Analysis on Common Defects of Castings

- 2021-12-24-

With the continuous improvement of the development level of the domestic manufacturing equipment industry, the equipment level of the die-casting machine has also been significantly improved, and the types of parts that can be manufactured are also increasing. It has been expanded, and the precision and complexity of die-cast parts have also been greatly improved. I believe that in the near future, die-casting parts will better serve our production and life!

The die-casting parts are widely used in various industies field.Die-casting parts can be manufactured as die-casting auto parts, die-casting automobile engine pipe fittings, die-casting air-conditioning parts, die-casting gasoline engine cylinder heads, die-casting valve rocker arms, die-casting valve seats, casting power accessories, die-casting motor end covers, die-casting shells, and die-casting pumps Housing, die-casting construction accessories, die-casting decorative accessories, die-casting guardrail accessories, die-casting wheels and other parts.

In order to know about the die-casting process, Let us analysis on common defects of die-casting together.

Causes of Defects Generation

● The design of the die-casting itself is unreasonable: the wall thickness, shape, rounded corners, draft angle, holes, etc. are inappropriate.

● Die-casting machine performance problems: insufficient injection force and clamping force, improper casting pressure and filling speed, mismatch of casting size and projected area with the capacity of the die-casting machine, etc.
● Die casting mold design and manufacturing problems: mold alignment, grinding, machining accuracy, the influence of movable parts on the size of castings, and improper arrangement of cooling water channels.
● Die-casting process problems: parting surface selection, gating system design, exhaust groove, die-casting process parameters, improper coating.
● Alloy materials and smelting problems: the composition of alloy raw materials, the ratio of new and old materials, and the smelting process are improper.
● Die-caster operation problems: material temperature, mold temperature control, process parameter control, improper injection, picking, production cycle, etc. If one of the above factors is incorrect, or the combination of several factors is incorrect, it will cause defects.

Defect analysis method [analyze by scientific means, prove with data]

State analysis

Frequency of defects:
① Often
② Occasionally

Defect location: 
① fixed at a certain position of the casting.
② not fixed at a certain position, free.

For defects that sometimes appear but do not appear most of the time, it may be an unstable state. 

For example:
① The material temperature is too high or too low;
② Mould temperature fluctuation;
③ Manual operation: improper production cycle for spraying, picking up parts;
④ Die casting machine malfunction.

For the defects caused by the unstable state, the main purpose is to strengthen the management and standard operation of the production site, which can be analyzed by monitoring the process parameters on the spot.

Chemical composition analysis

Advanced inspection methods such as spectrometer and atomic absorption analyzer are used to analyze the content of effective elements and impurity elements in the composition of zinc alloys to analyze their impact on the performance analysis of die castings and the quality of castings. 

①Are there any problems with the alloy material?
②Are there any problems with the smelting process?

Metallographic analysis

Cut the defect part and check the structure of the die-casting under a microscope. First judge the type of defect: If there are holes on the surface of the casting, are they pores? Shrinkage? Slag hole? Under the microscope, the defect can be accurately judged, and the cause of the defect can be further analyzed.

Analysis of pouring system

Whether the molten metal can flow smoothly in the runner and avoid entrainment, whether the way of filling the cavity is reasonable, and whether the gas is removed smoothly. In the production of die-casting, it is found that many casting defects are caused by the unreasonable opening of the gating system.

Bubble Defect Analysis

Zinc alloy die castings are currently widely used in various decorations, such as furniture accessories, architectural decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, various metal buckles, etc., so the surface quality of the castings is high, and Good surface treatment performance is required.

The most common defect of zinc alloy die castings is surface blistering.

Defect characterization: There are raised cannons on the surface of die-casting parts.

① Found after die casting;
② Revealed after polishing or processing
③ Appears after fuel injection or electroplating;
④ It appears after being left for a period of time.

Reasons for production

1. Caused by pores: mainly pores and shrinkage holes. The pores are often round, and most of the shrinkage holes are irregular.

Main reasons:
①In the process of filling and solidification of molten metal, due to gas intrusion, holes are formed on the surface or inside of the casting.
②The intrusion of gas from paint.
③ The gas content of the alloy liquid is too high and will precipitate during solidification.

Causes of shrinkage:
① In the process of metal solidification, shrinkage cavities occur due to the shrinkage of the volume or the failure of the molten metal to feed the final solidified part;
② Castings with uneven thickness or local overheating of castings cause a certain part to solidify slowly, and the surface will form a recess when the volume shrinks. Due to the existence of pores and shrinkage holes, the holes may enter water during the surface treatment of die castings. When baking is carried out after painting and electroplating, the gas in the hole is expanded; or the water in the hole will become steam and the volume will expand, which will cause blistering on the surface of the casting.

2. Caused by intergranular corrosion

Harmful impurities in the composition of zinc alloys: lead, cadmium, tin, will gather at the grain boundary to cause intergranular corrosion. The metal matrix is broken due to intergranular corrosion, and the electroplating solution accelerates this pot damage, and the parts subject to intergranular corrosion It will expand and lift up the coating, causing blistering on the surface of the casting. Especially in a humid environment, the intergranular corrosion will cause the casting to be shaped, cracked, or even broken.

3. Caused by cracks

Water pattern and cold barrier pattern: During the filling process, the molten metal that enters first contacts the mold wall to solidify prematurely; after the molten metal enters, it cannot be fused with the solidified metal layer, forming a moiré at the butt of the casting surface. Strip defects. The water mark is generally in the shallow layer on the surface of the casting; while the cold barrier may penetrate into the inside of the casting.

Thermal cracking:
① When the thickness of the casting is uneven, stress will be generated during the solidification process;
②The metal is ejected too early and the metal strength is not high;
③Uneven force during ejection;
④ Excessively high mold temperature makes the crystal grains coarse;

⑤ The presence of harmful impurities. The above factors may cause cracks. When there are water marks, cold barrier marks, and hot cracks in the die casting, the melt will penetrate into the cracks during electroplating, and will be converted into steam during baking, and the pressure will lift the electroplated layer to form blisters.