stress without breaking. In practice, it's the property that makes it possible to pull a metal until it's transformed into a thin wire through drawing, to elongate it before breaking and to absorb deformations in service without giving way suddenly.
A ductile metal, subjected to tension, elongates progressively and forms a necking, that is, a narrowing of the section, before breaking. A brittle metal, on the other hand, breaks abruptly, without appreciable deformations or evident signals.
Ductility:
Ductility is measured through the tensile test according to ISO 6892-1 for metals and ASTM E8/E8M, using two complementary parameters.
Elongation at break (A%)
It indicates the percentage plastic deformation measured on the specimen after breaking. An S235 steel with A% = 26, for example, has elongated by 26% before breaking. The higher the value of A%, the greater the ductility of the material.
Reduction of area coefficient (Z%)
It indicates the percentage reduction of the cross-section in the fracture zone and measures the deformation in the transverse direction. A Z% value of 60% means that the section has reduced by 60% before the fracture. Compared with A%, it's an indicator more sensitive to the metallurgical quality of the material.
The two parameters are complementary: A% measures how much the material elongates, while Z% measures how much it narrows. Both describe the metal's ability to deform plastically, but from different perspectives.
Malleability is a material’s ability to undergo plastic deformation under compressive stress
Malleability is the ability of a material to deform plastically under compressive stress without breaking. It's the property that makes it possible to crush a metal until it's transformed into a thin sheet through rolling, or to hammer, forge and coin it.
A malleable metal can be reduced into ever-thinner sheets without cracking. Gold is the most malleable metal: it can be transformed into leaves with a thickness below 0.1 µm.
Malleability:
Unlike ductility, malleability isn't defined by a universal standardised parameter comparable to elongation at break. In practice it's assessed through various tests.
Compression Test
A cylindrical specimen is compressed and the deformation reached before the appearance of cracks on the edges is measured. The greater the deformation withstood without cracks, the greater the malleability of the material.
Upset Test
It's a variant of the compression test used to assess forgeability. The sample is compressed to a predefined reduction of the height, typically of 50% or 75%, verifying that no cracks appear.
Reduction ratio in rolling
In industrial practice, malleability can be assessed on the basis of the minimum thickness achievable through rolling without cracking or of the percentage reduction obtainable per pass.
Bending Test
A flat specimen is bent around a mandrel of a defined radius and the possible appearance of cracks is verified. The minimum bending radius without cracking provides a practical indication of the material's ability to deform without breaking.
The two concepts are closely connected, because both describe a material's ability to deform plastically, but they indicate different behaviours.
| Aspect | Ductility | Malleability |
|---|---|---|
| Type of stress | Tension | Compression |
| Deformation | Elongation, reduction of section | Flattening, widening |
| Typical industrial process | Drawing, extrusion, deep drawing | Rolling, forging, minting |
| Standard measurement | A% and Z% in the tensile test | Compression, upsetting, bending |
| Example | Copper is very ductile and can be drawn into thin wires | Lead is very malleable and deforms easily under compression |
The key point is that ductility and malleability don't necessarily coincide. Most structural metals, such as mild steels, aluminium and copper, present both properties, but to a different extent.
The physical reason is that the deformation and fracture mechanisms at the crystalline level respond differently to the state of stress. In compression, cracks, porosity and internal defects tend to close up, allowing the material to withstand greater deformations. In tension, on the other hand, the same defects favour the formation and propagation of cracks, limiting the deformation before breaking.
Several factors determine the ductility and malleability of a metal. Knowing them helps to predict its behaviour during machining and in service.
Metals with an FCC (face-centred cubic) structure, such as copper, aluminium, nickel, gold, silver and austenitic steels, are generally very ductile and malleable because they have numerous slip systems along which dislocations can move easily.
BCC (body-centred cubic) metals, such as α iron, chromium, tungsten and molybdenum, have a behaviour more sensitive to temperature. Many materials with this structure can present a ductile-brittle transition at low temperatures, with a strong reduction in toughness and deformability.
