Introduction
Outline size of inverter housing is 560 mm×470 mm×110 mm, and average wall thickness is 3 mm. It mainly consists of two parts, one is to convert DC of single-chip battery into 30~50 kHz, 220 V AC through high-frequency modulation, and the other is to rectify, filter, and modulate AC into 50 Hz, 220 V AC. Both parts release a lot of heat when working. In order to ensure that operating temperature of inverter does not exceed 60 ℃, two water cooling areas (cooling water position 1 and position 2) are designed on inverter housing, and stirring grinding welding is required at cooling water position 2 to achieve sealing function. This study focuses on mold design of inverter housing casting and actual production, aiming to provide a reference for production of similar products.
Graphical results
Figure 1 shows inverter housing, which is made of AlSi9Cu3 alloy. Alloy is suitable for friction welding, with a tensile strength of 320 MPa, a housing weight of 7.80 kg, and a casting weight of 12.95 kg. It is produced using a 16,000 kN die-casting machine. Due to complex internal cavity structure of casting, large area of water channel and sealing part, and high density requirement, it is produced using a 1-cavity multi-stage sequential speed increase and medium-pressure die-casting process, and micro-spraying technology is used to achieve green die-casting without demolding wastewater.
Design of feeding system is shown in Figures 2 and 3. Mold is designed with 4 sliders, ordinary straight cooling water channels, and high-pressure point cooling devices, as shown in Figure 4. CAE mold flow analysis is shown in Figure 5. Mold requires 5 main feed ports and 2 side feed ports, and requires two "M"-shaped centralized exhaust blocks. Cold material requires 3 sets of slag bags and 2 sets of exhaust drainage channels to collect. 5 main feeds ensure smooth filling of two main water cooling parts of casting, avoiding air entrainment and confluence of multiple feeds at this position, and filling time must be controlled within 100 ms. Gate position in mold is selected at wall thickness position of casting, and aluminum liquid maintains a speed-up state before entering gate to reduce energy loss. Cold material and residual gas in cavity are pushed to set slag bag and exhaust block position.

 

Picture 1 Inverter housing

 

Picture 2 Feed and exhaust system

 

Picture 3 Cold material collection and exhaust system

 

Picture 4 Mold and high pressure spot cooling device
In the entire feeding system, speed of aluminum alloy liquid flowing through each section is inversely proportional to cross-sectional area:
S1v1=S2v2=S3v3
Where, S1 is cross-sectional area of barrel (handle); v1 is second fast speed of injection head; S2 is cross-sectional area of main channel; v2 is speed of aluminum alloy liquid flowing through main channel; S3 is the total cross-sectional area of inner gate; v3 is speed of aluminum alloy liquid flowing through inner gate. In mold design, it is necessary to avoid excessive speed changes in a single section in feeding system. Excessive speed increase will lead to rapid erosion of gate, and excessive deceleration will cause defects such as local cold shut or insufficient pouring. Design data of mold gate and exhaust system are shown in Table 1.

 

Figure 5 Filling time and temperature analysis

Table 1 Multi-stage speed-up feeding and exhaust system
In order to ensure that aluminum alloy liquid maintains speed increase during filling process, according to practical verification, pressure loss of alloy liquid is small when single-stage acceleration ratio is less than 4 during filling process. Cross-sectional area of mold barrel, cross-sectional area of main channel, and cross-sectional area of inner gate are optimized to 9.8:3.17:1. 5 main feed ports can meet needs of aluminum alloy liquid to quickly fill inverter housing body and water cooling position, but resistance to side wall is large. After on-site measurement, temperature loss is large, defects such as cold shut and mottled will occur on side wall. Therefore, two auxiliary gate feeds need to be added on the side of mold to change flow direction of cold material and high-temperature gas in cavity, reduce filling resistance, reduce casting pressure, and avoid expansion of mold. Burr of casting parting surface is controlled within 0.12 mm to eliminate burrs.
Mold is designed with ordinary cooling water and 3 sets of high-pressure spot cooling devices, as shown in Table 2, to achieve basically same temperature at core, slider, gate, etc., all maintained at around 180 ℃. High-pressure spot cooling conducts heat at local wall thickness of casting through pure water, and reduces mold temperature to around 160 ℃ within a die-casting cycle, achieving a dynamic balance of mold temperature. It is not necessary to lower mold temperature by spraying water, shortening spraying time, reducing local sticking phenomenon, and improving pores that may be caused by large-scale spraying.
Table 2 Cooling system design

Average wall thickness of inverter housing is 3 mm. CAE analysis data is compared with actual die-casting parameters to verify that casting pressure is 75 MPa, second fast speed is 4.60 m/s, feed speed at gate 1 is 45.08 m/s, temperature of aluminum liquid entering gate 1 is 630 ℃, and boost pressure is 90 MPa. AlSi9Cu3 alloy liquid has good fluidity at 630 ℃, low hydrogen content, small filling resistance, and the best casting forming effect; exhaust pressure of "M" type exhaust block is controlled at 6 MPa, cold material and residual gas in mold cavity need to push overflow port and exhaust block within 150 ms.
Mold is designed with 8 ordinary water transports in feeding system, mold core, slider and other feeding parts, and 3 sets of high-pressure point cooling devices are added to local thick wall parts to ensure that heat of mold core near gate and local high-temperature area can be quickly transferred to cooling water. After actual measurement by thermometer, temperature of mold core and slider parts is controlled at 180 ℃, and exhaust part is controlled at 140 ℃. Casting quality is ideal, see Table 3. Mold temperature can be automatically balanced, and there is no need to cool it by spraying. During production of this mold, only a spray-like release agent is required, and no water spraying is required. Die-casting process achieves clean production with zero demolding wastewater, and production cycle is shortened from industry average of 75 s to 65 s. Mold life is increased to more than 150,000 times.

Table 3 Cooling water temperature design

 

Figure 6 Inverter housing CT report
After optimized design and multi-stage speed-up die-casting process optimization, inverter housing mold has achieved mass production. Internal CT slice report of casting is shown in Figure 6. Two cooling water sealing areas of inverter housing that require stir friction welding shall not have pores larger than 0.2 mm, and CT results meet requirements. Mold feeding process ensures multi-stage speed increase, avoids excessive local speed increase, reduces filling resistance, slows down mold erosion, and increases mold life to more than 150,000 molds. Combination of ordinary cooling water and high-pressure spot cooling realizes waterless spraying through automatic mold temperature balance, shortening die-casting production cycle.