aluminium extrusion, aluminium profile, dimensional accuracy, online quenching, quenching (cooling) intensity, cooling rate, quenching sensitivity to cooling speed
In recent years, with the rapid development of economy, the increasing application of industrial aluminum profile results in higher demands, which are mainly in: 1. wider section and more complicated shape; 2. more accurate dimensional tolerance; 3. higher mechanical properties. To meet the requirements of the market for aluminum products, the production process and equipment should be more demanding, especially online quenching process. Because to achieve high mechanical properties, ensure fixed supersaturated solid solution, and reserve the hardening precipitates, it demands for fast cooling rate. However, the faster the cooling rate, the greater the residual stress and residual deformation of the quenched profile. Thus the dimensional accuracy of the profile can not be guaranteed. Given above three elements, we claim that more emphasis should be put on quenching process (especially online quenching): 1. with a various range of cooling rate to be managed to meet the requirements of quenching sensitivity to cooling speed and wall thickness of different alloys; 2. with adequate cooling rate, the deformation of the profile after cooling is effectively reduced to ensure the dimension precision.
The actuality of quenching technology in extruded aluminium
After nearly a decade of rapid development, machines including online quenching for aluminum extrusion has been greatly improved, yet it still can't answer to the development of aluminum products.
Most of the conventional online quenching in China is relatively backward
Failure to provide adequate cooling rate to meet the requirement of different alloys and wall thickness
Short of regulatory. With asymmetrical structure and uneven wall thickness, if the cooling rate can not be adjusted as per the section shape, it will cause deformation and unstable performance, for instance, thick-walled shapes often are cooled slowly, whereas thin-walled shapes are cooled quickly.
Difficult to operate. Some online quenching devices have the above mentioned functions, but the workers have difficulty in actual operation with poor accuracy
and high time consumption, so there is no way to compete the demand of use.
In view of large deformation, unreliable performance, high waste rate, and high cost, we conclude that it is critical for the domestic aluminum equipment industry to improve the online quenching equipment.
Online quenching devices in developed countries such as Europe, America and Japan are relatively advanced
It is a combination of air, mist and water cooling to provide a wide cooling range, so as to adopt proper cooling mode as per needs, like air cooling can be applied to 6063 alloy, high pressure spray can be applied to thick-walled 6061 alloy, and mist cooling can be applied to thin-walled 6061 alloy.
Good regulatory. 1. The nozzles are distributed around the section with independently adjustable columns, which makes it possible to apply proper cooling rate to
achieve optimal mechanical properties and correct dimensional tolerances. 2. Multi section cooling with independently adjustable sections. Due to the initial rate of quenching process dictates the initial metal hardness. Some shapes will warp if cooled too quickly. Therefore, a mild cooling rate is preferred at the initial of quenching process. When the profile is slowly cooled down to a certain temperature, then a fast cooling rate is allowed. Thus ensures adequate cooling intensity and small deformation.
Easy to operate. All these adjustments can be managed on man-machine interface, which is convenient and operable. The control system has automatic memory
function, which automatically records the process parameters of each production and can be automatically recalled when produce the same profile.
However, the extrusion process is intermittent. The effective cooling time of the profile in dead cycle is different from that in extrusion cycle, which will cause unbalanced quenching intensity, performance, and deformation. This problem also exists in the international advanced quenching devices.
Introduction of new online precise quenching technology and equipment in China
Adequate cooling range
To ensure fixed supersaturated solid solution and optimum mechanical properties, and reserve the hardening precipitates, it requires adequate cooling rate. As a result, a reasonable cooling source, cooling length and cooling density are claimed. Cooling source refers to the fan, water pump and cooling media. Cooling length refers to the length of air nozzle, spray nozzle, or water tank. Cooling density refers to the distribution density of the air nozzles or spray nozzles. The three elements are determined by the maximum linear density of the profile, the quenching sensitivity to the cooling speed and the extrusion speed. With bigger linear density, greater quenching sensitivity to the cooling speed, and faster extrusion speed, it needs bigger power of ventilator and water pump, longer quenching zone and cooler water temperature. When low temperature water can not meet the cooling rate, different solvents can be added to regulate the cooling capacity of water, such as, polyethanol is used to control the cooling rate by regulating the concentration of ethanol solution.
Consistent cooling speed
To ensure uniform performance and less irregularity and bending, it requires consistent cooling rate. Thus, it is necessary to adjust the cooling intensity to section shape. For example, thick-walled shapes are cooled using a high cooling intensity, while thin-walled shapes are cooled using a lower cooling intensity, which allows the profiles to be cooled to the same temperature at the same time.
It is necessary to ensure consistent cooling time so as to ensure balanced quenching intensity, that is, the effective cooling time of each section in quench zone is consistent.
Proper ratio change of width to height
The ratio change of width to height, particularly for large extrusion press, is great. When produce aluminum sheet, the ratio is more than 100:1, while produce aluminum tube or billet, the ratio may be 1:1. Under this circumstance, we need to manage adequate gap between nozzles of four sides to ensure effective cooling rate and reduce energy consumption.
High automation and easy operation.
Two new online quenching devices
To solve the above problems, we designed two types of online quenching devices. One is for large extrusion press (above 1650T), and the other is for small extrusion press (below 1650T). Because for relatively large section produced by the large extrusion press, it is easy to realize the cooling differential control of the four sides of profile. While for relatively small profile section, it is relatively difficult to achieve the cooling differential control in the four sides of profile. Therefore, the design should be accordingly.
