Ashley Stone, P. Eng.
CTO, Inventor, MAXImolding! Technology GmbH, Siegsdorf, Germany
Magnesium Semisolid Casting Part 1 - Status Quo of the Magnesium Die Casting Industry
Existing casting machines reach their limits in terms of quality. As a solution, a new vertical semi-solid machine will be presented, which enables a fully automatic digital foundry. The first part of the article series describes the development of the digital foundry and outlines the state of the magnesium die-casting industry.
Quality and Productivity
When output is low and input is high, productivity is very low. Improve quality and you reverse this situation. Output goes up, input goes down, and productivity shoots up. The idea to build quality by inspection is wrong and the results are poor quality and high costs. Dr. W. Edwards Deming is best known for his work in Japan, which commenced in 1950, and created a revolution in quality and economic production. In 1998 Maxicast, a division of Jacobsen Real-Time X-Ray Machinery Inc. adopted his approach - build quality by prevention of defects - by understanding and tuning up the manufacturing process, and focusing on the cause of the defects. In 2001 light alloy wheel manufacturing process was analyzed and fine-tuned at UBE factory in Sarnia, Ontario, Canada with specific focus on reducing the causes of the defects. To accomplish this, first manual x-ray picture feedback was used to set up the casting machines to produce fewer defects in the first place. While this approach is possible, it requires a very skilled level of people involved with setup of the machine and process. Firstly, the person interpreting images of wheel must be an expert in analyses of the x-ray imagery, and then the same person or different must be expert in process adjustments and interpretation of the inspection findings. The second person must take the quality inspected part and convert the knowledge about the actual part into a set of relevant parameters that can be applied to input of the machine to hopefully produce new part with better and more exact quality characteristics. This was a reason why this method was not adopted on a large scale. To mitigate these challenges, a further leap step improvement was necessary, but was not possible with existing casting machines. Therefore in summer of 2006 from scratch a totally new vertical semi-solid machine was developed. And finally in 2019 an operator on a casting machine was replaced and a fully automated digital casting factory was born.
State of Magnesium Die Casting Industry
Magnesium is the lightest of all the engineering metals, having density of 1.74 g/cm3. Magnesium is 35% lighter than aluminum and over four times lighter than steel. Magnesium alloys reduce vibration 25 times better than aluminium, giving this material a clear advantage by reducing road vibration. Most steering wheels in use today have a magnesium core, over-molded with vinyl, which improves driver comfort by preventing vibration from reaching the driver’s hands. Magnesium is the eighth most common element in nature, constitutes about 2 % of the Earth's crust by weight, and is the third most plentiful element dissolved in seawater (about 1.3 kg per cubic meter). It has good ductility, better noise and vibration dampening than aluminum, and excellent cast-ability. Alloying magnesium with aluminum, rare earths, zinc, or zirconium increases its strength to weight ratio, making the alloy very suitable wherever weight reduction is important, and where it is imperative to reduce inertial forces. The high specific strength and stiffness of magnesium are very desirable characteristics in applications wherever the weight of the product is an important feature. Besides having these innate characteristics, magnesium is also environmentally friendly. It is easily segregated from the recycling stream and is recoverable by re-melting. In addition, even if not recycled, it will naturally dissolve into magnesium oxide, which is harmless to people and to the environment.
Our research of the state of the art in the die-casting industry, as well as magnesium studies done by Dr. Frank Czerwinski show that the use of the magnesium alloys, where lightweight and material strength are required, will contribute to greater fuel economy and environmental conservation. Recent legislation limiting emissions (especially in Europe) has led to calls for mandating requirements to reduce the weight of car components, creating renewed and growing interest in magnesium. Leaders in this area are predominantly European and Japanese car manufacturers. In North America, magnesium accounts for less than 5 % of typical car production, but new studies show this trend will change under growing pressure from Millennials and Gen X, who demand higher quality products with minimal adverse effect on the environment.
North American manufacturers prefer aluminum to magnesium due to two factors: properties and price. The new semisolid casting technology will both improve the mechanical properties and reduce the cost of magnesium castings, making it a more attractive option. The parts supplier Magna International Inc. is advancing its research on low-cost magnesium parts for automotive markets. This trend continues but is progressing slowly for many reasons.
Lack of New Technologies and High Costs
One reason for the slow progress in the use of magnesium alloy in die casting is, in our opinion, general lack of new technologies and processes to make the wider use of magnesium easy. The cost of ownership and entry into the magnesium die casting field today is very expensive and the learning curve is long. This is mainly due to the lack of expertise in the trade and resistance to adopting new technologies. The prevailing opinion I have observed is “If it ain’t broke, don't fix it”. And, let’s not forget the old, backward looking and very inefficient saying in our industry, “This is how we’ve always done it”. This sentence contains the seven most expensive words in business. We know that past success is no guarantee for the future, especially when the only constant is change. Being stuck in a “culture of the past” can be very frustrating. Having inspected millions of die casting parts, we have observed universally adopted patterns of trying to improve quality by inspection, rather than by closing the loop by integrating inspection into process. Most often, the inspection process is separated from the die casting process, meaning that bad parts are just separated from good and re-melted. The energy cost of re-melting is added to part cost and passed on to the customer without longterm considerations of the effect of this practice on the longterm survival of the die casting industry. No wonder the plastics industry and not die casting, is the predominant employer these days and developing so rapidly. Our die casting industry is still stuck in fifties, while plastics has moved into the 21st century.
Magnesium alloys have some unique solidification characteristics, such as excellent fluidity, less susceptibility to hydrogen porosity, and thus, better castability over other well-known cast metals such as aluminum, zinc and copper. High-pressure die casting is the most common method of casting magnesium alloys. There are two types of high-pressure die casting processes: hot chamber die casting and cold chamber die casting.