Comptech for Rheocasting
The Commercial Breakthrough of Rheocasting
Since the discovery of thixotropy by Professor Flemings in 1971, semi-solid casting has never really left laboratories or technology supply offices around the world - until today.
The tipping point is electric vehicles (EV) and 5G telecom systems. 90% of all Discussions, Request for Quotations (RFQs) are based on designers looking for higher functionality to create cost-effective designs with lower weight, higher strength, better thermal conductivity, or leak-free components.
The disadvantages have changed from a lack of material properties to engineering capacity and to a few suppliers able to offer the process – a problem that has already been recognised on the market and can still be solved.
The Meaning of Rheocasting and Semi Solid Casting
Rheocasting is a member of the semi solid family of processes. The term 'semi-solid' means that the molten metal is placed in a state in which part of the melt is in a solid phase. In a mixture of solid and liquid, the phenomena of thixotropy occur, which means that the faster we shear the melt, the less force we need (as shown in Figure 1 in the picture gallery).
There are several different processes on the market today that all have their benefits and could be difficult to distinguish.
Three factors can be used to see the difference: solid fraction, slurry quality (homogenous) and process cost:
- The solids content must be above a certain value in order to obtain a laminar flow.
- Slurry quality means oxide size and distribution as well as homogeneity of the slurry.
- The process costs are based on a low investment and a fast sludge treatment
Semi-solid casting and rheocasting should be considered as melt preparation processes and not as a new process as they are 80 % similar to high pressure die casting (HPDC).
Why Is The Commercial Breakthrough Happening Now – 40 Years Later?
Over the years, the amount of experimentation, Research and Development (R&D) and other development projects have not led to massive capacity building, and the most important question is why:
- Semi solids have been expensive and are not giving the quality needed
- HPDC is good enough for yesterday´s products but not for EVs
- Need of better thermal properties at low cost is growing fast
The speed with which new systems were introduced was too high. There was no time for proper R&D activities and industrialization, which is why many promising processes failed due to expensive solutions and inferior slurries/melts. Through the efforts of the last 10 years these problems have been solved, why rheocasting as an example today causes the same costs as HPDC components.
Conventional engines with their drive trains and chassis structures were designed to be safe and economical. But with the introduction of electric cars and trucks, as well as hybrid vehicles, the focus on weight at the system level is increasing. For example, the need for parts cast with other alloys means that the eutectic alloys used in HPDC are not strong enough or do not have the same heat treatment properties as rheocasting Basically, we see many new applications such as electric motors, non-mechanical compressors, chassis with demanding welds, battery systems and battery boxes. All these new systems have some common design drivers: lightweight, leak-free and strong, which is why alloy selection has become a great discussion forum. Figure 2, for example, shows the + 9 % elongation in aerospace rheocasting.
There is a growing need for effective heating and cooling systems in 5G and electric vehicles to maintain batteries, interiors and waste heat from electric motors to optimize the work area. This is a new field for the automotive industry as the range becomes a very important USP. The use of heat sinks and other heat management components has been common for a long time, but when a higher thermal conductivity above 180 - 190 W/mK is required, the problems arise (see Figure 3 and 4 in the picture gallery).
The typical values that can be achieved with HPDC are the first problem. Not using HPDC in most cases results in an extruded part with costly machining or other processes that are often costly. For the automotive sector, low cost is a key factor, which is why semi-solid casting, which offers thermal conductivities well above 180 W/mK and is a viable option with component costs such as HPDC.
Porosity is a Problem
The disadvantage of using HPDC is that the turbulent metal flow flows back into the tool. The turbulence results in porosities that deactivate components for multiple sub-processes or after treatments that limit their use. Examples of possibilities opened up without these are:
- Stronger and lighter by heat treatment
- The hybrids require strong welds
- The Use of Small Machines
Strengths above 400 MPa require T6/T7 heat treatment. Because these treatments are based on high temperatures, the effect of trapped air bubbles is a bubble surface that causes all kinds of problems, from cosmetic to crack initialization sites (see Figure 5 in the picture gallery). Since today there is the welding of cast components, but if the weight is to be reduced, the part must be thinner, and therefore the quality of the weld must be better in terms of gas bubbles and other defects. Figure 6 and 7 show welded structures and chassis, as well as samples of welds.
The use of hybrids, i.e. components made of sheet metal, cast parts and extruded profiles, requires a high-quality weld seam because the cast part must be highly porosity-free. However, since hybrid components offer a variety of advantages, this development is rapid and we will probably see many welded structures outside the chassis area.
