Semi-Solid Metal Casting to Reduce Costs

Semi-solid metal casting (SSM) is a virtually net shape production process which gives manufacturers and users of copper alloy parts a substitute and, in several cases, an economical way to manufacture bulk quantities of parts with superior component quality compared to conventional pressure die casting technique.

The capability to apply SSM casting or die-casting with metal in a semi-solid state is an outcome of the successful evolution of a high temperature nickel-base alloy die system. This high temperature nickel based alloy die system prolongs the die life in die casting metals and alloys with high melting temperatures. As the process requires easily available cold chamber horizontal die casting machines as the casting unit; it has the potential for far-flung applications and uses by existing casting professionals equipped with such machines and technology.

Semi-solid metal casting is made with metal at a temperature between the temperatures of liquid and solid state, with the fraction solid being in the range of 30-65 percent approximately. The semi-solid billet holds its form and shape and is suitable for loading into the shot sleeve of a traditional die casting machine. For the semi-solid metal to have adequately low viscosity, the structure at the working temperature should comprise of a globular primary solid state surrounded by the liquid state. The technical and economical feasibility of the Semi-solid metal casting process are ascertained, to a great degree, by the approach used to develop the starting stock with the necessary precursor structure.

Advantages of Semi-Solid Metal Casting
As an advanced casting technique, SSM casting offers huge potential in saving costs, energy, and material, and in reducing the environmental impact of casting. The advantages are -

• Virtually net shape processing
• Reduced thermal fatigue heat, reduced mold or die wear, and reduced solidification shrinkage as a result of the reduced feedstock temperature.
• Lower shear strengths of semi-solid slurries that are related with lower forming forces than fitting operations for solid metal
• Potential for superior tolerance control because of the inherently tight process temperature control related with SSM casting and reduced thermal cycling of dies
• Finer, more consistent, micro structures resulting in higher mechanical performance
• Control of viscosity, which may result in less turbulent mold and die filling that, minimizes the gas entrainment, shrinkage, porosity, hot tearing, and other solidification shortcomings
• Superior material utilization in making small components because of the productivity and accurate introduction of metal into the forming dies
• As alternative for sand casting, SSM cast parts production gets rid of the environmental costs and troubles of reclaiming and disposing of lead-contaminated sands
• Increased casting speed compared to liquid processing due to lower thermal demands on the dies
• Allows for the lead content of red brasses to be highly reduced and, combined with a semi-solid charge, should enable alloys usually prone to hot tearing to be die cast

Best Foundry Practices

The metal casting industry has been, and continues to be, an integral component of the industrial backbone of several nations across the world. The industry provides employment to a large number of people and supplies huge amount of castings thereby impacting almost every other industrial and commercial sector. The metal casting industry is also an important recycler of metal scrap, a major energy user, and an important pollution prevention partner in many countries.

Good foundry practices address critical technology deployment requirements that will help foundries in saving costs and increasing profits. These practices help in reducing the energy consumption and environmental impact of the metal casting industry and improve its competitiveness.

Given below are some of the good practices that can be adopted for cost savings and increase in profits:

Energy Saving Practices
By adopting following measures and practices, foundries can make big cost savings –

Set variable frequency drives on motors
Getting rid of voltage imbalances helps in reducing losses from vibrations, mechanical stresses, torque pulsations and overheating.

Switch off equipment and lighting when not in use
Specifically place a start-up and shut-down procedures to control energy usage spikes.

Install high efficiency lighting
Using fluorescent lighting with magnetic ballasts instead of older ballasts is always cost-effective. Install targeted lighting at inspection points rather than less efficient ceiling lights.

Cut down the compressed air pressure set point by 10 percent
Maintaining excess air system pressure is very expensive. Set points can be cut down with proper maintenance of system and continual repair of air system leaks.

Upgrade motor drive belts
Using energy efficient cog belts instead of drive belts help in reducing energy requirements. Cogged belts can function on existing v-belt pulleys, but at lower temperatures.

Adopt superior melting practices
Specific recommendations may vary depending on the type of melting system used by the foundry. Furnace manufacturers can be of great help in helping to identify good melting practices for energy savings. The use of preheated air/oxygen and optimized burner designs has found to be good for gas-fired furnaces.

Improved compressed air practices 
Compressor should be properly sized, it should not smaller or over-sized. Use air storage systems to limit idling of compressors. Reduce leaks at valves, couplings and pipe joints.

Short cycle heat treatment
Majority of the heat treatment practices followed by foundries are overly conservative and waste energy. High efficiency furnaces and furnace linings are usually cost saving.

Low Cost Technology Practices
By adopting following measures and practices in various technical domains, foundries can save costs –

Melting
Melt cold, pour hot, pour fast – This will reduce the consumption of melting energy to minimum, while at the same time improves the quality of melt and reduces disfunctioning. It is usually much economical in the long run to completely preheat ladles than to melt hot and admit high temperature drops during the transfer of metal.

Molding
Upgrade the sand testing quality assurance – good control of sand systems ensure superior mold quality and reduction in scrap. Make consistent use of sand supplier testing capabilities. In several cases, improved molding practices can help reduce the biggest single contribution to casting scrap.

