Melting temperature of aluminum alloy is 580~740℃. In addition to withstanding such high temperature impact, aluminum alloy die-casting mold also withstands pressure shock injected into cavity, which brings about thermal erosion and thermal wear of contact portion of molten aluminum alloy, as well as cold and hot fatigue caused by cooling of mold itself, make working environment of mold harsh and working conditions are harsh. Test life of aluminum alloy die-casting mold is relatively low, 40,000~100,000 times, and main failure mode is thermal fatigue, accounting for 60%~80%. Such severe working conditions of aluminum alloy die-casting mold require properties such as heat-resistant fatigue, heat-resistant grinding, corrosion resistance, and impact resistance.
In order to solve this problem, many surface treatment techniques have been proposed, in which surface nitriding treatment is more and service life is also improved. However, it has not been seen so far that use of controlled nitriding quality improves service life, but only mentions that compound layer and vein structure in nitride layer affect service life. Through analysis, it is clear that compound layer and vein structure in nitride layer are main reasons for early failure of gate sleeve. Controlling and ensuring reasonable quality of nitride layer can improve service life of aluminum alloy die-casting mold.

1. Sampling

Service life of aluminum alloy die-casting mold sprue sleeve treated by conventional gas nitriding method is too low, and influence of nitriding quality on service life needs to be analyzed. For this reason, original nitride layer state and state of nitride layer after failure are taken and compared. As shown in Fig. 1, outer circular arc in the state of original nitride layer and inner circular arc in the state of nitrided layer after failure are taken from round-shaped gate of wall thickness of 45 mm, so as to compare and analyze state of two nitride layers.                                                                                  ) sprue bushing           (b) inner arc (failure surface)                (c) outer arc

2. Analysis results

(1) Macroscopic state of failure inner arc

Macroscopic state of inner arc observed by stereomicroscope is shown in Fig. 2, inner wall surface of sprue sleeve is severely cracked and dropped.

(2) State of original nitride layer and failed nitride layer

As shown in Fig. 3 and Fig. 4, original nitride layer of non-working surface of sprue sleeve is relatively thick (see Fig. 3a), compound layer (white bright layer) reaches 10~12μm (see Fig. 3b), Veins in diffusion layer are dense and high hardness region in nitride layer reaches about 150 μm (see Figure 4). As shown in Fig. 5, compound layer is not visible in arc-degraded nitride layer in working face of sprue bush (see Figure 5a), relatively dense block is seen in low-nitride layer (see Figure 5b). In high-nitridation layer, it is seen that brittle fragments are broken along grain boundary (see Fig. 5c).                                                                     (a) inner arc (50 ×)              (b) inner arc (100 ×)                  (c) inner arc (500 ×)

3. Analysis and discussion

Non-working surface arc nitriding state reflects original nitriding state of component. Compound layer in nitride layer is thicker, vein structure is denser, and hardness gradient is not smooth, indicating that original nitriding is in a nitriding state. Compound layer is a hard and brittle structure, which has a large difference from expansion coefficient of other parts. It is easily broken when subjected to external impact and thermal shock. Circular arc in working surface  fails, compound layer in nitride layer is broken by aluminum alloy melt injection impact, mold working cold and thermal impact, and compound layer is not visible.
Venous structure in diffusion layer mostly precipitates along low-energy grain boundary, making grain boundary fragile, and is a "microcrack" in nitride layer, which is easily broken by impact of aluminum alloy injection. When venous tissue forms a network, it is more prone to rupture and blockage. Nitrided reticular veins are prone to appear in the sharp corners of nitrided workpiece, automatic cracking and blockage often occur. When high hardness region of nitride layer is too thick, that is, when hardness gradient of nitride layer is not gentle, stress concentration is likely to occur, toughness is lowered, brittleness is increased, and nitride layer is broken. Quality of nitride layer failed to achieve good control. Compound layer was too thick, pulsed structure was too dense, and hardness gradient of nitride layer was not smooth, which was main reason for early failure of gate sleeve.

4. Nitrided layer quality control

Through analysis of early failure causes of a large number of fast hot forging die, it is found that detachment of compound layer and vein structure, unreasonable hardness gradient of nitride layer are main failure causes, which are basically same as failure process of aluminum alloy die-casting mold. After improving quality of nitride layer of fast hot forging machine, service life is obviously improved. Figure 6 and Figure 7 show quality control nitriding effect of nitride layer. We use current production of nitriding furnace, and improve quality of nitride layer of gate sleeve, which obviously improves service life and ensures smooth production of gate.                                                                (a) Controlled nitridation 100 ×                         (b) Controlled nitridation 500 ×

5. Conclusion

(1) Excessively thick compound layer, dense vein structure, and unreasonable nitride layer hardness gradient are main reasons for early failure of gate sleeve.
(2) Quality control of nitride layer is an important way to improve service life of various hot working molds.