What is the Coefficient of Thermal Expansion and Why Does it Matter in Die Casting
The coefficient of thermal expansion in die casting shows how materials like aluminium die casting alloys get bigger or smaller when they are heated or cooled. Expansion changes both the mould and the aluminium die casting part during casting. If die casting alloys and the mould get bigger at different speeds, thermal stresses can happen. These stresses might make cracks or make the shape less steady. Good dimensional stability in die casting helps keep the shape and quality of aluminium die casting parts. The right balance between the mould and die casting alloys gives strong thermal stability and fewer problems.
Coefficient of Thermal Expansion Basics
What is the Coefficient of Thermal Expansion
The coefficient of thermal expansion tells us how much a material changes size when it gets hotter or colder. In aluminium die casting, this number shows how aluminium alloys get bigger or smaller as the temperature goes up or down. When engineers talk about the coefficient of thermal expansion in die casting, they usually mean linear thermal expansion. This shows how much a material’s length changes for each degree of temperature change. Aluminium die casting alloys, such as ADC12 and A380, change size clearly when the temperature changes. These changes affect the physical and mechanical properties of the alloy and the finished part. If the expansion is not controlled, the die casting alloys can make stress or cracks in the mould.
How the Thermal Expansion Coefficient is Measured
Scientists find the thermal expansion coefficient by checking how much a sample grows or shrinks as the temperature changes. They use special tools called dilatometers to measure small changes in length. The main units for the coefficient of thermal expansion are per kelvin (K⁻¹) or per degree Celsius (°C⁻¹). The most common type is the linear thermal expansion coefficient. This shows the change in length for each degree of temperature change. The table below explains the types of thermal expansion coefficients:
Type of Coefficient | Definition | Formula | Units |
|---|---|---|---|
Linear (αL) | Change in length per degree of temperature change | αL = (1/L) dL/dT | K⁻¹, °C⁻¹, °F⁻¹, °R⁻¹ |
Area (αA) | Change in area per degree of temperature change | αA = (1/A) dA/dT | K⁻¹, °C⁻¹, °F⁻¹, °R⁻¹ |
Volumetric (αV) | Change in volume per degree of temperature change | αV = (1/V) dV/dT | K⁻¹, °C⁻¹, °F⁻¹, °R⁻¹ |
Note: For most aluminium alloys, the volumetric coefficient is about three times the linear thermal expansion coefficient.
Typical Values in Die Casting Alloys
Aluminium die casting alloys have higher coefficients of thermal expansion than many other metals. For example, ADC12, a popular aluminium alloy, has a linear thermal expansion coefficient of about 21 µm/m·K. This means that for every metre of length, the alloy gets 21 micrometres longer for each degree rise in temperature. A380, another common aluminium alloy, shows similar expansion behaviour. These values help engineers pick the right alloy for die casting. Tool steels, which are used to make dies, have lower coefficients of thermal expansion. This difference between die casting alloys and die materials can change the physical and mechanical properties of the final product. Engineers must think about these values to stop problems with expansion, cracking, or losing the correct size in aluminium die casting.
Coefficient of Thermal Expansion in Die Casting
Impact on Aluminium Die Casting
The coefficient of thermal expansion is very important in die casting. When the mould and alloys get hot, they both get bigger. The temperature rises fast during the process. The aluminium alloys inside the mould become very hot. The mould, which is usually steel, also heats up but not as quickly. This means the alloys and the mould expand at different speeds. The aluminium alloys usually expand more than the mould. When things cool down, both the alloys and the mould get smaller. They shrink at different rates. This can make gaps or tight spots. These changes affect how strong and tough the finished part is.
The table below shows what happens to the die when it gets hot during casting:
Parameter | Symbol | Value | Unit | Description |
|---|---|---|---|---|
Thermal expansion coefficient | γ | /°C | For die-casting die material | |
Temperature difference | ΔT | 300 | °C | Between room temperature and die temperature during molten aluminium injection |
Die length | L | 100 | mm | Length of the die considered |
Elongation due to heating | ΔL | 0.324 | mm | Calculated elongation of die |
Strain | ε | 3.24×10⁻³ | - | Calculated as ΔL/L |
Elastic modulus | E | 206 | GPa | Material property of die |
Calculated stress | σ = E·ε | 667.44 | MPa | Stress experienced by die during injection |
0.2% proof strength at 300°C | - | 2000 | MPa | Material yield strength at elevated temperature |
The thermal stress in the die is much less than the proof strength of the mould. This means the die will not change shape forever just from getting hot. But if the die is heated and cooled many times, it can still get damaged.
Die Casting Alloys and CTE Mismatch
Die casting alloys and moulds do not expand the same way. Aluminium alloys get bigger more than steel moulds do. This difference causes stress when casting. When aluminium alloys cool, they shrink faster than the mould. This can make the casting pull away or push against the mould. It can cause cracks, bending, or gaps where the alloys and mould meet.
Scientists have looked at how this difference affects casting. They saw that when alloys and moulds change size at different speeds, it causes distortion. Cameras and sensors checked the changes during heating and cooling. The studies showed that the mismatch in expansion causes stress and distortion. The expansion of the mould and alloys must be balanced to stop these problems. If not, heating and cooling again and again can make cracks, soft spots, and wear in the mould. This makes the mould last less time and the parts lower quality.
