Abstract: Production process of engine main bearing housing is analyzed, development difficulties are identified, and development process of engine main bearing housing is introduced, including raw material refining, die casting mold design, and die casting process. In raw material refining process, mechanical properties are improved by modification treatment; die casting filling process is optimized by CAE numerical simulation; and quality stability is verified by inspecting internal defects of main bearing housing, conducting strength and fatigue tests.
Main bearing housing is a key component of automobile engine, and its blank is produced by die casting of aluminum alloy. In engine structure, main bearing housing is connected to cylinder block, and engine crankshaft runs in the middle of it. During engine operation, main bearing housing bears a variety of variable loads, including bolt preload, bearing interference load, crankshaft dynamic load and thermal load, etc., so higher requirements are put forward for internal quality, static strength and durability of main bearing housing support part. This product is a domestically produced part. Throughout development process, while ensuring that both process quality and final product quality meet technical requirements, its results are superior to CKD (Completely Knocked Down) parts. This study takes engine main bearing housing as an example, introducing its development process and optimization to provide a reference for development of similar new products.

Main bearing housing part has an outer contour dimension of 410 mm * 184 mm * 87 mm and a product weight of 3.36 kg. A three-dimensional schematic diagram is shown in Figure 1. Main body has five support beams with a wall thickness of 22–24 mm. Each beam has a semi-circular surface with a diameter of ϕ55 mm at its center, and two bolt transition holes with a diameter of ϕ10 mm on both sides of each semi-circular surface. Bottom of product is surrounded by a thin-walled structure with an irregular shape and reinforcing ribs, connecting five support beams into one piece. Casting generally has a wall thickness of 4 mm, a transition fillet radius R of 3 mm, and a draft angle of 1.5°. Analysis of shape and structure of casting revealed that casting has a frame structure with uneven wall thickness and local thickness, resulting in poor casting processability. Technical requirements for part are that minimum load of bearing housing under pressure of reference pad is 6.7 kN, and porosity specification is ES1S7G-6F098-AA.

 

Fig.1 3D diagram of main bearing of bracket

Since main function of main bearing housing is to support engine crankshaft and ensure its normal operation, internal casting quality of bearing housing must be guaranteed to meet tensile strength and fatigue test requirements of bearing housing. However, due to serious uneven wall thickness of main bearing housing casting, wall thickness of casting is greater than 20 mm at the support beam where stress is greatest, shrinkage defects are easily generated during die casting process, which is a difficult point to be solved in die casting process.

Main bearing housing of engine is made of ADC12 alloy, which complies with JIS H 5302:2006 standard. Chemical composition of material is shown in Table 1. Fe content is relatively wide. Fe content (mass fraction, same below) below 0.6% easily leads to mold sticking, while above 1.2% reduces alloy fluidity and mechanical properties, affecting casting quality. Therefore, Fe content is controlled within 0.6% to 1.2%. ADC12 alloy has a tensile strength of 228 MPa, a yield strength of 154 MPa, an elongation of 1.4%, and an elastic modulus of 71 GPa. This material possesses good mechanical properties, machinability, and casting performance.
Tab.1 Chemical composition of ADC12 alloy %

Dongda Sanjian 2.0 t/h continuous aluminum melting and holding furnace was used for melting. During charging, Class I recycled material was used, with a mass less than 40% of the total melted aluminum alloy mass. Furnace gas temperature was set at 900 ℃, and aluminum melt tapping temperature was 720~770 ℃. After tapping, aluminum melt was poured into a ladle and refined using a refining degasser with nitrogen. During refining, Al-10Sr rods are added to control Sr content in molten aluminum to 0.03%–0.04%. Modification treatment is performed simultaneously with slag removal and degassing during refining. Figure 2 shows metallographic structure of ADC12 alloy before and after modification treatment. Modification treatment alters alloy's microstructure, transforming dendritic structure into a spherical structure. Refined microstructure is uniform, without mixed grain regions of varying coarse and fine grains, improving material's strength and elongation. Refined alloy was tested for chemical composition using a SPECTRD MX.06 direct-reading spectrometer, density using an HZY-A220 hydrogen analyzer, slag content using a "K-mold," and alloy temperature using an MXM-101 handheld thermocouple. Only alloys passing all these tests were transferred to die-casting machine's quantitative furnace for die-casting production.

