Condition for die-casting to meet brazing is that there should be neither pores nor hydrogen pores. Main production process of die-casting is aluminum alloy smelting - testing - transfer - insulation - die-casting, which requires that each link should follow strict process requirements to produce die-castings suitable for brazing. Dalian Yaming developed a thermal management system flow plate die-casting for a new energy vehicle company. Through improvement of traditional die-casting process, die-casting produced can meet brazing requirements and pass sealing test of 3 MPa pressure to verify feasibility of brazing of die-castings.
Flow channel plate uses Al-Ni alloy [composition (mass fraction, same below) is 0.07% Si, 0.97% Fe, 0.98% Mn, 0.04% Mg, 2.94% Ni, 0.01% Ti, balance is Al], and its solidus temperature is about 640 ℃, with good casting processability and welding performance. Controlling hydrogen content in aluminum liquid during aluminum alloy smelting process is one of key factors for success of brazing. Solubility of hydrogen in aluminum alloy liquid is very large, but solubility in solid alloy is very low. Therefore, during cooling and solidification of alloy, a large amount of hydrogen is precipitated to form hydrogen pores. Grade of solder is 4104, and brazing temperature is 600 ℃. Hydrogen will be amplified and released from welding surface during brazing heating process. Excessive release of hydrogen will affect fusion of die casting and solder, resulting in loose welding, affecting strength and sealing. During smelting process, hydrogen in aluminum liquid mainly comes from moisture, including undried aluminum ingots, refining agents, covering agents, modifiers, furnace linings, crucibles, inadequately dehydrated gases, coatings on tools, and refractory wool covered by launder. Moisture from these sources will react with aluminum liquid in the form of 2Al(1)+3H2O=Al2O3(s)+ 6[H]Al to generate atomic hydrogen dissolved in aluminum liquid. In order to control generation of moisture, melting furnace body must be fully preheated and ensure dryness. Preheating temperature is 730~750℃, and auxiliary tools must also be fully preheated and dried. At the same time, it is also necessary to avoid alloy materials, crucibles, tools, etc. from being contaminated by organic matter such as grease.
When melting furnace is heated by natural gas or heavy oil, hydrocarbons may also decompose to generate hydrogen and be absorbed by aluminum liquid. In order to reduce absorption of hydrogen and oxidation of alloy liquid, a non-flame direct heating method for melting aluminum alloys is adopted. Melting equipment uses LSN-1000H tiltable gas-fired crucible melting and holding furnace with a volume of 1 t and a melting capacity of 350 kg/h. Aluminum alloy uses clean and fully preheated alloy ingots, as well as purified recycled materials such as slag bags, material handles, scrapped parts, etc.
Hydrogen content in aluminum increases with increase of melting temperature, and casting temperature of aluminum liquid should be strictly controlled during casting process. Figure 1 shows solubility of hydrogen in pure aluminum. It can be seen that above 670 ℃, solubility of hydrogen in aluminum liquid increases slowly with increase of alloy temperature, and drops sharply below 670 ℃, while solid solubility is close to 0. Therefore, temperature needs to be strictly controlled during melting process, and it should not be too high. Melting liquid temperature is 730~750 ℃. Crucible melting furnace is equipped with an insulation cover that can be passed through inert gas, and high-purity nitrogen (99.999%) can be filled in for protection during melting process. After melting is completed, confirm that temperature reaches discharge standard, then transfer ladle can be used to discharge liquid. Schematic diagram of melting furnace is shown in Figure 2.

 

Figure 1 Solubility of hydrogen in pure aluminum

 

Figure 2 Schematic diagram of melting furnace
1. Aluminum ingot 2. Insulation cover 3. Crucible 4. Combustion chamber 5. Combustion nozzle
Move transfer ladle to refining station, and start equipment for refining after checking process conditions. Inert gas refining method is adopted, equipment is the XC 220-1 automatic rotary degasser, and refining gas is high-purity nitrogen (volume fraction is 99.999%). Refining parameters are determined through multiple tests: gas pressure is 0.2~0.3 MPa, rotor speed is 300~500 r/min, gas flow rate is 1.0~1.5 m3/h, and time is 6~8 min. After refining, use a skimmer to remove the slag, then use a sampling spoon to cast component test sample, and send it for inspection after cooling. After taking sample, HYCAL Mini online hydrogen meter is used to detect hydrogen content and temperature. Refining diagram is shown in Figure 3.