New applications are big - battery boxes, large housing parts and advanced antenna products are examples. The industry's response was to invest in large machines with a capacity of well over 4000 tons. However, a large tool in a large machine causes new problems such as metal flow length, tool wear due to extreme amounts of metal and speed, not to mention practical problems. However, the procurement problem can be greater than the technical problems with a large machine. First, the installed base is limited and the market is growing rapidly, making it difficult to find capacity. Second, the procurement risk will be extremely high as a tool failure will lead to massive supply disruptions, not to mention the bargaining power undermined by greater demand than the installed capacity can meet. Rheocasting offers two advantages to solve this: a lower metal pressure (40 - 50 %), which allows the use of smaller machines, and the ability to weld parts from smaller machines to a large complex part. In terms of cost, the end part will then be more cost effective as small machines are effective and under competitive pressure.
Benefits of Rheocasting
The advantages of using rheocasting result from three essential advantages:
- The solidification time in the tool
- Laminar flow
- Alloy freedom
The solidification time in the tool, which gives time for filling complex geometries. Due to longer filling at slow speed and solidification time, the designed geometries can be thinner/thicker and allow prefabricated features as prefabricated holes (M2.5 with 100 depth), this shows Figure 8 and 9.
Laminar flow that reduces porosity to very low values. This means that the melt is not sprayed into the mould or is turbulent, and this in combination with the possibility of post-dosing means that gas bubbles and shrinkage porosity can be minimised and in many cases completely avoided. The low degree of porosity opens up possibilities in the areas of safety parts, heat treatment, welding and leak-free castings.
Alloy freedom, allowing the use of more effective alloys. All alloys can be used, with the exception of eutectic alloys, see Figure 10, and this gives designers the opportunity to extend the functionality:
- Heat transfer, with very low Si content up to +190 W/mK
- Stronger alloys according to T6, for example A319, which gives 379 MPa (average) yield strength
- Cheap alloys, approx. -0,1 EUR/kg compared with EN 46 000
- Hard alloys with Si contents, 14 %-25 %, to avoid surface erosion and cavitation damage
Market Status of Rheocasting Right Now
The switch to e-mobility exerts strong pressure on the development of new systems and components, increasing the number of parts with SOP 2019/2020. From smaller quantities in electric buses, the process will be transferred to passenger cars at the end of 2019 and the number of parts will then increase below 2020.
The number of development projects for parts and pre-development is also increasing sharply, as there is a block for the verification of concept parts and also material data such as safety parts. The range of targets between ongoing projects is enormous: from battery compartments to two-stroke cylinders and from automotive to telecommunications, which represents an interesting transition as the world moves towards e-mobility to save the climate.
Another proof of the commercial breakthrough is that the sale of rheocasting equipment has already begun, even though the number of offers and calls from a few people is growing rapidly. This could be one of the strongest points as the equipment is initialized by the component purchasing company to increase capacity and protect technical data and capabilities for further development. With the multiplication of rheocasting sites, the speed of market development is expected to increase in 2019.
The Treats: Engineering Capacity and Production Capacity
These problems have evolved from technology and quality problems to more structural and capacity problems. As many R&D efforts will be completed in 2019, the data and results will be used to develop parts. Why new problems have been identified and discussed and which are the most important:
Lack of suppliers who can support component development: Rheocasting or any high-quality semi-solid casting represents a new way of thinking in terms of simulations, tool designs and also process parameters such as speed, pressure and temperature. For this reason, some partners are now concerned that learning and duplicating foundry engineers could be a problem that could affect companies' ability to support product development.
Failures and long industrialization times: 10 years ago, many attempts were made, and they failed by 90 %. The main reasons for the failure were the understanding of process parameters, errors in gating and tool design due to lack of simulation and inferior melting. The implementation of a rheocasting process is simple, but the know-how and learning curve must be respected, otherwise we will all have the same failure as 10 years ago.
The rheocasting process has made its commercial breakthrough. Based on the evidence in the commissions for mass production and the number of RFQs and R&D projects, there are clear indications that mass production will be established in 2019. The intensive development pressure is still high both in R&D projects for material data as pre-design work and in the development of components and applications. It is predicted that this pressure will increase as new insights come onto the market, as each result opens up opportunities for further applications.
Probably the strongest capacity driver is yet to come and will most likely be welding, as this will allow lighter chassis and their parts and reduce dependency on large machines, thus facilitating the pending procurement problem.
The number of installations is forecast to reach double digits in 2019, and as this happens, it is assumed that the number of people machining, developing and selling parts produced with Rheocasting will reach critical mass for creating a mass market. Limiting factors are capacity and efficiency, a problem that is likely to occur if the switch to EV´s as a mass market becomes larger than the competition where a major UPS could be the battery range between charging.
For example, Sven Hedlund, Cost Engineering Expert at Volvo Car Corporation says: "Volvo Cars has a strong interest in the SSM casting technology that develops our next generation lightweight components. The technology is very promising and combines increased mechanical properties and functionality with reduced costs."
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