Cause Analysis and Countermeasures of Defective Products of Aluminum Alloy Die Castings

Time：2024-03-29 16:02:21 / Popularity： / Source：

As light weight becomes a trend in the development of new energy vehicles and 5G communication and other fields in the future, many die-casting companies are developing layout and market competition. Aluminum alloy is widely used by die-casting enterprises as one of lightweight materials on the market. Do you know reasons for defective products of aluminum alloy die castings? What are specific common ischemia?

Today, we shares an article of technical dry goods for die-casting people in the industry. With above questions, let us understand together:

1. Classification of defects

1. Dimensional defects

Classification of Defects	Types of Defects	Defect characteristics
Size defect	Unqualified size	Due to various reasons, casting does not meet original design size
	Misalignment	Casting misalignment caused by misalignment of casting mold
	Core retreat due to mold opening	Due to expansion of casting mold and retreat of core, a thick flash is cast in casting part, which does not meet original size of product
	Deformation	Casting deformation changes original shape
	Fleshy, undercast	Casting is locally too thick or too thin and has local defects
	Partial defects	Partial defects in castings caused by gate and flash removal

2. External defects (appearance defects)

Classification of Defects	Types of Defects	Defect characteristics
External defects (cosmetic defects)	Poor flow of alloy liquid	Cavity has not yet been partially filled, and alloy has solidified
	Cold lines	Rough surface, shallow wrinkles, fluidity patterns caused by insufficient fusion of alloy liquid
	Cold shut	Where alloy liquids meet each other, junction line formed due to incomplete fusion
	Crack	Due to various reasons, cracks in parts of the product
	Shrinkage hole	Local depression produced on the surface of casting due to shrinkage of alloy liquid during solidification
	Bubble	Bubbles that develop on the surface of a casting as a result of gases contained in casting
	Scrape	Scratches on the surface of a casting as it is ejected from mold
	Adhesion traces (sticky mold)	Alloy liquid is deposited on the surface of casting mold, and surface of casting is pulled and damaged when it is demolded, becoming rough or losing flesh.
	Cavity scars, thermal cracks	Bruises and cracks on the surface of mold cavity, traces or cracks and burrs formed on the surface of casting as it is during casting.
	Corrosion defects in cavity	Defects on castings due to localized corrosion of mold cavity used
	Stomata	Small holes that occur on the surface of casting.
	Bruised	Scars caused by bumps during handling and picking up goods
	Pores at gate	A small hole that occurs at gate of a casting when gate is removed
	Tool scars	Injuries caused by operators using tools, etc.

3. Internal defects

Classification of Defects	Types of Defects	Defect characteristics
Internal defects	Shrinkage hole	Cavities created inside casting when alloy liquid shrinks after solidification
	Loose	Coarse spongy texture locally present on castings
	Stomata	Because gas in cavity and gas generated by paint are wrapped in alloy liquid, large voids formed inside casting remain after solidification.
	Pores in the center of thick wall	A small spherical hole in the center of a thick wall

4. Material defects

Classification of Defects	Types of Defects	Defect characteristics
Material defects	Hard point	Particles that appear in castings with high hardness and prevent normal processing
	Bad material	Chemical composition of casting does not meet requirements and there are impurities
	Oxide	Castings mixed with oxides

Other defects

Classification of Defects	Types of Defects	Defect characteristics
Other defects	Poor physical and chemical properties	Physical and chemical properties such as strength and corrosion resistance do not meet requirements
	Poor pressure resistance	Liquid and air pressure are added to casting, and there is a partial pressure leakage phenomenon
	Missing inserts	Products that forgot to install inlays
	Casting error or product error	Wrong mold is used or wrong assembly of mold is used to produce unwanted products
	Improper refurbishment of castings	Unfinished products caused by processing failures, processing errors, or processing omissions

