Post-processing die casting dies

On my blog already in 2021, I published the theoretical course of the production process of the die-casting dies. https://www.jstconsultancy.cz/jak-na-zihaci-procesy-u-form-na-tlakove-liti/.  Since time has advanced, I am returning to this issue again from the perspective of new technologies.

Tab.1 – List of all heat treatment processes on a die casting tool

Tab. 2 – Legend to the table

It can be seen from the table that if we were to follow all the recommended steps, and assuming that during the production of the insert we will also need welding, then in order to achieve a perfect condition, it is necessary to undergo 18 different thermal operations, including the re-activation of compressive stresses on the surface of the insert by one of methods below.

If we evaluate these operations in terms of time, then the time consumption for the production and maintenance of one insert is a total of 124 hours according to the following table. Of this, 40% of the time, a total of 50 hours, must be devoted to various annealing processes to remove stress.

Out of the notified 124 hours, 88 hours (71%) will be included in the price of the tool, as these are activities for which the tool manufacturer is responsible, at least 36 hours (29%) will have to be devoted to the die by the user of the tool as part of its maintenance. But this time can be longer if there are more than 3 stress relaxation processes.

In practice, of course, it will be a little different, because we usually heat treat several inserts at once in one cycle, but it is sufficient for our idea. And if it were GIGA tools, then this time consumption will be much higher, because each heat cycle with an insert over 1 ton will take not 8 hours, but even 24 hours, and there will always be only one insert in the furnace.

Tab. 3 – Theoretical time consumption for heat treatment of the inserts for HPDC

Tab. 4 – High-temperature, low-temperature, and no-temperature cycle requirements

The analysis of the theoretical workflow also shows that 72% of the processes take place in low-temperature furnaces up to 750°C and only 11% of the cycles represent actual hardening at temperatures above 1,000°C. These are quenching in oil and tempering for evaluating the input quality of the material, and the quenching and tempering in the gas stream of the real part. The remaining 17% of the cycles are tied to technologies that are able to introduce forced compressive stress into the surface.

The analysis shows that the need for low-temperature furnaces for tempering and annealing during the lifetime of the die is 7 times higher than the need for quenching furnaces.

If we take a look at what is the responsibility of the tool maker and what is the responsibility of the die casting foundry, then we see that the need for a foundry is only and exclusively tied to low temperature LT furnaces. But the foundry usually does not have these furnaces. It is interesting because if we talk about tools for extrusion of aluminium profiles, the situation is completely opposite. Every ALU profile manufacturer has a NITREX furnace as a standard, where it periodically performs a thermal process, in this case nitriding, which simultaneously acts as a relaxation of internal stresses from thermal fatigue and an activator of compressive stresses on the surface.

Repetitive annealing to reduce thermal fatigue stress has another problem in addition to the LT furnace requirement. We have to disassemble and reassemble the tool, and at the same time reseal all the cooling channel plugs. These are additional costs and, at the same time, pressure to meet deadlines. It is common for the foundry to disassemble the die on Friday, and ask the commercial heat treater to take it away annealed for reassembly on Monday.

However, the larger the die, the more difficult it will be to carry out this thermal work on time. If I delve into my favourite topic of “GIGA Casting”, then I will say frankly that I can’t imagine it very well. This is because each individual insert will weigh a ton or more, and the annealing process for each one will take 16 to 24 hours, assuming we actually have such a furnace at our disposal. And each tool could have up to 8 of these GIGA inserts. 4 on the fixed side and 4 on the movable side. So, we are talking about the need to anneal to remove stress from thermal fatigue of an insert weighing 8 to 16 tons.

Fig . 1 – Clamping plate of the IDRA GIGA press with a clamping force of 9,000 tons (https://www.teslarati.com/tesla-cybertruck-idra-9k-ton-giga-press-teaser-images-video/)

There are indications that this may not be the case.

a) Annealing to remove stress is not only related to the thermal process, but also has a variant in the vibration method of removing stress from thermal fatigue (e.g. https://www.jstconsultancy.cz/vibracni-zihani-na-odstraneni-pnuti/

Fig . 2 – Equipment for vibration release of stress from thermal fatigue from the company BONAL (https://www.bonal.com/)

b) Even the methods for the forced induction of compressive stresses in the surface of the insert do not have to be tied to its disassembly and, in theory, can be carried out directly in the foundry.

I will mention these methods in a little more detail, leaving out the most well-known method, which is gas nitriding. This has a small disadvantage if we weld the insert in the future. Even this problem can be solved today, e.g. with the help of a DMG Mori Lasertech hybrid machine tool, when we machine the nitrided layer and then restore the shape using DED (Direct Energy Deposition) powder additive technology.

As far as inherent compressive stresses are concerned, the first method is offered by Bohler under the brand name ABP®. This is a special process of blasting the surface of the insert, by which we achieve zero or even compressive stresses. This process can also be applied in the ABPplus® or ABPdualplus® variant, when gas nitriding is also applied, or post-oxidation. In this case, however, the insert must be disassembled and assembled.

Fig. 3 – The principle of ABP process (https://www.voestalpine.com/highperformancemetals/cs/cs/sluzeb/tepelne-zpracovani/abp-abp-plus/)

The second method is offered by HILASE (https://www.hilase.cz/ ), a workplace of the Institute of Physics of the Academy of Sciences of the Czech Republic. This method uses the shock wave of a high-powered laser beam to introduce compressive stresses into the surface and is known as Laser Shock Peening. Unfortunately, even for this method, the tool must be disassembled.

Fig. 4 – Mold parts and forging die for the Laser Shot Peening method

But the third method interested me the most. This is a completely new technology offered on the market by the Czech startup PSP Technologies (https://www.plasmashockpeening.com/ ) under the name Plasma Shock Peening. The method was developed by the Institute of Thermomechanics of the Academy of Sciences of the Czech Republic and is globally patent protected. The device itself is portable and can be installed on an industrial robot. It is therefore possible to imagine that the device can be installed directly in the foundry, and without disassembling the mold, we will generate compressive stresses to the surface of the tool according to the program with which we will program the robot. Of course, there will be geometric limitations, e.g. deep and narrow shapes cannot be affected in this way, but we can still modify a significant part of the mold surface in this way.

Fig. 5 – Plasma source on a robotic arm for the application of the PSP method

So, what does that mean? Ignoring the mandatory thermal processes of the tool manufacturer, its user, the die casting foundry, can fulfil the conditions for periodic removal of thermal fatigue stresses and the subsequent application of compressive stresses without having to disassemble the die. Both the Vibration Stress Relief method and Plasma Shock Peening can be applied cold. Not only will we save wasted time disassembling and assembling the tool, we will also save significant energy needed for heating and cooling the inserts.

The only downside is that no one has tried it yet. So, it’s actually a big challenge for developers. What about Elon Musk? Since the GIGA die costs 4 million USD and more, and has a lifetime of 30 to 70,000 shots, the cost of testing the vibration generator from BONAL at around €40k is so small investment that it will be paid for by just increasing the life of the tool by only 700 pcs. And the price for Plasma Shock Peening? That’s what PSP has to say, they have a great effort to establish their method worldwide. I really like them.

Inquiries can be made to the PSP CEO, Tomáš Slavík, tomas.slavik@plasmapeening.com

20240527 PL Plasma Shock Peening CZ

Jiří Stanislav

29. července 2024