In view of fact that shrinkage of aluminum alloy die castings are internal defects with high frequency and difficult to be observed and identified, macro and micro morphology of shrinkage holes are displayed and improvement of shrinkage holes is explained. In production, die casting process is optimized by adopting local extrusion technology, high pressure spot cooling technology, contour weight reduction technology, reduction of excess processing technology and mold temperature control layout technology to solve shrinkage defects of die castings.

People's requirements for automobiles are increasingly tending to high performance, low pollution, low energy consumption, etc. Weight of car plays a decisive role in fuel economy. For every 100kg reduction in vehicle weight, fuel consumption can be reduced by 0.7L/100km. Using lightweight materials to manufacture automotive parts is an effective way to reduce weight of car. Aluminum alloy die castings have excellent material properties, convenient forming and lightweight, making them the first choice. Application of aluminum alloy die castings in automobiles is mainly concentrated in housing parts, engine parts and other non-engine parts. With development of automobile and other industries, output of aluminum alloy die castings has increased by nearly 13% annually, accounting for more than 75% of output of non-ferrous alloy die castings. At present, aluminum alloy die castings are developing towards large, complex, thin-walled, high-precision and integrated directions, which has promoted development of aluminum alloy die casting technology.

Shrinkage defects are common defects of die-cast aluminum alloy parts, mainly concentrated in the center of thick and large hot section of product, as shown in Figure 1 (a). Microstructure of shrinkage defects shows irregular and rough hole shapes, dark and rough surfaces, as shown in Figures 1 (b) and 1 (c). This defect is mainly caused by insufficient shrinkage of alloy liquid during solidification process. Shrinkage defects cause mechanical properties of aluminum alloy die castings to deteriorate to a certain extent, small shrinkage cavities that appear inside general aluminum alloy die castings are difficult to detect by X-ray flaw detection.

 

Die-casting process is a process that organically combines three major elements of die-casting machine, die-casting mold and alloy for comprehensive use. According to process of metal filling cavity during die-casting, it is a process that unifies process factors such as pressure, speed, temperature and time. At the same time, these process factors influence and restrict each other, and complement each other. Only by correctly selecting and adjusting these factors, making them coordinated can expected results be obtained. Therefore, in die-casting process, it is necessary not only to pay attention to processability of casting structure, advancement of die-casting mold, excellence of die-casting machine performance and structure, adaptability of die-casting alloy selection and standardization of smelting process, but also to pay attention to important role of process parameters such as pressure, temperature and time on quality of casting. In die-casting process, attention should be paid to effective control of these parameters.

In design of local extrusion system in die-casting mold, it is necessary to consider extrusion point, extrusion force, extrusion pin diameter and extrusion depth, as well as extrusion delay and extrusion duration. In actual production, local extrusion is achieved by directly designing a booster cylinder in slider of mold to directly extrude part where shrinkage cavity is generated. When molten metal is in a semi-solid state in cavity, a certain pressure is applied to last solidified part through an extrusion pin of a certain diameter to force shrinkage compensation to reduce or eliminate shrinkage cavity defect at that place, thereby improving internal quality of die casting.
There are three main types of local extrusion forming methods: one is to apply pressure to outer surface of casting. Pressurized part of casting is a certain distance higher than actual height of product to avoid squeezing cold material on the surface layer of casting into interior of casting. Higher part is directly removed by subsequent machining. At the same time, side wall is left with more than 2mm on one side to avoid hard layer, as shown in Figure 2 (a); the other is to perform local extrusion on thick wall part of casting, directly extrude bottom hole of casting, then obtain hole with required accuracy through machining, as shown in Figure 2 (b); the third is to perform local extrusion on pouring system near thick part of casting, and control shrinkage direction by thickness of both sides of runner, so that metal of pouring system can shrink thick wall part of casting, as shown in Figure 2 (c).