 

Figure 3 Schematic diagram of aluminum alloy refining
1. Rotary degasser 2. Graphite rotor 3. Asbestos plug 4. Teapot ladle 5. Bubble
After refining and passing test, aluminum liquid is transferred to insulation furnace with a sealed teapot-shaped turnover bag protected by an inert gas atmosphere. Spout is sealed with an asbestos plug during transfer process.
ZCD 1200 quantitative insulation furnace is used. It must be fully preheated to 730~750℃ before use, and inert gas protection is used during pouring. Insulation furnace is equipped with an automatic rotary degassing device, which can continue to remove hydrogen from aluminum liquid to ensure that hydrogen content remains stable during continuous production. When stopping production, it must be protected by an inert gas atmosphere to avoid hydrogen absorption. Schematic diagram of insulation furnace is shown in Figure 4.

 

Figure 4 Schematic diagram of insulation furnace
1. Lifting pipe 2. Graphite rotor 3. Blowing hole 4. Bubble
Sealing method is optimized from mold design to ensure that expected vacuum degree can be achieved during die casting production. First, mold parting surface is processed into a dovetail groove to form a completely closed area, and a heat-resistant rubber strip is installed. Secondly, push rod is sealed with an annular groove plus an O-ring. Thirdly, material cylinder is sealed with a sealing strip and a high-temperature resistant sealant, and a vacuum punch is used to minimize leakage that may occur during vacuuming process. Finally, mold vacuum valve adopts a washboard type with reliable performance and low failure rate.
Vacuum pump can be selected in a combination of 60 m3/h and 100 m3/h. It was finally determined that 800 L vacuum tank is equipped with a 60 m3/h vacuum pump, and 1 000 L vacuum tank is equipped with a 100 m3/h vacuum pump. 1 000 L system double circuit corresponds to two circuits of cavity, and 800 L system single circuit corresponds to material cylinder. Start-stop control mode of vacuum system is: for cylinder circuit A, vacuum starts when punch moves forward to close pouring port - vacuum is closed when punch reaches cylinder vacuum interface; for cavity vacuum circuits B and C, vacuum starts when punch moves forward to close pouring port - vacuum is closed when punch reaches end position of injection. According to calculation results, vacuum system is configured. Schematic diagram of vacuum system configuration is shown in Figure 5.

 

Figure 5 Vacuum system configuration diagram
1, 9, 10, 11, 12. Sealing strip 2. Push rod 3. Moving mold 4. Vacuum valve 5. Fixed mold 6, 13. Vacuum system 7. Sealing punch 8. Cylinder

Table 1 Main process parameters

Table 2 Vacuum degree record
Blowholes are one of main defects of traditional die castings, and for brazing, blowholes in die castings are almost unacceptable. Porosity mainly comes from two aspects. One is mechanical air entrainment during die casting, which is generated by air entrained during filling process; the other is gas emission from auxiliary products, such as mold release agent and punch oil; the third is abnormal intrusion of hydraulic oil, heat transfer oil, cooling water, etc.
To solve problem of pores, common means is to optimize process conditions first and use a vacuum system to form a high vacuum environment. Secondly, it is necessary to accurately control amount of gas-generating products to avoid excessive residues. Thirdly, it is necessary to ensure that hydraulic oil, heat transfer oil, cooling water, etc. do not leak and will not enter material cylinder or cavity. After removing gate and cleaning flash burrs, qualified parts produced by die casting need to be hydrogenated. Process conditions are: heating to 600 ℃, keeping warm for 2 hours, then cooling to room temperature. Parts with bulges on the surface after hydrogenation treatment should be scrapped, and parts without bulges should be transferred to subsequent processing, as shown in Figures 6 and 7.

 

Figure 6 Parts with bulges after heating

 

Figure 7 Qualified parts after heating
After many rounds of improvement and testing, small batch welding yield rate exceeded 90%, meeting customer's use requirements. Weld with hydrogen pores after brazing is shown in Figure 8, and qualified weld is shown in Figure 9.

 

Figure 8 Weld with pores

 

Figure 9 Qualified weld
Conclusion
Impact of pores in die castings on brazing quality is that first, blistering will occur during brazing heating process, which cannot meet appearance requirements, and secondly, it affects fusion of weld, resulting in unqualified sealing. Pores come from hydrogen in aluminum liquid and various pores generated during die casting process. Pores generated during die casting process can be avoided by higher vacuum and reasonable process conditions. Hydrogen pores need to be controlled from melting, refining, and insulation links. Through multiple rounds of process verification, continuous improvements have been made from two aspects mentioned above to produce die castings that meet requirements.