Heating temperature control of aluminum alloy forgings is the core link to ensure the quality of forgings. Excessive temperature may not only cause cracking, but also cause various defects. The following is an analysis of temperature control technology, temperature influence mechanism and preventive measures:
I. Precise control technology of heating temperature
1. Temperature threshold setting based on alloy grade
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Example: When a company forges 7075 battery shells, it uses segmented temperature control: in the preheating stage, it is kept at 400℃ for 2h, and then heated to 430℃±5℃ constant temperature to ensure that the β phase (MgZn₂) is fully dissolved, while avoiding the melting of the low melting point eutectic (475℃) at the α+β phase boundary.
2. Heating equipment and temperature control system
Gas furnace segmented temperature control: a three-chamber continuous heating furnace (preheating chamber 400℃, heating chamber 450℃, and equalizing chamber 430℃) is used, with an infrared thermometer (accuracy ±3℃), and the furnace temperature uniformity is controlled within ±10℃.
Precise control of electric heating furnace: the vacuum resistance furnace uses the PID intelligent temperature control system to heat up to the set temperature at a rate of 5℃/min, and the fluctuation in the insulation stage is ≤±5℃, which is suitable for sensitive alloys such as the 7 series.
Dynamic compensation of induction heating: For complex-shaped forgings (such as multi-cavity structures of battery shells), medium-frequency induction heating (frequency 20-50kHz) is used to locally compensate the temperature through the eddy current effect, so that the cross-sectional temperature difference is less than 15°C.
3. Temperature field simulation and real-time monitoring
CAE simulation before forging: Deform-3D is used to simulate the heating process and predict the temperature distribution of the billet. For example, the simulation of a certain L-shaped battery bracket forging shows that the temperature at the corner is 20°C lower than that on the plane. In actual production, it is compensated by partition heating coils.
Online infrared thermal imager: Scanning speed 100 frames/second, real-time generation of temperature cloud map, when local over-temperature is detected (such as > set value 15°C), the system automatically starts the air cooling device to cool down.
II. Analysis of the mechanism of cracking caused by excessive temperature
1. Structural defects caused by thermal damage
Three characteristics of overburning:
Oxidation triangles appear at grain boundaries (when the temperature is greater than the eutectic melting point, Mg₂Si and other phases melt);
Grain boundaries widen and form a network (for example, when 6061 aluminum alloy is heated at 560℃ for 20min, the liquid phase ratio at the grain boundaries reaches 3%);
Remelting balls appear between dendrites (7075 aluminum alloy is kept at 480℃ for 1h, and the Al-Zn-Mg phase between dendrites melts).
Granular and weak grains: When the temperature exceeds the upper limit of the recrystallization temperature (such as 460℃ for 7075), the grain size grows rapidly from 10-20μm in the forged state to more than 500μm, the plasticity decreases by 40%, and cracks occur along the grain boundaries during forging.
2. Stress concentration induces cracking
Temperature difference stress cracking: When the heating rate is too fast (e.g. >15℃/min), the temperature difference between the surface and the core of the forging is >50℃, generating thermal stress (σ=EαΔT). When σ> material yield strength (e.g. 7075 at 400℃ σs=120MPa), cracking occurs.
Phase transformation stress superposition: When the 2-series aluminum alloy is heated to 500℃, the dissolution rate of the θ phase (CuAl₂) is uneven, and the local phase transformation stress is superimposed on the forging stress, causing the crack to extend along the grain boundary.
III. Anti-cracking process countermeasures
1. Gradient heating and insulation control
Step-type heating curve:
Low temperature section (200-300℃): heating rate 5℃/min, eliminate the internal stress of the billet;
Medium temperature section (300-400℃): rate 10℃/min, promote uniform distribution of the second phase;
High temperature section (400 - set temperature): rate 5℃/min, ensure uniform temperature.
Insulation time calculation: according to billet thickness (mm) × 1.5-2min/mm, for example, 100mm thick 7075 billet, 430℃ insulation for 2.5-3h, so that the strengthening phase is fully dissolved.
2. Die preheating and isothermal forging
Mold temperature matching: before forging, the mold is preheated to 250-300℃ (6 series) or 180-220℃ (7 series) to reduce the temperature difference stress caused by rapid cooling of the forging.
Isothermal forging technology: Forging at a low speed of 0.01-0.1mm/s on a servo press, while the built-in heating rod in the mold maintains the billet temperature at ±3℃, which is suitable for complex thin-walled battery shells (wall thickness <3mm).
3. Crack prevention and detection
Surface treatment before heating: Remove the oxide scale on the surface of the billet (when the thickness is >0.2mm, the microcracks under the oxide scale will expand at high temperature), and use shot peening or alkali washing for pretreatment.
Non-destructive testing control: 100% ultrasonic flaw detection (frequency 2.5-5MHz) after forging to detect grain boundary loosening caused by overburning (reflection amplitude ≥φ2mm flat bottom hole equivalent).