 

Size of extrusion pressure depends on diameter of booster cylinder. If cylinder diameter is too small, local extrusion pressure is insufficient, which will cause insufficient shrinkage; if cylinder diameter is too large, cylinder volume will be large, resulting in waste. According to actual production experience, extrusion pressure is generally more than 3 times casting pressure. Taking product in Figure 2 (b) as an example to introduce actual use of local extrusion, casting pressure of die casting is 95MPa, extrusion pressure of extrusion mechanism is selected as 300MPa, and extrusion pin diameter is selected as 10mm according to bottom hole processing allowance. Therefore, according to formula: F cylinder = F extrusion, it can be obtained: P cylinder * π (D cylinder / 2) 2 = P extrusion * π (D extrusion pin / 2) 2
Where:
P cylinder is equipment system pressure, 16MPa;
P extrusion is extrusion pressure to be achieved, 300MPa;
D extrusion pin is extrusion pin diameter, 10mm;
D cylinder is required extrusion cylinder piston diameter.
Extrusion cylinder diameter D cylinder = 43.3mm is calculated, and standard cylinder piston diameter is 50mm.
Extrusion time is divided into extrusion delay time and extrusion duration. Extrusion delay time refers to time interval from the end of metal liquid filling to start of local extrusion. The best extrusion effect can be obtained by applying extrusion pressure when molten metal is in a semi-solid state. When extrusion delay time is too short, molten metal is still in a flowing state, thick and large parts have not solidified and crystallized, causing extrusion pin to become a core and no extrusion effect; when extrusion delay time is too long, molten metal solidifies and forms a hard shell on mold wall, causing extrusion pin to be blocked and unable to compensate for shrinkage and achieve extrusion effect. Usually, extrusion delay time needs to be determined by experiments during mold trial to determine effective parameter range. Local extrusion delay of product in Figure 2 (b) is set to 1s±0.5s.
Extrusion duration refers to time from start of extrusion until extrusion pin retreats. If extrusion duration is short, extrusion pin retreats too early, casting cannot be completely solidified, and crack defects are prone to occur in extrusion area; if extrusion duration is too long, clamping force between fully condensed casting and extrusion rod will be too large, resulting in a reduced extrusion rod life. Local extrusion holding time of product in Figure 2 (b) is 7-10s.
Extrusion depth is amount of pressurization, which determines amount of aluminum liquid pressed in. Depth is directly related to volume of shrinkage required for thick and large parts of die casting. If extrusion depth is too small, metal liquid filled into thick and large parts is not enough, and shrinkage is insufficient, which cannot achieve purpose of locally reducing or eliminating shrinkage defects; if extrusion depth is too large, not only a large pressure extrusion cylinder equipment is required, but extrusion pin is also easily damaged due to excessive temperature, and extrusion pin is easy to bend or even break. Extrusion depth of product in Figure 2 (b) is set to 18mm. In actual use, local extrusion pin is usually spray-cooled, extrusion pin spray is extended by 1s±0.5s, spray time is 8s±1s to prevent extrusion pin from overheating and bending and breaking during extrusion process, thereby increasing service life of extrusion pin.
By adopting local extrusion technology, internal structure of casting is dense, shrinkage defects in processing hole are solved. State after processing is shown in Figure 3.

 

In order to further improve dimensional accuracy of castings and solve problems of local shrinkage, ordinary spot cooling is implemented on ordinary parts of mold to ensure thermal balance as much as possible; high-pressure spot cooling is implemented on molds of important parts of corresponding products to increase thickness of quenching layer and ensure internal quality of product. Water pressure of ordinary cooling methods is about 4kg, which is difficult to penetrate cooling water pipes below φ6mm, while water pressure of high-pressure spot cooling can reach 10kg, which can not only penetrate water channels as low as φ3mm, but also adjust start and end time of water flow, which has a significant effect on preventing shrinkage.
High-pressure spot cooling technology can adjust residence time of cooling medium in mold by setting a timer, and control mold surface temperature to always maintain optimal state. This process can also ensure the life of the small core, making it not easy to break. Because the high-pressure spot cooling water pipe is thin, ordinary industrial water contains a lot of scale, and high-pressure spot cooling pipe is prone to blockage during production, so that it cannot achieve desired cooling effect. Therefore, high-pressure spot cooling technology should be equipped with a pure water machine to pass pure water to high-pressure spot cooling water pipe to ensure spot cooling effect.
For thick-walled processing parts on castings, such as threaded holes, shrinkage holes may cause product leakage after processing. When shrinkage holes in thick-walled parts cannot be solved by local extrusion due to mold structure limitations, core can be cooled rapidly by passing high-pressure cooling water through core to solidify surrounding tissue of core first, forming a dense layer and reducing tendency of shrinkage holes.
Figure 4 (a) shows a valve chamber cover. The two isolated slender bosses in the middle are M6 threaded holes for fixing respirator. In the early stage of production, proportion of shrinkage holes in threaded holes is relatively high, as shown in Figure 4 (b) and Figure 4 (c).