HCP (hexagonal close-packed) metals, such as magnesium, zinc and α titanium, have fewer active slip systems at room temperature and are generally less deformable. Their workability improves significantly with the increase in temperature.
In general, ductility and malleability increase with temperature, because the movement of dislocations becomes easier and further deformation mechanisms are activated.
Hot forging exploits precisely this principle. Steel around 1,100 °C is much more deformable than at room temperature, while many titanium alloys are formed at high temperatures to facilitate their working.
Ferritic steels can present a transition from ductile to brittle as the temperature decreases. The threshold depends on composition, microstructure, thickness and deformation rate. For cryogenic applications, austenitic steels are often used, whose FCC structure maintains a good toughness even at very low temperatures.
In steels, the increase in carbon content tends to reduce ductility and to increase strength and hardness. Alloying elements too can hinder the movement of dislocations, although their effect depends on the composition and the microstructure obtained.
Heat treatments profoundly modify deformability. An annealed steel is generally more ductile than the same material quenched and tempered, while a predominantly untempered martensitic structure presents high hardness but very reduced ductility.
The deformation rate can significantly influence ductility. In many metals, very rapid stresses such as impacts and shocks favour a less ductile behaviour compared with quasi-static tests, although the effect varies according to the material and the temperature.
For applications subject to impact, therefore, the values reported in the technical data sheets obtained with quasi-static tests don't always completely describe the real behaviour.
Non-metallic inclusions, porosity, segregations and surface defects can act as fracture-initiation points, reducing ductility and malleability. For the same composition, a steel with greater metallurgical cleanliness generally presents a better ability to deform without breaking.
|
Metal / Alloy |
Indicative A% |
Notes |
|
Pure gold |
45–50 |
Exceptionally ductile and malleable |
|
Pure copper (Cu-ETP) |
30–50 |
Standard conductivity and ductility |
|
Pure silver |
40–50 |
Ductile, used in jewellery and electronics |
|
Platinum |
35–45 |
Ductile, expensive |
|
35–60 |
Among the most ductile steels |
|
|
Pure aluminum |
35–45 |
Ductile but with low mechanical strength |
|
14–22 |
Ductile structural alloy |
|
|
Aluminum 6082-T6 |
6–12 |
Heat-treated, less ductile |
|
10–26 |
Standard structural steel |
|
|
20–22 |
Stronger, slightly less ductile |
|
|
Gr2 Titanium |
24–30 |
Commercially pure, good ductility |
|
Titanium Gr5 (Ti6Al4V) |
10–15 |
High-strength alloy, less ductile |
|
CW508L Brass (CuZn37) |
15–45 |
Wide range, depends on the state |
|
Quenched and tempered 42CrMo4 steel |
12–16 |
High strength, moderate ductility |
|
Spheroidal graphite cast iron EN-GJS-500-7 |
7–10 |
Ductile for cast iron |
|
Tungsten |
2–4 |
Brittle at room temperature |
|
Gray cast iron EN-GJL-250 |
< 1 |
Practically brittle |
There's no standardised scale equivalent to the one used for ductility. For pure metals, an indicative order of malleability is the following.
Gold, can be beaten into leaves with a thickness below 0.1 µm
Silver, easily reducible into very thin sheets
Aluminium, highly rollable and used for sheets a few micrometres thick
Copper, easily rollable and forgeable
Tin, very deformable and historically used to produce thin metal sheets
Lead, very malleable thanks to the low resistance to deformation
Zinc, little deformable at room temperature but more malleable around 100 and 150 °C
Iron and mild steels, easily forgeable and rollable
Nickel, ductile and malleable, especially in the annealed state
Titanium, more difficult to deform cold but much more workable hot
Malleability varies significantly with temperature. A material that's little deformable at room temperature can become much more malleable during hot working.
For the designer, ductility and malleability aren't abstract properties: they directly influence the structural behaviour and the machinability of the component.
A component made with a ductile material can deform significantly before breaking, providing an evident signal of the overload. A brittle material, on the other hand, can fail with very limited deformations. For this reason many structural standards provide for minimum requirements of ductility, in addition to those of strength.