Scrap Reduction
Improved reporting and analysis of scrap – The single effective way to reduce scrap is to identify the scrap and its root causes in a proper manner. Lack of attention to detail in reporting of scrap may badly affect the bottom line; for instance, it is crucial to identify ‘sand’ or ‘slag’ as the causes of scrap instead of simply grouping them together as 'dirt'.

Lighting
Good housekeeping & lighting results in improved quality – A little extra KW of energy use, specifically in inspection areas, actually results in money savings. Good housekeeping leads to pride among workers and improved quality of products.

Data Collection
Decisions should be based on sound data collection - Make sure that the data, which you have collected has sufficient gage R&R; it is useless to collect data if you are not going to use it. Employees will be much more accurate and efficient collection of data if they know how that data is going to be used.

Training
Training helps in enhancing the skills and productivity of employees – Make use of all internal and external training resources to improve the work skills of your employees. If production employees are better trained they are found to be better problem solvers.

Profitability
Superior costing & pricing systems – This is essentially a challenge especially in the jobbing foundry, but is essential to ensure the long term success of an organization.

Novel Casting Process Could Change the Way Complex Metal Parts Are Made

A Georgia Tech research team has developed a novel technology which could transform how industry designs and casts complex, expensive metal parts. This new casting technique makes possible faster prototype development times, as well as more efficient and cost-effective manufacturing processes after a part moves to mass production.

Suman Das, a professor in the George W. Woodruff School of Mechanical Engineering, has developed an all-digital approach which allows a part to be created directly from its computer-aided design (CAD). The project, sponsored by the Defense Advanced Research Projects Agency (DARPA), has received $4.65 million in funding.

Suman Das displays a ceramic mold produced directly from digital designs using large area maskless photopolymerization (LAMP) technology. In his right hand, he holds a single-crystal superalloy turbine airfoil which was cast using a ceramic mold of the kind he holds in his left hand. 

"We have developed a proof-of-concept system that is already turning out complex metal parts, and which fundamentally transforms the way that very high-value castings are made," said Das, who directs the Direct Digital Manufacturing Laboratory in Georgia Tech's Manufacturing Research Center (MaRC). "We're confident that our approach can reduce costs by at least 25 % and reduce the number of unusable waste parts by more than 90 %, while eliminating 100 % of the tooling."

The approach being used by Das and his team focuses on a technique called investment casting, also known as lost-wax casting. In this method, which dates back thousands of years, molten metal is poured into an expendable ceramic mold to form a part.

The mold is made by creating a wax replica of the part to be cast, surrounding or "investing" the replica with a ceramic slurry, and then drying the slurry and hardening it to form the mold. The wax is then melted out – or lost – to produce a mold cavity into which metal can be poured and solidified to create the casting.

Investment casting method is used to design precision parts across a wide range of industries including aerospace, energy, biomedical and electronics. Das's current efforts are focused on parts used in aircraft engines. He is working with turbine-engine airfoils – complex parts used in jet engines – in collaboration with the University of Michigan and PCC Airfoils.

Today, Das explained, majority of the precision metal castings are designed on computers, using computer-aided design software. But the next step – creating the ceramic mold with which the part is cast – presently involves a sequence of 6 major operations requiring expensive precision-machined dies and hundreds of tooling pieces.

"The result is a costly process which normally produces many defective molds and waste parts before a useable prototype is achieved," Das said. "This trial-and-error development phase often requires severl months to cast a part that is accurate enough to enter the next stage, which involves testing and evaluation."

By contrast, Das's approach involves a device that builds ceramic molds directly from a CAD design, completing the task much faster and producing far fewer unusable parts. Called Large Area Maskless Photopolymerization (LAMP), this high-resolution digital process accretes the mold layer by layer by projecting bitmaps of ultraviolet light onto a mixture of photosensitive resin and ceramic particles, and then selectively curing the mixture to a solid.

The technique places one 100-micron layer on top of another until the structure is complete. After the mold is formed, the cured resin is removed through binder burnout and the remaining ceramic is sintered in a furnace. The result is a fully ceramic structure into which molten metal – such as nickel-based super alloys or titanium-based alloys – are poured, forming a highly accurate casting.

"The LAMP process reduces the time required to turn a CAD design into a test-worthy part from a year to about a week," Das said. "We eliminate the scrap and the tooling, and each digitally manufactured mold is identical to the others."

A prototype LAMP alpha machine is at present building six typical turbine-engine airfoil molds in six hours. Das predicts that a larger beta machine – currently in construction at Georgia Tech and scheduled for installation at a PCC Airfoils facility in Ohio in 2012 – will produce 100 molds at a time in about 24 hours.

Though the present work focuses on turbine-engine airfoils, Das believes the LAMP technique will be effective in the production of several types of intricate metal parts. He envisions a scenario in which companies could send out part designs to digital foundries and receive test castings within a smaller period, much as integrated-circuit designers send CAD plans to chip foundries today.