 

Fig.2 Metallographic structure before and after metamorphic treatment

Main parameters of die-casting process include speed, time, and pressure. Due to special nature of die casting production process, quality of products obtained by die casting is mainly determined by characteristics of process parameters. Therefore, CAE analysis is required in the early stage of mold design, that is, to simulate die casting process, observe and analyze filling sequence of molten metal, injection speed and cooling process, find high-risk areas that are prone to defects such as porosity, shrinkage cavities and porosity. Effective preventive measures should be taken in design stage to improve first-pass yield and reduce development costs.
CAE flow field simulation numerical analysis was performed using AnyCasting software. First, based on structural characteristics of casting itself and experience of similar product parts, gating system of die casting was designed. Then, multiple simulation analyses were performed using different process parameters. During this process, shape of horizontal runner and thickness of ingate were optimized. Final determined process parameters are shown in Table 2. Results of optimized simulation filling sequence and filling speed are shown in Figure 3. A detailed solidification shrinkage process analysis was performed to address development difficulties of casting, namely casting defects that may occur in thick-walled area. Solidification sequence and shrinkage defect analysis results are shown in Figure 4.
Tab.2 Die casting process parameters

 

Fig.3 Filling process analysis of ADC12 alloy

 

Fig.4 Solidification process analysis of ADC12 alloy
As shown in Figure 3, molten metal flows into cavity from ingate during filling process. After first four support seats are filled, melt flows to fifth support seat under pressure. When filling reaches 98%, only overflow groove and venting channel of last support seat are not filled. With help of end vacuum system, filling process is relatively stable. When filling speed of ingate is 48 m/s, there will be no erosion of cavity. However, since ingate is directly opposite core, it will erode core and affect flow direction of molten metal, which is prone to air entrapment. When designing mold, it is necessary to appropriately shorten height of core and leave a channel for flow of molten metal. Figure 4 shows solidification process analysis of ADC12 alloy. Figure 4 shows that casting will have different degrees of shrinkage cavities and porosity in thick-walled area of five support seats. This is mainly due to structure of product. When local volume of casting is too large, volume shrinks during solidification of molten metal. If it cannot be replenished by external molten metal, shrinkage cavities and porosity will occur. Analysis shows that shrinkage cavities and porosity that may occur in the middle of support seat are basically more than 10 mm away from machined surface of bearing bush, and have little impact on performance of casting. To reduce shrinkage cavities and porosity in thick-walled middle section, mold design should consider thickness of ingate and reserve sufficient filling channels to ensure pressure feeding before molten metal is fully filled and solidified. Figure 5 shows a comparison of distribution and state of shrinkage porosity in casting after increasing thickness of ingate and shortening length of fixed mold core. When ingate thickness is 4.0 mm and fixed mold core length is 15 mm, internal shrinkage porosity of casting is significantly reduced compared with ingate thickness of 3.2 mm and fixed mold core length of 30 mm.

 

Fig.5 Shrinkage cavity distribution of ADC12 alloy

According to numerical simulation scheme, gating system of engine main bearing housing die casting mold adopts longitudinal bidirectional filling. Large-capacity slag collection bags are arranged on both sides of each bearing housing journal. Damped vacuum is used at the end of filling. Schematic diagram of mold structure is shown in Figure 6. Cavity adopts an integral insert structure. Forming part is machined by high-speed milling CNC, local irregular shape and small rounded corner are machined by electrical discharge machining. According to product structure and shape, forming part of casting is pushed out by push tube, sprue and slag collection bag are pushed out by push rod. According to simulation analysis results, when designing mold, on the basis of meeting cross-sectional area of ingate, thickness of ingate is appropriately increased, and length of fixed mold core is shortened to leave a channel for molten metal to compensate for shrinkage, as shown in Figure 5b.