 

Later, ordinary core was changed to a high-pressure spot cooling core and core was lengthened. Shrinkage holes in the bottom holes of two isolated bosses in the middle were eliminated, and internal quality was significantly improved, as shown in Figure 5.

 

Thick parts of product that are prone to shrinkage holes should be structurally optimized, and profiling design is usually used to reduce casting heat nodes. As shown in Figure 6, three concentrated φ12mm bolt holes in the middle of gear chamber are 72mm thick, which is an isolated thick and large thermal node. It is very easy to produce shrinkage defects in the middle. Back of product is hollowed out by using a contoured weight reduction design to eliminate thermal node and optimize product structure, so that high-quality products can be produced using low-cost processes.

 

As shown in Figure 7, there are many threaded holes on gear surface of a reducer housing. In order to improve quality of thread, contoured weight reduction groove design is used around precast bottom hole of threaded hole to reduce thermal node and eliminate shrinkage defect. At the same time, main reinforcement design is highlighted to fully reduce weight while improving structural strength of product.

 

As shown in Figure 8, a heavy-duty engine generator bracket, key feature of part is that oil channel hole air tightness requirement is: maintain pressure for 1min under a pressure of 600kPa, and no leakage is allowed. Oil channel hole diameter of this product is required to be φ22mm and 228mm deep, which is a deep and long hole. During product trial production stage, the roughcast oil channel hole was formed by a long core with a core draft angle of 1°, a large end diameter of φ22.6mm, and a depth of 228mm. Small end diameter of oil channel hole was calculated to be φ14.6mm, that is, single-side machining allowance reached 2.7mm. The two bolt holes and end faces of holes near oil channel holes need to be processed to ensure hole diameter and hole position; at the same time, main ribs on the back of oil channel holes are removed near middle position due to assembly interference. After hard layer on the surface of casting is removed, density of organization is destroyed, resulting in 100% leakage at oil channel hole.

 

After using local extrusion and high-pressure spot cooling technology, leakage rate has been significantly improved, but leakage problem caused by shrinkage defects has not been completely solved. Finally, leakage of oil channel hole is reduced by reducing processing amount and protecting dense hardened layer formed by die casting on the surface of aluminum casting by bolt hole without processing; core draft angle of oil channel hole is reduced to 0.5° to reduce processing amount in oil channel hole; at the same time, processing amount of the two bolt holes and end surface of hole near oil channel hole is reduced, and anti-interference position processing on main rib is cancelled, as shown in Figure 9.

 

Mold temperature control refers to heating and maintaining mold at a certain working temperature to ensure stability of casting quality, shorten injection cycle and extend mold life. Temperature of mold is a key factor in heat dissipation of metal solution, filling and solidification of casting. In many cases, inappropriate mold temperature will cause casting defects such as shrinkage.
Heat exchange of die casting mold can be considered to go through following process: In each injection cycle, heat is in a quasi-equilibrium state, heat dissipated by mold and heat brought out by heat transfer oil is balanced with heat transferred to mold by liquid metal and heat emitted by casting cooling. Heat is dissipated to surroundings by heat radiation and heat conduction, and is transferred to mold plate by heat conduction.
During mold design stage, casting simulation software is combined to arrange cooling water for high-temperature area; during production debugging process, an infrared thermal imager is used to obtain mold temperature distribution, so as to adjust mold cooling water flow to ensure mold temperature balance. By rationally arranging cooling water channel in high-temperature area and oil channel in low-temperature area, controlling flow of cooling water and oil channels, temperature of each position of mold is balanced to ensure internal quality of product. Distribution of oil channel and water channel of dynamic and static molds is shown in Figure 10.

 

(1) Local extrusion technology can effectively improve density of internal structure of casting and solve internal shrinkage defects in local thick position. Extrusion pressure, extrusion delay time, extrusion duration, and extrusion depth jointly determine extrusion effect.
(2) For shrinkage defects in hole, high-pressure spot cooling technology is preferred. High-pressure spot cooling technology can ensure internal quality of product hole by increasing thickness of quenching layer on the surface of casting.
(3) In the early stage of product structure design, weight-reducing grooves and contoured structures should be used to reduce local heat nodes; when developing die-casting process, priority should be given to reducing processing allowance at locations prone to shrinkage holes, or even eliminating processing to reduce damage to dense layer; when designing mold, focus should be on combining casting simulation software, arranging cooling water for high-temperature areas and controlling flow of cooling water to ensure mold temperature balance, thereby ensuring a product with better internal quality.