Dimensional Stability and Defects
Dimensional stability is very important in aluminium die casting. The way the alloys and mould get bigger and smaller changes the final size and shape. If the alloys do not expand like the mould, the part may not fit right. The shrinkage of the alloys must be thought about when making the mould. If not, the part can have problems like bending, cracks, or gaps.
The physical and mechanical properties of the alloys also change with heat. Getting bigger and smaller during casting can make stress inside the part. This stress can cause problems in the finished part. The expansion of the mould and alloys must be controlled to keep the part the right size. Good control of the coefficient of thermal expansion helps stop problems and makes sure the part is made correctly.
Tip: Engineers can help keep the part the right size by picking alloys and moulds that expand at similar rates. They can also design the mould to let it shrink and expand as needed during casting.
Managing Thermal Expansion in Die Casting
Material Selection Strategies
Picking the right alloy and mould is very important. Engineers try to use alloys that expand like the mould. This helps stop stress and cracks from forming. Aluminium die casting uses alloys like AlSi12. These alloys flow well, shrink less, and do not expand much. Changing how much silicon is in aluminium alloys can change how they expand and how strong they are. Adding copper, about 2-3%, makes the alloy stronger when hot. It does not make it heavier. Good CTE data for alloys like Al7075, titanium TC4, and stainless steel 316L helps engineers choose well. Tests show that different steels and special inserts in the mould change how tightly things press together and how heat moves. More pressure and cooling channels close to the mould surface help cool things down and control expansion. These ideas help manage CTE changes and make things work better in cars and planes.
Tests show that raising contact pressure from 0 to 30 bar cuts thermal contact resistance by up to 33%.
Cooling channels closer to the mould surface, at 6.5 mm instead of 9.5 mm, lower resistance by about 28%.
Aluminium die casting alloys are used a lot in many fields because they do not expand much and are strong.
Design for Thermal Expansion
How the mould is made is very important for controlling expansion and shrinking. Engineers use special ways like Design of Experiments and the Taguchi method. These help them find the best shape for the mould and where to put cooling channels. These methods help control how the metal hardens and lower the number of holes inside. The table below shows how changing the design can lower damage from heat and defects:
Aspect | Description |
|---|---|
Design Parameters Optimised | Cooling channel heat transfer, mould shape, process settings |
Key Findings | Better cooling and mould design lowers holes and controls how much the part shrinks |
Impact on Casting Quality | Holes drop below 5%, and the mould handles alloy changes better |
Implication for Thermal Fatigue | Better heat control means less damage from heat and longer mould life |
Engineers also make moulds so they can expand and shrink in a controlled way. They change the mould design to match how much the chosen alloy shrinks. This keeps the final part the right size and stops problems.
Quality Control and Best Practices
Quality control checks that expansion and shrinking do not cause problems in aluminium die casting. Engineers use different checks and tools to watch and improve quality:
Secondary Dendrite Arm Spacing (SDAS) checks the tiny structure and strength of the alloy.
How fast heat leaves the melted alloy, controlled by mould cooling and coatings, changes how the metal hardens and how many problems happen.
Quality control charts, like Six Sigma charts, track how many problems there are.
Design of Experiments, fishbone diagrams, and Pareto charts help find and fix problems like holes and shrinking.
Demerit Control Charts (DCC) put all problem levels into one score for better choices.
These checks help control what happens when things get bigger or smaller, so there are fewer problems and parts last longer. Checking often and changing the process when needed keeps the mould and alloys working well together for the best results.
Knowing about the coefficient of thermal expansion helps engineers. It lets them make better die casting parts. If they manage CTE well, there are fewer problems. The dies also last longer. Picking the right materials and design keeps products good.
Tip: Engineers must check CTE values before starting new projects. This helps parts stay strong and the right size from the beginning.
FAQ
What does the coefficient of thermal expansion mean in die casting?
The coefficient of thermal expansion tells us how much a material gets bigger or smaller when it heats up or cools down. In die casting, this number helps engineers know what will happen to parts and moulds as they get hot or cold.
Why do aluminium alloys and steel dies expand differently?
Aluminium alloys have a higher coefficient of thermal expansion than steel. This means aluminium gets bigger and smaller more than steel when the temperature changes. Because of this, stress or cracks can form in the finished part.
How can engineers reduce problems from thermal expansion?
Engineers pick alloys and die materials that expand at almost the same rate. They also make moulds that can move a little. Careful cooling and good control of the process help lower stress and stop defects.
What happens if the coefficient of thermal expansion is ignored?
If this value is not checked, cracks, bending, or bad fitting can happen in die cast parts. The mould might wear out sooner. The finished part may not be the right size or quality.
Can the coefficient of thermal expansion change with temperature?
Yes, the coefficient can change at different temperatures. Most materials get bigger faster when they are hotter. Engineers watch these changes to keep die cast parts strong and the right size.
See Also
Exploring The Advantages Of CAE In Die Casting Design
Key Elements That Affect The Price Of Die Cast Moulds
A Guide To Aluminium Extrusion And Die Casting Methods
Understanding Anodising And Its Use In Die Casting Items
Comprehending Shrinkage And Machining Allowances In Casting
About Hunan Puka
Established in 2016 and based in Hunan, China, with a liaison point in Berlin, we are a Tier 2 supplier for the automobile industry. We specialize in the production of customized aluminum die-casting parts designed for machines with a closing force ranging from 280 to 1250 tons, with subsequent manufacturing process CNC machining and surface treatment. Our commitment to quality is reflected in our accredited quality management system, certified by ISO9001:2015 and IATF16949:2016 standards.
