Application of quality control method for die casting

1 Overview
In the ISO 9001:2008 standard, a special process is defined in such a way that when the output of the production and service delivery process cannot be verified by subsequent supervisors or measurements, so that the problem occurs after the product is used or after the service is delivered, the organization should respond to any such The process is confirmed in real time.
It can be known from the above definition that the die casting process is a special process. The quality of die castings cannot be controlled in the process of die casting. All process parameters are only indirect measurements of external control. The true quality of die castings can only be verified by post-test. At this time, the quality of the die-casting part has been formed, and the inspection result cannot be changed to the quality of the die-casting part. Therefore, in order to ensure the quality of the die-cast product, it is necessary to take necessary control measures for the die-casting process, such as strict control of the stability of the process parameters in the die-casting process and the consistency of personnel operations. However, it is particularly important in the production process to find unqualified die-casting parts in time and reduce waste loss as much as possible.
Some parts produced by our company are products with air tightness and safety requirements. The internal air holes, shrinkage holes and air leakage defects are the main reasons for product scrapping. Therefore, in order to ensure and improve the quality of products and meet the requirements of mass production, some quality control measures must be taken from the production preparation stage to exert some benign influence on the quality of die-casting parts to ensure the quality of die-casting parts produced in large quantities. Reliability and stability. This paper intends to briefly discuss the quality control measures of die castings from the three stages of die casting preparation, die casting process, and sample inspection (before, during, and after).
2. Die casting preparation stage
(1) Aluminum alloy composition and smelting control production practice shows that improving the quality of aluminum alloy melt is a key factor to improve the quality of die-casting parts. The melting process of aluminum alloy has clear regulations on melting temperature, smelting time and proportion of returning materials. . In the aluminum alloy smelting process, the proportion of refining agent and slag removing agent required for the aluminum liquid used in the refining material is larger than that when only the aluminum alloy ingot is melted, and the ratio of the increasing ratio to the proportion of the reclaimed material has a corresponding relationship. For the smelting metal liquid, the slag removal and degassing work should be carried out in a timely manner. During the concentrated melting process, the chemical composition of each pot of molten metal should be tested to ensure that the chemical components of the product are qualified during the melting process. In addition, long-term smelting should be avoided, otherwise the hydrogen content in the alloy liquid will increase, which will affect the strength and air tightness of the die-cast product.
For the production of castings with air tightness requirements, it is also necessary to monitor the degassing effect to ensure that the gas content in the alloy liquid can meet the requirements of use, and if necessary, the hydrogen content of the aluminum liquid is qualitatively measured and used. Density measurement is used to monitor the quality of the aluminum liquid, as shown in Figure 1.

(2) Verification of slow shot speed The effect of slow shot is to use the slow movement of the punch to accumulate the metal melt at the front end of the punch without generating a jump wave. As the punch advances, the melt level gradually rises. At the top of the pressure chamber, the gas in the pressure chamber is removed from the mold vent. For this reason, in the slow injection preheating mold stage, it is necessary to verify the slow injection speed set by the process.
Generally, a simple method can be used to confirm the slow injection process parameters. When the injection piston slow injection starts to reach the fast injection start position, the injection rod automatically stops moving forward (this function can be customized on some die casting machines). After the alloy liquid is cooled, it is ejected, and it is judged by cutting and turning.
At the correct slow injection speed, when the molten metal reaches the inner runner, the molten metal in the pressure chamber has completely become a complete cylinder. When the internal structure is found to be relatively dense, there is no visible pores at this time. Slow shot speeds are generally suitable. Inappropriate slow injection speeds will result in severe air entrainment. It can be seen from Fig. 2 that when the slow injection speed is 0.17 m/s, there are serious holes inside the handle. This will have an adverse effect on the quality of the casting.

3. Die casting process control
(1) Consistency in the operation of the production process Since the automation degree of die-casting production is generally low, it is basically in the state of semi-manual operation. The quality of the die-casting parts and the operation skills, working environment and mental state of the operators are extremely great. relationship. In order to make these factors as stable as possible, the main measures taken by our company in this regard are: the “three-principle principle” of setting people, setting up, and setting products, that is, arranging specific people to produce specific products on specific equipment. Once any of the three factors have changed, they must be confirmed and monitored.
(2) Consistency confirmation of multiple adjustments of process parameters Under normal circumstances, the production of each product is carried out in batches, and the consistency of process adjustment after each re-replacement of the mold must be clearly confirmed. The current common speed adjustment method on die-casting equipment is relatively simple. The slow injection speed is basically not detected and controlled. The fast injection speed is often adjusted by handwheel (see Figure 3). According to the handwheel scale provided by the manufacturer and two The speed comparison table is used to check the process parameters of this time. This kind of control itself has great quality risks, the main performances are as follows:

