Welding Phenomenon in Aluminum Alloy Die Casting Mold

Among all the main reasons leading to the failure of aluminum alloy die-casting molds, the problem of welding on the surface of the molds has gradually attracted attention. “Welding” is a term in the die casting industry, which refers to the reaction between the mold and the die casting alloy.

Once welding occurs on the surface of the mold, a complex Fe-Al intermetallic compound phase will be generated and cause defects on the surface of the casting in the next die-casting cycle. Hard intermetallic phases will also accumulate on the surface of the mold, so production must be interrupted and the welding products must be removed by polishing, which will lead to extended production time, waste of labor, and will also reduce the life of the mold.

According to the different forms of welding, “welding” can be divided into two types.

The former type of welding is called “impact welding”, that is, the welding occurs at the entrance of the mold surface or the gate to the cavity. These areas are generally subjected to violent impact of the molten metal flow during filling, the surface temperature is higher, the pressure is greater, the protective layer is easily damaged, and the mold protective layer fails and the metal matrix is ​​exposed under the continuous flushing of the die-cast alloy. The alloy reacts with the matrix material to form a complex intermetallic compound phase. The intermetallic compound is hard and not easy to deform. Its cracking and shedding during die casting will not only cause casting quality defects, but also take away the matrix material and expose the fresh surface. After this, the welding phenomenon gradually deepens, and in serious cases, the mold surface will be affected. Corrosion and loss of mold material. Therefore, it is necessary to remove and repair damaged surfaces in a timely manner early in the occurrence of welding.

The second form of welding is called “deposition welding”, that is, the welding position is away from the gate or away from the runner. These areas are usually where the surface treatment or mold lubricant cannot reach. Therefore, their surface conditions, temperature distribution, and pressure conditions are different from other places.

Generally, the temperature of the die-cast alloy is lower after reaching these areas, and its fluidity also becomes worse, which is easy to solidify earlier. The contact time of the hot semi-solid alloy with the surface of the mold becomes longer, and the surface state of the mold itself is not ideal here, so It is easy to form FeAl intermetallic compounds. In multiple die-casting cycles, intermetallic compounds will gradually deposit in these areas with poor fluidity, and then form a serious weld, which will affect the production of die-casting.

Although different forms of welding will occur in different areas of the aluminum alloy die-casting mold, the welding takes place has some common features-that is, the welding area on the surface of the mold generally shows a silver-white gloss.

The composition of the solder layer is often a complex Fe-Al intermetallic compound, and because the intermetallic compound that makes up the layer is thin, there are certain difficulties in analysis.

However, foreign researchers ZWChen and DTFraser used X-ray diffraction to analyze the intermetallic compound structure generated by dipping H13 steel in molten Al-11Si-3Cu die-cast aluminum alloy. They believe that the welded layer is composed of a composite The intermetallic compound αbcc-(FeSiAlCrMnCu), the hexagonal αH-(Fe2SiAl8) intermetallic compound in the outer compact layer and the rhombic η-Fe2Al5 intermetallic compound in the inner compact layer. The Fe-Al interface structure they photographed is very similar to the Fe-Al interface morphology obtained by the author’s experiment of “dipping H13 steel in ADC12 die-cast aluminum alloy”.

The amount of intermetallic compounds is very small, the welding surface layer is extremely thin and the analysis method is limited. At the current stage, domestic and foreign researchers can only carry out a rough qualitative analysis. As for the generation and development of the welded layer, the quantitative analysis of intermetallic compounds will be the focus of future researchers’ work.