1 Introduction
The integrated exhaust technology of the cylinder head is a big breakthrough in the casting technology of the cylinder head, and the integrated casting of the cylinder head + exhaust manifold is realized
In recent years, it has become the preferred technology for mainstream energy conservation and emission reduction. In the traditional engine design, the cylinder head and the exhaust manifold are designed separately, the exhaust manifold development investment is large, the manufacturing cost is high, the casting process is subject to the requirements of the exhaust channel flow field and thermal stress, and the scrap rate is also high, resulting in the high cost of the whole machine, the large space size, and the product opening Difficulties such as long hair cycle. With the improvement of emission requirements, this technology has become the standard technology of China VI emissions, and its effect on reducing engine cold start emissions is excellent, which not only realizes the comprehensive effect of energy saving and emission reduction, but also further reduces engine costs. Although the integrated exhaust technology of the cylinder head has many advantages, its casting difficulty is also incomparable with the traditional cylinder head, and the waterway structure extending in all directions puts forward higher requirements for the casting process, especially low-pressure casting. At present, the forming methods of IEM cylinder heads mainly include gravity casting, low-pressure casting, etc., among which low-pressure casting is also known for It is widely used for high automation rate and high material utilization. However, in actual production, there will be many casting defects on the surface and inside of the casting, such as cold separation, cracks, insufficient pouring, shrinkage porosity, shrinkage porosity, oxidation inclusions, etc. This paper analyzes the causes of cold insulation defects in a typical low-pressure casting IEM cylinder head molding process The prevention and control measures were proposed, which improved the product qualification rate and obtained good economic benefits. These measures have the same effect on similar defects of similar products, and have the same effect on solving similar quality problems Instructive.
1 Products and molding processes
2.1 Materials and Construction
A series of engine cylinder heads of our company are small cylinder heads produced by low-pressure metal mold casting process, and the materials are similar AC4B aluminum alloy, the total weight of the casting is about 16Kg, the size is about 458288142, and the waterway, air passage and oil chamber are all made of sand core The sand core material is resin sand. The appearance structure of the product is shown in the figure below.
2.2 Molding process
This product adopts the bottom-injection low-pressure casting process, the upper and lower, left and right, front and rear parting metal molds are formed, the casting pressure, temperature, time, cooling, etc. are all automatically controlled by the pouring machine, and the appearance quality is manually checked after the casting is completed. The waterway sand core is formed by the hot core box process, and the core-shooting temperature, pressure, time and sintering time are automatically controlled by the core making machine, and the sintering thickness and sand core appearance are manually checked after sintering.
3 Problems that arise
In the process of rapid ramp-up of the engine, the workshop feedback cylinder head cold isolation is serious, especially the position of the small-end sensor, the average scrap rate is as high as 1%, which seriously affects the achievement of the quality and cost goals of the workshop, and needs to be solved quickly.
4 Cause analysis
4.1 Mechanism analysis
The cold insulation is mainly caused by the poor fluidity of the molten aluminum and the unreasonable temperature of the mold during the filling and solidification process of the casting Defects caused by the obstruction of the flow of molten aluminum in the cavity. The defect is mainly manifested as the surface defect or incomplete contour of the defective part of the cylinder head, and the defect affects the overall strength, machining quality and product performance of the casting to a certain extent, resulting in product scrapping. In the face of the existing problems, the project team started from the mechanism of cold separation and determined two improvement directions:
1) Improve the fluidity of molten aluminum;
2) Improve the rationality of the temperature field distribution of the mold; At the same time, the project team brainstormed and further analyzed the cold insulation defects, and suspected that the factors affecting the cold insulation defects on the outer wall of the small-end sensor may be: mold differences, equipment differences, and labor Time, Filling Pressure-Time, Cooling Time-Flow Rate, Molten Aluminum Temperature, Paint Shedding State, Mold Exhaust Groove, Small End Structure.
4.2 Analytical ideas
Through the preliminary investigation and fishbone diagram analysis, the factors have been quickly locked to a certain range, but further analysis is still needed to solve the problem, and the project group analyzed each factor one by one and verified it in small batches through the y=f(x) expansion chart (as shown in the figure below).
4.3 Exception troubleshooting
Through two-factor ANOVA, there was no obvious difference in the cold insulation defect rate of different molds on different equipment, which ruled out that the cold insulation defect rate was abnormally high due to the difference between a certain equipment and mold.
4.3 Cause analysis and small batch verification
4.3.1 Improve the rationality of the temperature field distribution of the mold
1) Labor time analysis
If the pouring cycle is too long or too short, it will lead to changes in the mold temperature field, which will increase defects such as strain, cold separation, shrinkage porosity, etc. The pouring cycle time consists of process time and manual operation time:
(1) The process time is automatically controlled by the PLC of the pouring machine, which is a fixed time, and the process time of each product is consistent; (2) Artificial time: the sum of the time of manual action in the pouring process, mainly including sand blowing, sand core and other action time, the manual operation time of each product is inconsistent, and there are differences between different people, which will eventually lead to certain fluctuations in the beat. According to the query of the 8 pouring machines in the Thingworx system from May 11th to May 15th, the 4# pouring machine with the largest beat fluctuation is the 4# pouring machine, and the 6# pouring machine with the smallest beat fluctuation is the 6# pouring machine, and the product quality of the 4# and 6# pouring machines from May 11th to May 15th is analyzed and compared, and the results are as follows:
The P value=0.753>0.05 was analyzed by the double ratio test, indicating that the artificial time fluctuation was not the key factor affecting the cold isolation defect on the outer wall of the small-end sensor. 2) Cooling time-flow analysisThe distribution of the mold temperature field will affect the fluidity of the molten aluminum, and when the local temperature of the mold is too low, the fluidity of the molten aluminum in this area will be reduced, which may cause cold insulation defects. In order to verify the effect of cooling time-flow rate on the temperature field at the little-end, the 7.8-way air-cooled (290+80) air cooling of the upper mold closest to the small-end was turned off and CAE analysis was performed.