2. What are characteristics of defects in die castings?

1. Because die castings are obtained by hydraulically injecting alloys into precision casting molds at high pressure and high speed, causes and countermeasures of defects that occur are clearer than those of other types of castings.
2. Because die-casting is a mass production method, when a certain defect occurs, it will also occur in future products, and there is a danger of a large number of defective products in a short period of time.
3. Die-casting mold is mainly composed of two parts: a dynamic mold and a static (fixed) mold. To change casting scheme, it is necessary to change parting surface of mold. Therefore, casting plan cannot be greatly changed. In the case of large changes, there is a risk of non-recasting.
4. Because dimensional accuracy of die castings is very high, even small deformations often lead to failure to assemble, or become defective products due to insufficient machining allowance. Dimensional failures will occur most of time.
5. For die-casting method, coating layer used on the surface of casting is very thin, so cooling rate of casting mold to alloy liquid is high, and cooling is fast. On large die castings and thin-walled castings, surface defects unique to die castings such as pockmarks and cold shuts are prone to occur.
6. Generally, during die casting, because of high pressure and high speed casting, gas inside mold is compressed and stored in casting, so pores are unavoidable. In addition, when welding castings, it will be difficult to weld due to expansion of gas. During heat treatment, due to gas expansion, bubbles will bulge on the surface of casting and accumulate to form blisters. It is difficult to perform T6 heat treatment.
7. For die castings, hard spots are unique defects of die castings.

3. What is relationship between classification of defects and defective products?

Defects of die castings vary widely, and some are difficult to be sure that they are defects. Even if it is same type of defect; in terms of practicality, performance, strength, etc., there are often cases where it does not hinder the use. It is a very complicated issue by what standard is judged as a defective product or a qualified product.
Furthermore, determination of defects is often different due to subjective judgment of inspector. It is necessary to exchange standards and establish samples. Important thing here is to classify defects in advance and specify inspection method

4. Direct cause of defects in die castings

1. Shape of product does not meet requirements,
2. Die-casting machine and filling conditions are not appropriate.
3. Mold and casting scheme are incorrect.
4. Casting conditions Improper casting operation.
5. Incorrect materials and smelting techniques.
6. Operator is inappropriate.

5. Indirect causes of defects in die castings

As far as cause of defect is concerned, direct cause is the greatest impact of poor technology. So special efforts are required to improve technology, but indirect causes also have a greater impact. Sufficient attention should also be paid to following indirect causes.
1. Engineering organization is unreasonable and engineering management is inappropriate.
2. Quality management (procedures, work standards, inspection standards, etc.) is not perfect enough.
3. Operator is derelict of duty (insufficient ideological education)
4. Managers lack enthusiasm for work (managers are not educated enough)
6. Reason analysis and countermeasures for unqualified dimensions

Reasons and countermeasures for unqualified dimensions
	Classification	Reason	Countermeasures
Dimensions fail	Mistake	Wrong drawing size Check is not meticulous Casting error	1. Improve drawing review system and carefully review drawings; 2. Check carefully during trial casting, especially for wall thickness of joint of curved surface, etc., it should be cut open or checked by other methods. 3. In order for error not to be missed because inspector did not see it unilaterally, a double check should be done. 4. In mold repair document, modified part should be written clearly, repair should be checked after repair, and mold should be tried again.
	Dimensions fail	Improper assembly of mold	1. Check whether there is any error in the transformation assembly, and correct it if there is any. 2. Check whether the bolts are loose, whether the assembly is correct, and repair them. 3. Check whether the backing plate and contact mounting surface are uneven, and repair them. 4. Check whether the surface condition of the mold mounting surface is good, and adjust the clearance of the backing plate.
		Mold wear	1. Repair worn parts. 2. Study and modify the position of gate, mold structure and casting scheme. 3. Study and modify material and hardness of the mold.
		Shrinkage changes caused by changes in mold temperature, casting temperature, etc. Partial overheating of mold	1. Adjust casting conditions such as pouring temperature, casting cycle, mold opening time, mold temperature, mold cooling, etc., and strictly follow prescribed standards. 2. Check and adjust composition of alloy liquid. 3. When local overheating and local shrinkage are too large, cooling amount, gate position and alloy composition of this part should be modified or adjusted.
		Mold is not strong enough	1. Research on strengthening strength of mold. 2. Study and modify design of mold. 3. Study and modify shape of product.

Causes and Countermeasures of Wrong Modes
	Classification	Reason	Countermeasures
Misalignment	Misalignment Core lock	There is a gap between guide post and guide sleeve There is a gap between backing plate and mounting surface of mold There is a gap on wedging surface and sliding surface of movable core	1. Check whether guide post and guide sleeve are worn, and repair them if there is a gap. 2. Check whether there is a gap between backing plate and mold installation surface, and repair it if there is a gap. 3. Check wedging surface of wedging block, and repair it if there is a gap. 4. Check matching state of sliding surface of movable core, and repair it when there is a gap.

Gud Mould Industry Co., Ltd