Surface Treatment Solutions for Die Casting Mold

Posted on : Sep 3 , 2022 By  GREFEE

Die casing model is one of the tools that are utilized to cast the metal parts, which is specifically completed on the die casting die forging machine. The basis process of die casting include: the metal liquid is injected into the mold cavity in a slow or high speed. The mold with moving mold cavity surfaces will be forged under pressure as the cooling of liquid metal, which eliminates the porosity of blank, but also makes the internal structure of the blank reach the broken grains in the forging state. The comprehensive mechanical properties of blank is improved significantly. The material selections for die casting are various, like aluminum alloy die casting, magnesium alloy die casting, zinc alloy die casting, and other metals which need to be melted at high temperature before casting.

Thus, the working environment for die casting mold is server, requiring surface treatment. Currently,  there are different kinds of surface treatment for die casting mold, but can be divided into three main categories generally.

(1) Improved technology of traditional heat treatment process;

(2)Surface modification technology, including surface thermal diffusion treatment, surface phase transformation strengthening, EDM strengthening technology, etc;

(3) Coating technology, including electroless plating, etc.

Die casting mold is a large category of mold. With the rapid development of automotive and motorcycle manufacturing industry, the needs for die casting parts increase, too. In the meantime, it also has high requirement for the comprehensive mechanical properties, service life, etc. at present, It is difficult to continual improve the use performance requirements by relying on the applications of new mold materials, which have to adopt different surface treatments into the die casting mold to meet the requirement of high efficiency, high accuracy and long service life.

In different kinds of molds, the working conditions of die casting molds are strict. It will constantly get contact with the hot metal during the process, so the die casting mold must have high thermal fatigue resistance, thermal conductivity, wear resistance, corrosion resistance, impact toughness, red hardness, good demoulding property, etc. Therefore, the requirement of the surface treatment of die casting mold is gradually higher and higher.

1. Improved technology of traditional heat treatment process

The traditional heat treatment process of die casting molds is the quenching-tempering, and later developed surface treatment technology. Due to the varieties of material of die casting molds, the same surface treatment skill and process application will bring a different effect when using different materials. On the base of the traditional process, appropriately processing technique are introduced to different mold materials to improve the properties and extend the service life of molds.

Thermal treatment technique improvement is another development direction, which is the combination of the traditional thermal treatment and advanced surface treatment process to extend the service life of the die casting molds, such as the chemical heat treatment method carbonitriding, NQN combined with conventional quenching and tempering process, which is carbonitriding quenching carbonitriding composite strengthening, can not only obtain higher surface hardness, but also can effectively increase in hardened layer depth. The distribution of hardness gradient of infiltrated layer is appropriate, and the tempering stability and corrosion resistance are improved, so the die casting mold can obtain great core properties and the surface quality and properties are enhanced significantly.

2. Surface modification technology

Surface thermal diffusion technology

It includes carburizing, nitriding, boriding, carbonitriding, sulfur carbonitriding, etc.

Carburizing and carbonitridingThe carburizing process can be applied into cold and hot work and surface strengthening of plastic molds, which can extend the service life of mold, such as the 3Cr2W8V steel die casting mold, of which the surface hardness can be HRC56～61 after carburize first, then quench at 1140 ~ 1150 ℃, and temper twice at 550 ℃. The service life of die-casting non-ferrous metals and their alloys is increased by 1.8 ~ 3.0 times. During the carburizing treatment, the major treatment methods include solid powder carburizing, gas carburizing, vacuum carburizing, ion carburizing and carbonitriding formed by adding nitrogen element in carburizing atmosphere, etc. among these methods, the vacuum carburizing and ion carburizing is the new technologies in recently years, which has advantages of fast infiltration rate, uniform infiltration layer, gentle carbon concentration gradient, and small workpiece deformation, which gradually plays an important role in the surface treatment of high precision mold.

Nitriding and related low temperature thermal diffusion technology.

It includes nitriding, ion nitriding, carbonitriding, oxygen nitriding, sulfur nitriding, and sulfur carbon nitrogen, oxygen nitrogen sulfur ternary nitriding, etc. these methods are simple to process, and has strong adoptability, with low diffusion temperature at 480-600 degrees. The deformation of workpieces are small, especially for the surface strengthening of high precision molds, and the nitrided layer has high hardness, good wear resistance and good mold adhesion resistance.

After tempering and nitriding at 520 ~ 540 ℃, the service life of 3Cr2W8V Steel die-casting mold is 2 ~ 3 times longer than that of non nitriding mold. Most of the die casting molds in America made of H13 steel require the nitriding treatments, and nitriding is used instead of tempering. The surface hardness of the H13 is up to HRC65～70, and the hardness of the bottom is low, but good rigidity for obtaining the great and comprehensive mechanical properties.

The nitriding treatment is a common surface treatment of the die casting molds. When there is thin and brittle whitening layers on the nitriding layer, it cannot resist the effect of alternating thermal stress, so it is easy to find micro cracking on the surface, which reduces the thermal fatigue resistance. Thus, during the nitriding process, we should strictly control the process to avoid the generation of brittle layers. Some western centuries introduced the second and multiple nitriding processes. With the repeated nitriding, it can decompose the nitride white layer that is easy to produce microcracks in the service process, increase the thickness of the nitriding layer.

Meanwhile, a very thick residual stress layer on the surface of the mold, will extend the service life of mold significantly. In addition, salt bath carbonitriding and salt bath sulfur nitrogen carbonitriding are also adopted, such as the Tfi+abi process is to impregnate in alkaline oxidizing salt bath after nitrocarburizing in salt bath. The color of the surface of workpiece is black when the oxidation occurs. Its wear resistance, corrosion resistance, thermal resistance are all significantly improved. The service life of the aluminum alloy die casting mold increases by hundreds of hours after this processing.

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