Excerpt from the March 2014 issue of Metalworking World by Fabio Boiocchi
The working instruments and the dies for forming metal parts, especially in the HPDC (High-Pressure Die casting) technology, are very complex components subjected to strong stresses. In those applications, different failure mechanisms can, therefore, occur: thermal fatigue, cracking, abrasive wear, plastic deformation, transfer of die-cast metal on the die surface. Those problems can cause frequent downtimes of productive lines. It is assessed on average a productivity decrease equal to at least 15%, which causes 15% of the volume of production rejects. This has naturally relevant economic and environmental implications related to the process, efficiency, and energy and raw materials consumption.
One of the most effective means to delay the soldering reaction provides for the modification of the surface characteristics of the die through coatings, heat treatments and surface reactions. In particular, the hard nanocomposite coatings based on nitrides or transition metal carbides can protect the surface against the erosion, soldering and other damaging mechanisms, thus improving the surface functionality and increasing the die service life by 10 times. At the same time these coatings drastically reduce the refrigeration and detaching agents. Over the last few years, they have developed the single layer, the multilayer and micro-structure gradients with chemical and stoichiometric tests of different coatings, they represent cutting-edge solutions in the coating technology.
The modifications of the die surfaces result in an improvement of the resistance to wear and, consequently, they allow obtaining a reduction of costs in the forming process. This article is presenting the results of a study concerning two nanocomposite coatings (AlSiTiN and AlSiCrN) deposited on H11 steel and on nitride H11. Coatings have been analyzed in terms of hardness, friction adhesion and resistance to wear at high temperatures (in this case, equal to 450° C).
When the coating gets hard
All nanocomposite coatings show high hardness. AlSiTiN and AlSiCrN alloys, in particular, show comparable values on the different H11 and H13 substrates. Nitrided coatings are the cases with higher hardness in both. Good adhesion properties were observed for all coatings. The thin layer deposited on the H11 substrate and above the nitrided layer shows the same type of yield. In the beginning, it is possible to observe some parallel cracks along the scratching channel and some internal chipping. Besides, at the beginning of the track it was not possible to detect any acoustic emission. AlSiCrN_H13 and AlSiTiN_H13 underwent the spallation of the interface at the edge of the track due to the scratching. The progressive propagation inside the track leads then to the complete final detachment. AlSiTiN, deposited on the H13 sample, presents the highest critical charge and the nitriding causes a decrease of the adhesion.
When comparing the friction evolution, the deposited AlSiTiN alloy turned out to be the one with minor friction coefficient. The nitriding determines an increment of the friction coefficient. If we compare the depth at the end of the tests at 5,000 and 15,000 revolutions, the thin AlSiCrN coating shows a high resistance in comparison with the AlSiTiN alloy in all the cases considered in this analysis. For this reason, AlSiCrN has not lost the coating layer at the end of all tests. Besides, it is worth pointing out that the depth is constant and homogeneous for all the traces due to wear.
Nitrided materials feature higher resistance to wear; and this is a likely consequence of the higher hardness that they can boast in comparison with others. We have treated two nitrided inserts coated with AlTiSiN and two with AlTiN and two die cavities with AlTiN treatment. The duration of the production process to which the pieces were subjected consists of 100,000 initial cycles, which allow highlighting the present modifications and defects. The result was, however, that the coated cavities did not show any improvement in comparison with untreated cavities; the inserts coated with AlTiN showed evident defects on the surface and, finally, the AlTiSiN-coated inserts did not show any kind of defects. The test was then continued up to 175,000 cycles, then until the time in which inserts coated with AlTiN revealed to be so defective that their replacement became necessary.
On the contrary, AlTiSiN inserts were still in good condition. Even at 175,000 cycles it was not possible to identify any difference between coated and uncoated cavities. At the end of something like 375,000 cycles AlTiSiN inserts and AlTiN cavities underwent some further inspections again. And they permitted to point out that in this case AlTiSiN inserts showed some surface defects. The comparison remained on the contrary unchanged for cavities. In short, it was observed that the life cycle of an insert nitrided and coated with AlTiN can approximately bear a total of 175,000 cycles. And, contextually, that the life cycle of an insert nitrided and coated with AlTiSiN approximately corresponds to 350,000 injection cycles. An aluminum injection die was composed by four parts differently treated (one part coated with CrN, one part coated with AlTiSiN and, at the bottom, the uncoated parts).