Protective atmosphere continuous aluminum brazing furnace is a kind of brazing equipment for aluminum radiator. It is suitable for brazing aluminum radiator, condenser, evaporator, intercooler and oil cooler. It is widely used in vehicles, electronics, household appliances, led, aviation, military equipment, generator set, construction machinery, thermal power station and wind power equipment.
Aluminum brazing is based on the fact that the melting point of Al Si alloy is lower than that of AI metal material. The filler metal is usually placed near or inside the welding surface of the parts to be connected. The brazing flux is coated between the parts and the welding surface and the surface. After installation and drying, the parts are heated to the temperature of brazing and welding finance under the protection of nitrogen atmosphere. At the same time, the flux breaks the oxide layer of aluminum. After cooling, the solder will form a metal bond with the contact surface of the parts.
The process of continuous brazing in protective atmosphere furnace is a special process. The performance of brazing furnace is directly related to the quality stability and service life of brazing products.
2. Relevant standards for temperature management of brazing furnace
For the temperature management of brazing furnace, the following standards can be referred to gb/t 80825-2016, AIAG heat treatment evaluation system standard cqi-9 (Third Edition), American aviation material standard AMS 2750e high temperature measurement, etc.
The actual temperature displayed on the control cabinet of brazing furnace is the temperature control precision, which is the deviation from the set temperature. The temperature control accuracy is generally required to be ± 1 ℃.
System accuracy (SAT): after reasonable compensation, the temperature system, which is composed of thermocouple, compensating conductor and display instrument, is compared with another set of temperature system composed of thermocouple, compensating conductor and display instrument with known error to determine whether the temperature measured by the instrument system of brazing furnace meets the requirements.
The accuracy of the temperature display system is tested during the system accuracy test. The thermocouple installed on the furnace no longer requires removal for calibration. As long as the SAT test is conducted regularly, the K-type thermocouple installed on the brazing furnace shall be replaced every year.
For brazing furnace with temperature uniformity of ± 5 ℃, the maximum error of system accuracy is 0.3% t or ± 1.7 ℃. The maximum allowable correction amount of the process temperature system of the brazing furnace is ± 3 ℃, and the thermocouple with the correction amount exceeding 3 ℃ shall be replaced. It is usually tested once in two weeks, and once a month after several tests are qualified.
The temperature uniformity of brazing furnace is required to be ± 5 ℃, in gb/t 30825-2014, the furnace is a type II furnace, and the instrument type is D. the system accuracy shall be tested once a week. It is required to test the uniformity of furnace temperature once a month, and once every two months after 8 consecutive tests are qualified. Two tests can take more than three days. In cqi-9, SAT is required to be tested once a quarter, and temperature uniformity is required to be tested once a year. After furnace maintenance or furnace modification, it must be tested. In AMS 2750e, for brazing furnace with temperature uniformity of ± 5 ℃, the instrument type is D type, sat requires weekly test and tus test once a month.
3. System accuracy test (SAT) and management of control thermocouple
There must be a reserved hole beside the control thermocouple that can be inserted into the SAT test thermocouple. When designing and manufacturing the brazing furnace, the position must be reserved.
SAT test thermocouple, compensation lead and test instrument must be within the validity period of calibration. The maximum service life of SAT test thermocouple is one year.
The control thermocouple is no longer removed for recalibration. The maximum service life is one year. The thermocouple must be replaced after one year.
4. Temperature uniformity test (TUS) and analysis of actual temperature curve
4.1 SMT furnace temperature tracker
Number of channels: 12 channels (12 channels measure external temperature, one channel is used for instrument internal temperature)
Temperature measurement range: 0 ~ 900 ℃
Thermocouple: Type K
Resolution: 0.1 ℃
Measurement accuracy: 0.5 ℃ or 0.2%
Instrument size: Φ 76 * 200
Operating temperature: – 40 ℃ to 250 ℃
Incubator size: 400 (L) x 200 (W) x 150 (H)
Holding temperature and time: when the instrument is put in the incubator, it can be used continuously for 1 h at 630 ℃, including 25 min at 400 ℃, 15 min at 600 ℃ and 10 min at 800 ℃
The SMT furnace temperature tracker needs to be calibrated once a year, and the thermocouple needs to be replaced every six months.
Data size: 252000 lines of data
Recording time: all 12 channels are recorded once in 1s, which can record 70h data. After full recording, the instrument does not stop recording data (FIFO), and the oldest data is replaced by the latest data.
20 groups of brazing data can be recorded at the same time
Sampling period: sampling period: 1s, 2S, 3S, 5S, 40s, 60s, set by software
Battery: the battery works continuously for more than one year, and the software has battery power display
Support system: support XP, win7, win8, win10
4.2 test curve and process improvement
In fact, the control thermocouple is not in the effective heating zone of the brazing furnace. There must be a temperature difference between the temperature of the effective heating zone and the temperature of the control thermocouple.
The required temperature curve of the products in the brazing furnace is shown in Figure 3. Generally, it is required that the time greater than 600 ℃ is 3min, and the temperature uniformity in the brazing process is required to reach 600 ℃± 5 ℃.
5 conclusions and suggestions
5.1 for newly designed or modified brazing furnace, thermocouple holes for SAT test shall be reserved.
5.2 for newly put into operation or revamped brazing furnace, acceptance of brazing furnace shall be carried out by testing temperature curve.
5.3 conduct SAT test regularly, usually once a month. The temperature control thermocouple should be replaced regularly, and it is not allowed to calibrate again. The K-type thermocouple should be replaced once a year.
5.4 according to the requirements of customers, tus test shall be conducted regularly, once a month. The furnace temperature tracker for TUS test is required to be calibrated once a year, and the thermocouple needs to be replaced every half a year.