For structural steels, values such as the elongation at break A% and the ratio between tensile strength and yield strength Rm/Re contribute to defining the material's ability to deform plastically before breaking. The values required depend on the grade, the thickness and the applicable standard.
In a statically indeterminate structure, the most stressed zones can plasticise and redistribute part of the load towards the less stressed ones. This behaviour requires sufficient ductility. In brittle materials, on the other hand, the redistribution capacity is much more limited and a local fracture can propagate rapidly.
Ductility and malleability influence workability by plastic deformation, so operations such as bending, deep drawing, stamping, forging and drawing. A sufficiently ductile metal can be bent or deep-drawn without developing cracks, while a good malleability facilitates rolling and forging.
The minimum bending radius, fundamental in the design of sheet metal, also depends on ductility. A 304 stainless steel, very ductile, can generally be bent with tighter radii than a 7075 T6 aluminium alloy, significantly less deformable.
In applications where the component has to absorb energy (protections, bumpers, crash boxes, guardrails), ductility is fundamental. The energy absorbed corresponds to the area under the stress-strain curve and therefore depends on the combination between mechanical strength and the material's ability to deform before breaking.
Ductility also influences workability on machine tools, but not always in a favourable way. Very ductile materials, such as pure copper and aluminium, tend to generate long, continuous chips that can wrap around the tool and worsen the surface quality. Grey cast iron, on the other hand, generally produces short, easily evacuable chips.
Among the materials with good CNC machinability are those whose composition and microstructure favour the breaking of the chip, such as brasses, free-machining steels and various machining aluminium alloys (2xxx and 6xxx series treated T6).
For example, for a milled part, aluminium 6082-T6 is often the most balanced choice when machinability, corrosion resistance, weldability and cost count: it's suited to brackets, plates, supports, casings and general mechanical components.
Aluminium 7075-T6 is preferable when the component has to maximise the strength/weight ratio or withstand higher loads and fatigue cycles, for example in aerospace and high-performance mechanical parts. It offers superior mechanical performance, but costs more, is less resistant to natural corrosion and is difficult to weld.
Rule of thumb: 6082 for the best compromise between performance, machinability and versatility; 7075 when the mechanical strength justifies the cost and the greater design attention. In Weerg's CNC machining service, you can compare materials, finishes and production times starting from the 3D file.
The term malleability is little used for polymers, because it belongs mainly to metallurgical terminology. Ductility, on the other hand, is applicable to polymer materials too and describes the ability to develop significant deformations before breaking, often assessed through elongation at break.
Some indicative values for materials used in on-demand production:
| Material | A% | Behaviour |
|---|---|---|
| PA12 MJF | 20% | Good ductility for a component produced through MJF |
| PA11 Gen 2 MJF | 27.5% | High deformability at break |
| Medical ABS FDM | 31% | High ductility and good toughness |
| PEEK Annealed FDM | 9.1% | Moderate ductility and high stiffness |
| PEEK CF FDM | 3.9% | High stiffness and reduced deformability at break |
| ECOtech FDM | 2.8% | Rigid material with low ductility |
| TPU MJF | 291% | Elastomer with extremely high elastic deformability |
In the case of TPU, a high elongation at break doesn't necessarily correspond to a high ductility in the strict sense. Being an elastomer, much of the deformation is elastic and reversible, while in ductile materials the deformation before breaking includes a significant permanent plastic component.
Ductility and malleability describe two distinct aspects of a material's ability to deform plastically without breaking. Ductility concerns mainly the behaviour in tension, while malleability describes the ability to deform under compression. In design, ductility is often the most used parameter because it influences structural safety, formability, impact behaviour and workability on machine tools. Malleability, on the other hand, is particularly relevant in the processes of rolling, forging and plastic deformation.
Knowing parameters such as elongation at break A%, reduction of area coefficient Z% and minimum bending radius, together with the factors that influence them, makes it possible to dimension more reliable components, choose the production processes correctly and assess materials with coherent engineering criteria.
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