Functional features of online quenching device for large extrusion press
Combination of air, air & mist, mist, high pressure spray cooling
We use the combination of air, air and mist, mist, high pressure spray cooling, and each function can be adjusted as per requirement. The cooling intensity is adjustable for different wall thickness and different alloys. For instance, thin-walled 6063 alloy uses air cooling, thick-walled 6063 alloy or thin-walled 6061 alloy uses air and mist cooling, medium and thin-walled 6061 alloy uses mist cooling, thick-walled 6061 alloy uses high pressure spray.
The capacity of the cooling source is determined by those factors such as the ratio of weight to length, extrusion speed and quenching sensitivity to cooling speed. With higher ratio of weight to length, higher extrusion speed and greater quenching sensitivity to cooling speed, it requires bigger power of the ventilator and water pump, and longer quenching zone.
The main reason of using air and mist combined cooling is that the environment temperature in the extrusion workshop is higher. When using air cooling only, even with high air pressure and big air volume, fast cooling rate cannot be attained and energy is costly. When using mist cooling or spray cooling only, the cooling rate of thin-walled shapes is relatively high and deformation is easily caused. When using air and mist combined cooling, the better cooling effect can be achieved and the energy is saved.
Distribution of columns and differential adjustment of cooling intensity
Distribution of columns and differential adjustment of cooling intensity
Sectional adjustment and switch on/off in sequence
The cooling intensity of each section can be adjusted independently. When some shapes are particularly easy to bend during quenching, while the above two functions are not capable, you can reduce the cooling intensity of the front section, and increase the cooling intensity of the rear section to ensure adequate cooling and less deformation.
For the sake of obtaining balanced quenching, the nozzles should be divided into sections (see fig 2). Each section has an independent control valve. During the dead cycle, the section is switched off in sequence as the direction from handling table to extrusion press. During the extrusion cycle, the section is switched on in sequence reversely. In this way, the cooling time is basically consistent, thus to ensure uniform performance and small bending.
Sectional adjustment and switch on/off in sequence
Due to big ratio change of width to height, the distance between nozzles and profile surface is changeable. If the nozzles on top are designed independently and moveable (see fig 3), the distance between nozzles and profile surface can be adjusted properly according to the specific ratio to improve precision and save energy.
Subsidiary puller head
During quenching operation, especially water-cooling operation, the puller is not allowed to enter the quenching zone. If the puller enters the quenching area, the quenching device will not work properly and the desired hardness will not be well obtained, which will cause the waste of the section. As the puller does not need to enter the quenching zone, and the traction can be done simultaneously, the subsidiary puller head (see fig 4) is adopted. When produce the first billet, the subsidiary puller head moved by the puller is used to pull the profile outside of the quenching zone.
Man-machine interface control and data memory function
To facilitate the control of operators, all online quenching device and process parameters are controlled by man-machine interface, which is simple and convenient (see fig 5). In order to improve the efficiency and reduce the waste generated by improper adjustment process, the control system is designed with automatic memory function. For each production, the quenching process parameters can be saved. When produce the same profile, inputting the alloy code, the system will automatically recall relative parameters.
Remote debugging, monitoring and maintenance
The control system has remote monitoring interface to facilitate debugging, monitoring and maintenance. When necessary, the remote debugging, monitoring and maintenance can be conducted via internet.
These functions have been applied to the online quenching device for 2500T and 3600T extrusion line in Guangdong Haomei Aluminum Industry (see fig 6). After quenching and artificial aging, the mechanical properties and the dimension precision of profile are guaranteed.
Functional features of online quenching devices for small extrusion press
Apart from above mentioned functions (under clause a, b, c, f, g) for large extrusion press, the following functions of online quenching devices for small extrusion press are put (see fig 7)
The nozzles are designed to swing and rotate.
To achieve accurate positioning of water spray onto the small section, the spray nozzles of water and mist are designed to swing and rotate (shown as α and β in fig 7). It is to adjust the rotating angle or swing angle to adjust the spray width, even align with the center line of the extrusion press, so water and mist are amassed to a certain shape of the profile.
The nozzles are designed to move in radial direction
The nozzles can move a certain distance in radial direction, which is for better cooling positioning and lower loss of water pressure. The design has been applied to the online quenching equipment for 1450T and 1800T extrusion line in Shenzhen Fusheng Hardware Products Co., Ltd.
Further development of online quenching
At present, online quenching device is controlled by men. Operators select proper cooling mode and cooling rate based on alloy composition, cross section and wall thickness. Nevertheless, this will not only affect dependence on man, but also affect efficiency and accuracy. Therefore, the intelligent control should be developed. To input the alloy code and cross-section diagram into the control system, the control system will automatically select the corresponding cooling mode and cooling speed.
Convert offline quenching into online quenching
Because offline quenching requires secondary heating and high energy consumption, it is a must to convert offline quenching into online quenching to significantly reduce energy consumption and improve efficiency.
“Foshan Nanhai Save Aluminium Equipment Co., Ltd.” obtained the name of “Guangdong Save Aluminium Equipment Co., Ltd” on Apr 25, 2012.
Mr. Zhutang Wang , Mr. Rongzhang Tian Aluminium Alloy and Its Processing Manual Zhongnan University Press (The 3rd Edition) 2005