After CAE analysis, the cooling-flow change near the small end had little effect on the change in the mold temperature field at this part The group discussion concluded that the cooling time-flow rate was not the key factor affecting the cooling separation, and decided not to conduct experimental verification of the cooling time-flow rate.
One of the main functions of low-pressure casting mold coatings is mold insulation. As production continues, the coating on the surface of the mold will Gradual peeling off leads to a decrease in the thermal insulation ability of the paint peeling part, which in turn produces cold insulation defects.
3) The paint is peeling off
4.3.2 Improve the fluidity of molten aluminum
1) Molten aluminum temperature
When the temperature of the molten aluminum is low, the fluidity of the molten aluminum is reduced, which will form defects such as cold isolation and unsatisfactory filling on the surface of the casting; When the temperature of molten aluminum is high, it is easy to produce defects such as shrinkage porosity and shrinkage porosity, and at the same time, it will prolong the pouring cycle. Select the 2# mold to test on the 8# pouring machine, and set the molten aluminum temperature at 705°C, 710°C, 715°C (process Range 695°C~715°C, current setting value 700°C) production for 4 days, statistics and analysis of cold insulation defects of the small end sensor of the cylinder head are as follows:
3) Mold exhaust groove
When the mold exhaust is poor, the molten aluminum will be blocked at this position, and the fluidity will be reduced, and the surface of the molten aluminum at this position will be rapidly oxidized to form an oxide film during solidification, so that the surrounding molten aluminum can not be completely integrated, and eventually lead to cold isolation at the place. In order to strengthen the exhaust at the position of the small-end sensor, the project team discussed that the exhaust groove was planned to be added to the place to extend the original exhaust groove. The 3# pouring mold was selected for improvement and optimization, and the exhaust groove on the inner wall of the small-end sensor was increased, and the outer part of the small-end sensor was increased Wall exhaust groove, statistical analysis to optimize the quality of the front and rear castings, the details are as follows:
The P=0.674>0.05 value was found by the double ratio test, indicating that there was no significant difference in the cold isolation defect rate of the small-end sensor before and after optimization, indicating that poor mold exhaust was not the key factor affecting the cold insulation defect on the outer wall of the small-end sensor.
4) Filling pressure – time
According to the analysis of casting theory, by adjusting the filling pressure and solidification pressure, that is, appropriately increasing the specific pressure, the fluidity of the molten aluminum can be improved, and the cold insulation defects can be improved or eliminated. Through the discussion and analysis of the project team, it was finally determined that the specific pressure was increased on the 3# pouring machine, and the comparative analysis of the process before and after adjustment is as follows:
The double-ratio test of the defect rate of the 3# machine before and after the adjustment found that P=0.035<0.05 indicated that the cold insulation defect before and after the adjustment was obtained
There is a significant difference in the rate, and the adjusted defect rate is lower, indicating that the specific pressure is the key factor affecting the cold isolation defect on the outer wall of the small-end sensor, and the specific pressure (filling pressure-time) can be improved by increasing the specific pressure (filling pressure-time).
5 Improvement measures
5.1 Improvement measures: Mass verification
After the Y=F(x) analysis of each suspicious factor and the verification analysis of small batches, the cold isolation of the small-end sensor will finally be affected The factor was locked to the filling pressure-time and small-end structure, and the two factors were verified in large quantities.
5.1.1 Filling pressure-time
Through temporary process changes, the specific pressure switching process production was increased by half a month with small batch verification conclusions, and the production after the change was tracked
The case for high-volume verification is as follows:
5.1.2 Little-endian structure
5.2 Improvement measures curing
After a large number of verifications, it is determined that the measures of the preliminary analysis are effective, and then the improvement measures are solidified, first of all, the process and
Control changes to the plan to ensure the consistency of process execution, followed by 3D changes to products and molds to ensure the consistency of product data and on-site execution and the transfer of experience.
6 Verify the results
The above measures have been promoted and implemented simultaneously on the rest of the series of models and newly developed models, and the improvement effect is remarkable, the defect rate is significantly reduced, the defects are basically eliminated, and the improvement measures are remarkable.
7 Conclusion
1) When designing products, full consideration should be given to avoiding possible problems in the process of filling and solidifying castings, and in the design of process structure Combined with the preliminary project and CAE analysis and other means, the process structure such as exhaust grooves and stiffeners should be reasonably designed, and the corresponding problem list should be formed to provide experience accumulation for subsequent models.
2) In the process of problem analysis and solution, you can make full use of Six Sigma and other analysis tools to systematically classify the causes and problems Analysis and verification is conducive to rapid discovery and systematic problem solving, and avoids the recurrence of problems caused by inadequate analysis.