Email：cast@ebcastings.com
Heating temperature control of aluminum alloy forgings is the core link to ensure the quality of forgings. Excessive temperature may not only cause cracking, but also cause various defects. The following is an analysis of temperature control technology, temperature influence mechanism and preventive measures:
I. Precise control technology of heating temperature
1. Temperature threshold setting based on alloy grade
|
Example: When a company forges 7075 battery shells, it uses segmented temperature control: in the preheating stage, it is kept at 400℃ for 2h, and then heated to 430℃±5℃ constant temperature to ensure that the β phase (MgZn₂) is fully dissolved, while avoiding the melting of the low melting point eutectic (475℃) at the α+β phase boundary.
2. Heating equipment and temperature control system
Gas furnace segmented temperature control: a three-chamber continuous heating furnace (preheating chamber 400℃, heating chamber 450℃, and equalizing chamber 430℃) is used, with an infrared thermometer (accuracy ±3℃), and the furnace temperature uniformity is controlled within ±10℃.
Precise control of electric heating furnace: the vacuum resistance furnace uses the PID intelligent temperature control system to heat up to the set temperature at a rate of 5℃/min, and the fluctuation in the insulation stage is ≤±5℃, which is suitable for sensitive alloys such as the 7 series.
Dynamic compensation of induction heating: For complex-shaped forgings (such as multi-cavity structures of battery shells), medium-frequency induction heating (frequency 20-50kHz) is used to locally compensate the temperature through the eddy current effect, so that the cross-sectional temperature difference is less than 15°C.
3. Temperature field simulation and real-time monitoring
CAE simulation before forging: Deform-3D is used to simulate the heating process and predict the temperature distribution of the billet. For example, the simulation of a certain L-shaped battery bracket forging shows that the temperature at the corner is 20°C lower than that on the plane. In actual production, it is compensated by partition heating coils.
Online infrared thermal imager: Scanning speed 100 frames/second, real-time generation of temperature cloud map, when local over-temperature is detected (such as > set value 15°C), the system automatically starts the air cooling device to cool down.
II. Analysis of the mechanism of cracking caused by excessive temperature
1. Structural defects caused by thermal damage
Three characteristics of overburning:
Oxidation triangles appear at grain boundaries (when the temperature is greater than the eutectic melting point, Mg₂Si and other phases melt);
Grain boundaries widen and form a network (for example, when 6061 aluminum alloy is heated at 560℃ for 20min, the liquid phase ratio at the grain boundaries reaches 3%);
Remelting balls appear between dendrites (7075 aluminum alloy is kept at 480℃ for 1h, and the Al-Zn-Mg phase between dendrites melts).
Granular and weak grains: When the temperature exceeds the upper limit of the recrystallization temperature (such as 460℃ for 7075), the grain size grows rapidly from 10-20μm in the forged state to more than 500μm, the plasticity decreases by 40%, and cracks occur along the grain boundaries during forging.
2. Stress concentration induces cracking
Temperature difference stress cracking: When the heating rate is too fast (e.g. >15℃/min), the temperature difference between the surface and the core of the forging is >50℃, generating thermal stress (σ=EαΔT). When σ> material yield strength (e.g. 7075 at 400℃ σs=120MPa), cracking occurs.
Phase transformation stress superposition: When the 2-series aluminum alloy is heated to 500℃, the dissolution rate of the θ phase (CuAl₂) is uneven, and the local phase transformation stress is superimposed on the forging stress, causing the crack to extend along the grain boundary.
III. Anti-cracking process countermeasures
1. Gradient heating and insulation control
Step-type heating curve:
Low temperature section (200-300℃): heating rate 5℃/min, eliminate the internal stress of the billet;
Medium temperature section (300-400℃): rate 10℃/min, promote uniform distribution of the second phase;
High temperature section (400 - set temperature): rate 5℃/min, ensure uniform temperature.
Insulation time calculation: according to billet thickness (mm) × 1.5-2min/mm, for example, 100mm thick 7075 billet, 430℃ insulation for 2.5-3h, so that the strengthening phase is fully dissolved.
2. Die preheating and isothermal forging
Mold temperature matching: before forging, the mold is preheated to 250-300℃ (6 series) or 180-220℃ (7 series) to reduce the temperature difference stress caused by rapid cooling of the forging.
Isothermal forging technology: Forging at a low speed of 0.01-0.1mm/s on a servo press, while the built-in heating rod in the mold maintains the billet temperature at ±3℃, which is suitable for complex thin-walled battery shells (wall thickness <3mm).
3. Crack prevention and detection
Surface treatment before heating: Remove the oxide scale on the surface of the billet (when the thickness is >0.2mm, the microcracks under the oxide scale will expand at high temperature), and use shot peening or alkali washing for pretreatment.
Non-destructive testing control: 100% ultrasonic flaw detection (frequency 2.5-5MHz) after forging to detect grain boundary loosening caused by overburning (reflection amplitude ≥φ2mm flat bottom hole equivalent).
Email：cast@ebcastings.com
