Vol. 4, No. 12, December 2020


Case Study Of A Quality Complaint On Aluminum Die Casting Products
- The Importance Of Process Quality Capability Improvement
To Ensure Stable Product Quality (Ⅱ)

(Continued from the last issue)

Before we start to analyze this case, interested readers can read “How to Ensure your Long-Term Suppliers’ Continuous Quality Compliance” in this year’s May, June, and July editions of the Newsletter at which discussed the concept of “process quality capability”, also known as engineering capability and process capability. Referring to the ability to meet the quality requirement in the process, it is the ability to produce qualified products with processes under control. It measures the internal consistency in the process to achieve the minimum quality fluctuation under the most stable state.

Based on the writer’s understanding, here are two possible states for the mould and black spots on the surface of die casting parts: one is the pollution of the internal composition of the part, caused by the color, size and position of the die structure and the degree of the aluminum liquid being polluted and oxidized in the process of die casting, and appearing from the inside to the outside. The other case is that process disciplines are not strictly implemented during the shot blasting, machining, cleaning, passivating, air-tight testing, packing and delivering processes, resulting in the black mildew spots with different sizes, different positions, and different color depths on the surface.

Aluminum alloy is an active metal that will be naturally yet slowly oxidized in a dry environment, but it will be easily oxidized and eroded at higher temperature and humidity. This characteristic of aluminum alloy determines that any non-standard operating methods or environments in production, packing, storage, inspections and transportation will lead to the surfaces of the product being oxidized, with the appearance of black mildew and black spots.

This article mainly reviews the production process (raw material inspection – smelting – die casting – shot blasting – machining – cleaning/passivation – air tightness testing – packing – warehousing – shipping) of the aluminum die casting of thermostat housing and analyzes the influence (quality failure) and risks caused by the process, parameters, implementation compliance, as well as process change, verification and process quality capability. The goal is to explore and summarize the factors that may lead to black spots on the surface of die castings for references when purchasing aluminum die casting parts.

▍The impact of alloy material on black spot on the casting parts

Most die casting alloys are easily oxidized aluminum alloys, especially at high temperature. In the process of pouring and filling the die, it will oxidize and produce defects of oxide slag inclusion. During melting and taking the liquid, the longer the time, the more rolling of the alloy liquid, resulting in higher level of oxidation. Therefore, we need to pay more attention to the chemical composition of the aluminum alloys, apart from the product’s mechanical properties, in the selection of die casting alloy. Different chemical compositions have different anti-oxidation properties, which could lead to different degree of blacks on the product surfaces. Try to select, for production, alloys that are not easy to oxidize. We can also add a small amount, 0.1% – 0.3%, of sodium fluor aluminate or sodium fluorosilicate during slag cleaning and refining to prevent the oxidation discoloration of the die castings.

(To be continued)

By Felix SS YUAN



Turbocharger is a mechanical device which uses the high temperature and high pressure exhaust gas produced by internal combustion engine as power to compress the air entering the engine. After the engine is installed with a turbocharger, the air flow into the engine is increased, enabling fuller combustion and higher thermal efficiency. It is an important means of environmental protection and energy saving for fuel vehicles in recent decades. Nearly 70% of the diesel vehicles in the market use turbocharger.

Primary failure modes:
Noise, insufficient intake pressure, hysteresis, surge, oil leakage, air leakage, etc.

The main causes for these failures:
● The materials of impeller, seal ring and bearing are not up to standard;
● Problems in machining, bearing precision and size matching;
● The intrinsic frequency of the product is not matched with the engine;
● Failure in dynamic balance calibration.

In conclusion, the failures of turbochargers basically involve all aspects of the production, especially the selection and inspection of materials, the adoption of special and necessary equipment and long-term design improvement experience in order to ensure stable and continuous quality.



Approved Vendor List (AVL)

Approved Vendor List (AVL), or Approved Supplier List (ASL), is an important requirement of a sound quality management system.

ISO 9001:2015 8.4.1 requires organizations and companies to establish criteria for the selection, evaluation and re-evaluation of suppliers. The standards also require records of supplier evaluation results be maintained. This includes any necessary actions taken as a consequence of the evaluations conducted, such as the removal of a supplier from the AVL or changed approval status.

The primary purpose of the AVL is to ensure the placement of purchase orders or contracts are limited to those suppliers that meet the organization’s established criteria for supplier selection, evaluation, and re-evaluation.


Cost of raw materials:
Exchange rates:

USD/RMB: 1:6.531

EUR/RMB: 1:7.9066

RUB/RMB: 1:0.0873

December 2020

The above information is for reference only


With this December issue, we will soon say goodbye to 2020 which has been without doubt the most challenging year in decades globally.

As we are all excited about the availability of the COVID-19 vaccines, we are also experiencing the worst moments in the almost year-long fight against the virus. The COVID-19 situations in Europe and the US are getting increasingly serious by the day, and more sporadic cases are also being reported in China and many other Asian countries.

The impact of COVID-19 has also been made much more painful to the world business communities by the skyrocketing shipping costs and lack of containers for global ocean freight as we speak, the sharp appreciation of Chinese Yuan against the US$ and the prospect of further depreciation of the US dollar in 2021, and the alarming rise of the raw material costs in China. 2021 Q1 looks even more challenging to many companies.

In the long-term, however, as the COVID-19 vaccinations become widely available, we do expect to see gradual return to a new “normal”, hopefully, later in 2021.

Again, we are most grateful to all of our readers whose support has enabled us to continue the publication of this newsletter throughout this year. Your comments are always important and we’d be most pleased to hear from you.

Happy Holidays!

SHENTOU SUPPLY CHAIN MANAGEMENT CO. LTD. is a Shenzhen, China, based company serving international automotive clients in the implementation of their China strategies and programs. CHINA AUTOMOTIVE SUPPLIER QUALITY MANAGEMENT BRIEFING is a bi-monthly newsletter published by Shentou to address the specific and unique quality challenges and concerns international automotive companies face with suppliers in China. Comments are welcome at Click here to subscribe.

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