 

Fig. 6 Structure diagram of die casting
1. Ejector tube 2. Moving mold insert 3. Fixed mold core 4. Casting 5. Casting 6. Ejector rod 7. Moving mold core 8. Cooling channel
Mold temperature directly affects quality of casting and service life of mold. Especially for thick-walled castings, as die casting mold number increases, temperature of mold cavity increases accordingly. It is necessary to set up a reasonable cooling system to cool mold and remove excessive heat transferred to mold by molten metal in die casting production so that mold reaches optimal working temperature. Main bearing seat die casting mold adopts water cooling, which has low operating cost and fast cooling speed. Cooling water channel adopts a combination of series and parallel connection, and water dividers are used to change depth of cooling water in local deep cavity. Since forming part is concentrated in moving mold cavity, cooling water channels in moving mold cavity are relatively dense. Multiple water-distributing cooling mechanisms are designed at each journal of main bearing housing. Cooling depth of water-distributing plates is adjusted according to cavity depth. Mold cooling scheme is shown in Figures 6c and 6d.

Equipment used for die casting production of main bearing housing mold is a ZDC-900TCS, equipped with a Sigma quantitative furnace. Aluminum liquid temperature is set at (665±15) ℃. Slow injection speed is 0.25 m/s, fast injection speed is 3.5 m/s, boost pressure is 110 MPa, injection boost time is 10 s, and cooling time is 15 s. A self-made vacuum system is used in die casting process, achieving a vacuum degree of 10–20 kPa. Mold cavity temperature is monitored during continuous production and kept within process requirements range of (180±30) ℃. A trial production of 500 pieces was conducted, as shown in Figure 7. As can be seen, casting blanks are free of external defects such as cold shuts, marks, and scratches. After removing gates and cleaning burrs, casting blanks underwent shot blasting, as shown in Figure 7a. 100% of trial production parts underwent X-ray flaw detection, with a focus on inspecting internal quality of main bearing housing journal. X-ray flaw detection results are shown in Figure 7b. Simultaneously, 10 blanks were randomly selected and journals were sectioned; section surface quality is shown in Figure 7c. Through X-ray flaw detection combined with section surface quality analysis, it is evident that there are no shrinkage cavities larger than 2 mm within casting, internal quality meets customer requirements and is superior to CKD parts. Internal quality of 3rd journal (intermediate journal) and 5th journal (final filling) in 5th journal of main bearing housing is slightly worse.

 

Fig.7 Appearance and internal quality of castings

During engine operation, besides being subjected to static loads, most failures of main bearing housing are caused by fatigue. Therefore, fatigue tests and static strength failure tests are required for main bearing housing to determine whether casting quality meets design requirements. Samples of main bearing housing of an automotive engine were taken and labeled. X-ray inspection revealed that journals 1, 3, and 5 were relatively weak points in terms of internal quality. Journals 1, 3, and 5 were selected for both fatigue and static strength failure tests. Fatigue tests were conducted on parts numbered 1 to 9 to determine fatigue limit (corresponding to cyclic stresses > 10⁷) under different average stresses. Fatigue tests were performed using a PLG-100 microcomputer-controlled high-frequency fatigue testing machine, with output system being East China Vibration Testing System. Test conditions and results are shown in Figure 8 and Table 3. Fatigue test results showed that no fatigue failure occurred under set fatigue test loads. Static strength failure tests were conducted on main bearing housing using same fixture. Figure 9 shows a photograph of bearing housing after failure test, and failure test results are shown in Table 4. Minimum fracture stress was 347 MPa, and minimum 0.2% endurance was 146 MPa. These results meet quality requirements of castings and are superior to CKD parts.

 

Fig. 8 Fatigue test
1. Strain gauge 2. Component 3. Indenter 4. Fixture
Tab. 3 Fatigue test results

 

Fig. 9 Images of bearing housing after destructive test
Tab. 4 Static destructive test results

(1) Using CAE software to simulate and analyze process scheme of castings can quickly identify defects in die castings, allowing for effective measures to reduce possibility of defects during design phase, shortening development cycle and reducing development costs.
(2) Based on structural characteristics of castings, a reasonable gating system is designed, reasonable filling and pressure-boosting channels are created by appropriately increasing thickness of ingate and shortening core length, reducing generation of shrinkage cavities in thick-walled areas of castings.
(3) For locally thick-walled areas of castings, local cooling capacity of mold needs to be increased to maintain optimal mold working temperature. Reasonable working temperature for thick-walled aluminum alloy castings is (180±30) ℃.
(4) Use of a modification treatment process in melting process of aluminum alloy raw materials, combined with good internal quality of castings, can improve mechanical properties of castings and ensure that fatigue strength meets quality requirements.