First, the hand wheel opening degree of the equipment of different manufacturers is inconsistent with the two speed comparison curves.
Second, the actual speed of the same model of the same manufacturer and the hand wheel opening are also inconsistent.
Third, there is an inconsistency between the actual speed of the same equipment at different stages and the hand wheel opening.
These inconsistencies will make the process personnel feel at a loss. At the production site, the die-casting parts produced by the original process are qualified, but according to the same production process, no qualified products can be produced. The actual reasons for this performance are still two. The process parameters of the secondary production are inconsistent.
The simple way to solve this problem is to use a linear speedometer to measure the slow injection speed and the fast injection speed during actual production. Each production can be checked according to the determined value. Of course, for a device with a shot curve, the shot speed can be easily detected on the device, and each adjustment can be performed.
4. Sample post-production inspection
(1) The internal quality of the trial-processed die-casting part is the most convenient and quick way to detect by X-ray inspection, but it has the disadvantages of large investment and unintuitive result. The most commonly used and most direct inspection method is trial processing. After the trial processing, the quality of the machined surface can be observed more intuitively, and a reasonable judgment can be made on the quality of the product.
For enterprises that simultaneously produce blanks and carry out subsequent machining, it is practicable to directly deliver the produced blanks to the production line for trial processing to judge the internal quality of the blanks. However, for the factory that only produces the blank, the method of trial processing of the product, although the product quality of the blank is inspected, the product of the trial processing will be directly scrapped, and if the product part is not properly obtained during the trial addition process, The control will also be a big loss. Trial processing is mainly for those products with high internal quality requirements and air tightness requirements.
Figure 4 shows the products of the automotive air-conditioning compressor casing produced by our company. These products are subjected to 100% airtightness test after machining, and more stringent inspections are required after assembly. If the air leak is detected, the whole machine will be disposed of, and the damage caused will be borne by the responsible party. For such products, strict, efficient and practical real-time testing methods must be adopted to ensure the quality of the products.
For this type of compressor casing products, our company generally only supplies blanks to be machined by the main engine factory. The machining cost of one-piece products is much higher than the value of the blanks. The OEM has strict rejection rate for each batch of products. control. In order to control the internal quality of the die casting, it is necessary to conduct a test leak test on the product parts. Since the leak test process can be carried out after the blank machining, it is necessary to process the corresponding parts of the blank before the test leak can be performed. In order to speed up the inspection, we have simplified the processing during machining. The main methods are as follows:

First, all threads are treated with round holes of equal diameter.
Second, the inner bore is machined to the required dimensions.
Third, the machining allowance is not less than the design margin.
By adopting the method of leak test after machining, we can find out the problems of the product parts in time, and adjust the die-casting process accordingly to ensure the product quality of the die-casting blanks.
(2) Destructive inspection The so-called destructive inspection is self-evidently a method of testing some dimensions and properties of a product after it has been destroyed. The direct consequence of this inspection method is that the product is scrapped. Can not be used again. However, it allows the inspector to more intuitively observe and measure some of the internal features of the die casting. For example, a certain type of steering gear housing produced by our company is shown in Figure 5.

It can be seen from Fig. 5b that the middle is a long non-through hole (about 140 mm long and ≤ 25 mm in diameter). Due to the limited product structure, the core above the mold must be in a cantilever state. As we all know, the die-casting process is a high-temperature, high-pressure, high-speed metal liquid filling process, which will inevitably cause a large flow impact on the core of the cantilever here. The long-term metal liquid impact and the removal of the mold parting surface are not timely. Other factors can cause the core to bend, the positioning is not allowed, and so on. Once the core is bent or misaligned, the product will be scrapped. Due to the inconvenience of disassembly and assembly of the core on the mold, in order to find out possible quality hazards in time, we adopted a method of destructive inspection of the steering gear housing, and saw the produced die-casting parts from the location of the smallest end of the core. The shape after sawing is as shown in Fig. 5c, and it can be judged whether the core is bent or deformed by measuring the wall thickness of the circumference of the inner hole.
5 Conclusion
In the die-casting production, by adopting the auxiliary testing means described in this paper, the inherent quality problems of the product parts can be found in time, and the die-casting process, the mold, the equipment and the raw materials are adjusted accordingly, thereby avoiding the production of large quantities of defective products to ensure the products. The quality of the pieces.
About the author: Cui Aijun Luoyang Gaoxin Xinhua Welfare Processing Co., Ltd., deputy chief engineer.

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