Preparation before vacuum brazing
After the joint structure, brazing material and brazing method are selected, it is necessary to realize the brazing of parts through a reasonable process. The brazing process is an important link to form the brazing head and ensure the quality of brazing, so it is necessary to make a reasonable brazing process according to the brazing method, material and structure adopted. Soldering process refers to the brazing process during the production and the floorboard of the technical requirements, including brazing former parts surface preparation, parts assembly and positioning, add solder and fixed, qian welding thermal cycle and brazing after processing, etc.
1. L surface preparation
Stay in front of the vacuum brazing welding parts processing, storage and transportation process, inevitably will be covered with oxide film, or grease, dirt, etc., they will interfere with solder on the base of spreading and caulking, and cause unnecessary pollution to vacuum brazing furnace, thus to ensure the smooth progress of vacuum brazing, vacuum brazing before surface preparation necessary to treat the welding parts. Parts surface preparation process is mainly the removal of surface oil, dirt and oxides, moreover, in some cases, to ensure the quality of vacuum brazing, still need the brazing material surface coating processing.
1.1.1 removal of grease and organic matter on the surface of parts
All the organic matter on the parts in a vacuum brazing heating gas, and may affect the solder wetting left in the surface of parts, so the surface grease and other organic matter such as paint, marker pen mark, crossed blue shall be cleared.
The following methods can be used to remove grease and dirt from the surface:
(1) cleaning or cleaning with organic solvent;
(2) cleaning in vapors of organic solvent;
(3) chemical cleaning of aqueous degreasing solution;
(4) electrochemical degreasing, etc. When using liquid cleaning, ultrasonic wave can be used as auxiliary cleaning method.
For single and small batch production, the easiest way is to use organic solvent cleaning or scrub. Commonly used solvent such as acetone, gasoline and ethanol. In mass production, dichloroethane, trichloroethane, trichloroethylene and other organic solvents can be used to remove oil. They dissolve grease well and regenerate easily. Among them, the most commonly used is trichloroethylene, which can dissolve most grease and organic matter and is not inflammable, and can use high cleaning temperature to improve the cleaning efficiency. However, for titanium and zirconium alloys, trichloroethylene should not be used for cleaning, only non-chlorine solvents can be used. Parts can be cleaned by solvent immersion, solvent vapor cleaning and composite method. Immersion washing is generally used for small parts and parts with serious oil content on the surface. Steam cleaning is mostly used for large parts and parts with light oil pollution on the surface. The combined method of immersion washing and steam cleaning is more effective.
Chemical removal of oil by tomb-water solution is also a common method to remove surface oil pollution. The better effect can be achieved by using water base solution together with ultrasonic wave when cleaning. The cleaning process of water base solution is simple, low cost and good effect. The disadvantage is that the solution sometimes needs to be heated, it is difficult to regenerate after use, and some metals have corrosive effects on the need for dry treatment. The suitability of the component matrix to the solution should be taken into account in selecting the chemical degreasing method.
Table 1-1 shows the components of the commonly used chemical degreasing solution. In addition, the commercial oil remover can have a good effect. For example, the lxf-52 oil remover can have a good oil removal effect at room temperature and has a certain corrosion inhibition effect.
For paint, marker pen mark on the surface of the parts, such as line blue usually adopt chemical scrubbing solvent removal, commonly used solvents such as ethanol, acetone, petrol, generally need to be carried out after oil removal. For thicker paint, mechanical removal is sometimes required.
1.1.2 removal of surface oxidation film
The removal of oxides on the surface of parts can be carried out by means of mechanical removal and chemical cleaning according to the requirements of material, oxide film thickness and precision.
There are many ways to remove machinery, such as file, scraper and sandpaper grinding, but the production efficiency is very low, only suitable for single or small batch production. With wire brush, metal wire wheel, sand cloth wheel, sand wheel grinding clean, slightly higher efficiency, suitable for small batch production. For large area and mass production of parts, can be used to blow sand cleaning, blowing sand removal efficiency is high, generally used for ferrous metal, nickel-based alloys and so on. Blown sand and sand paper burnish hind parts residue on the surface or embedded in sand to a certain extent, will affect the solder wetting brazing, should be removed, can use methods such as ultrasonic cleaning, solvent to wipe purify. For soft non-ferrous metals such as copper and aluminum alloy, it is not suitable to use sandpaper grinding or sand blowing to remove the surface oxidation film.
In addition to mechanical removal, the most commonly used method of chemical etching is to remove surface oxides by pickling or alkaline cleaning. Compared with mechanical removal, chemical etching has the advantages of high production efficiency, good removal effect and easy quality control, especially for aluminum, magnesium, titanium and its alloys. Therefore, it is the main method used in mass production. However, the process control is complex, the equipment and equipment cost is high, and the improper disposal of waste liquid is easy to cause environmental pollution.
For different materials, different chemical solutions are used due to different surface oxidation film properties. Even with the same material, there are often multiple solution formulations. For steel, nickel base alloy, copper alloy, titanium alloy, etc. generally need to be pickling, while for aluminum alloy, magnesium alloy, etc. need to be alkaline washing, and then passivation treatment in acid solution.
Solder on the surface of the oxide film can also adopt the method of chemical etching, chemical etching solution solder surface oxidation film formula can be reference to its composition is close to the substrate materials used in the solution formula, but for thin strip solder, due to its size is thinner, need to consider the effect of chemical etching on the solder size, and take corresponding measures to compensate.
1.1.3 surface plating
Before the vacuum brazing of parts surface plating metal is a special soldering process measures, are usually based on simplified vacuum brazing process or requirements for improving the quality of vacuum brazing, but in some cases is the main technology to realize the parts good vacuum brazing. The function of metal coating on the surface of parts can be divided into three types: process coating, protective coating and solder coating. The process methods of metal plating include electroplating, chemical plating, hot dip plating, pressure coating, physical vapor deposition (PVD), chemical vapor deposition and so on.
Technology of coating is mainly to improve or simplify the vacuum brazing process conditions, used for surface more susceptible to oxidation and surface oxidation film stability, in particular under the condition of vacuum brazing process is not easy to be solder wetting of the parent metal. Plating metal for vacuum brazing technology commonly good metal, such as nickel, copper, gold, silver, etc., surface plating, can be in relatively low vacuum brazing process conditions to obtain good braze welding head, improve the quality of joint quality and stability. After coating, it can also realize the smooth brazing connection between the solder materials, such as ceramic, tungsten, molybdenum materials and different materials, which are difficult to be wetted under the normal vacuum brazing process. In order to improve the strength of the joint, most of the process coatings on the parts should be dissolved by the solder.
The role of protective coatings is to inhibit some harmful reactions that may occur during vacuum brazing. For example, under the action of solder, the self-cracking of the base material, the formation of brittle phase between the filler metal and the base material, as well as the changes in the composition and performance of the base material. It is hoped that the coating can be well wetted by the solder and the dissolution reaction degree will be small in order to achieve better isolation and protection effect.
The coating can also be used as solder directly. Implementation in the form of coating solder to add mainly for the following purposes: one is the simplified solder adding process, in the large area, the structure of the brazing seam simplified in the production of vacuum brazing filler metals of add, fixed vacuum brazing process, improve the efficiency of production, such as vacuum brazing of aluminium alloy heat exchangers with composite brazing plate, etc.; Secondly, the precise control of the amount of solder can be realized. The third is to realize the reliable addition of difficult solder, such as large area addition of difficult to be processed into foil band solder. Solder coating including a single set of RMB yuan or more groups of solder, solder by multi-layer coating or more groups of yuan alloy coating technology to achieve, sometimes also can be in the process of heating and the parent metal reaction liquid solder, such as silver plated copper surface eutectic reaction brazing, etc.