Moreover, he said, direct digital manufacturing enabled by LAMP should allow designers to  produce increasingly sophisticated pieces capable of achieving greater efficiency in jet engines and other systems.

"This process can produce parts of a complexity which designers could only dream of before," he said. "The digital technique takes advantage of high-resolution optics and precision motion systems to achieve extremely sharp, small features – on the order of 100 microns."

Das also noted that the new process not only creates testable prototypes but could also be used in the actual manufacturing procedures. That would allow more rapid production of complex metal parts, in both low and high volumes, at lower costs in a wide range of industries.

"When you can produce desired volumes in a short period without tooling," he said, "you have gone beyond rapid prototyping to true rapid manufacturing."

Important Rules to Make Good Castings

In the recent few years, we have witnessed a considerable improvement in our knowledge of metal casting methods and techniques. With this enhanced understanding, the list of requirements has been consistently corrected as they have become known. Industry experts have identified several rules and practices, which incorporate the latest technology to manufacture quality castings.

The ten rules that have been identified are proposed as essential, but not, of course, adequate. Manufacturers should follow these rules along with the conventional technical specifications, including type of alloys, strength and traceability via quality standards, and other traditional foundry control measures, such as casting temperature, etc.

Though these rules have not been yet tested on all types of cast materials, there are reasons, which suggest that these rules have general validity and are suitable for all types of metals and alloys. The metals and alloys include those based on zinc, aluminum, cast irons, magnesium, steel, air- and vacuum-cast nickel, titanium and cobalt. In spite of all this, experts believe that these rules will probably benefit all metal casting applications, while some applications will be benefited more, others will be less affected.

Formulated to assist the casting manufacturers and designers, these rules are expected to speed up the process of producing the quality castings right the first time and in the significant reduction of scrap when the casting goes into production. Thus, these rules are expected to help the metal casting industry to raise its quality standards without any considerable increase in costs.

While on the other hand, these rules constitute a draft process, which the metal casting buyers may demand if they wish to make sure that they are buying the best possible casting quality. If the buyers ask their casting sources to follow these casting rules, the reliability and quality of castings would be better than what could be achieved by any quality control casting practice.

Rules -

• Provide a Good Quality Melt

• Avoid Liquid Front Damage

• Avoid Arrest of the Liquid Front

• Avoid Bubble Damage

• Avoid Core Blows

• Avoid Shrinkage Damage

• Avoid Convection Damage

• Plan Segregation Distribution

• Control Residual Stress

• Provide Location Points

Improve Induction Melting With Gas Diffusers

Also known as porous plugs, gas diffusers have been used for many years in ladles to enable gas purging of steel melts before streaming the metal into molds. The purging process degasses the melt and makes cleaner steel that results in a repressed scrap rate in castings.

According to experts, the proper use of gas diffusers provides a variety of benefits in induction melting. These include reduced scrap rates, more efficient melt, and a considerable improvement in furnace operational life.

For safe and effective functioning of a gas diffuser, several factors have to be taken into account. The chosen refractory material should be able to withstand penetration when in touch with liquid metals. The gas diffuser should be designed in a way so that it supplies little quantities of inert gas to the induction furnace melt in a controlled manner. The diffuser must also be compatible with the lining materials of the induction furnace. The diffuser must also be designed in a way so as to ensure easy installation, operation and long service life.

If possible, the diffuser must be fitted in the center of furnace base, or as close as possible to the center.

The furnace should have a gas supply to connect to the gas diffuser, and a there should be a proper gas-flow control arrangement. Such a system can be as simple as a pressure regulator on an argon-gas bottle with an inlet needle valve and flow meter; or as complex as a PLC package linked to a computer-controlled process control arrangement.

The induction furnace lining should be sintered before using the gas diffuser; this helps the gas to pass through the lining without disturbing it. In order to ensure proper sintering, gas diffusers should not be used early in the melting process. According to experts, best results can be obtained by bringing in the gas to the diffuser during the 3rd melt and onward.

Rapid Tooling - A Key to Production Improvement & Cost Savings

Rapid prototyping is fast emerging as a preferred manufacturing method among global manufacturers to bring products faster to the market. As a part of this process, manufacturers are using varied tooling methods to design pre-production models, limited parts of production runs, and even production quality tools of the rapid prototyping process. Die casters are using a variety of rapid tooling methods to produce pre-production models, limited runs of production parts and even production quality tools subject of running up to 100,000 parts in certain cases.

It was during the year 1980, when developments in rapid prototyping started with the help of computer aided design or animation modeling software. These tools were used to convert the virtual designs into cross-sections constituting the physical dimensions of the model. The procedure can be compared to that of a topographical model where the layers represent to the elevations in the model. The virtual model is then transformed into a real design through either additive or subtractive prototyping.

Additive Prototyping

In this method, the model is created by producing crosssections using sequential micrometer or millimeter-thick layers of liquid plastic, powdered plastic or any other engineering material. The normal interface between the CAD software and rapid prototyping machines is the STL (stereolithography) format that is similar to printing. The layers posited by the prototyping machine that fit to the virtual cross section from the CAD model, are fixed together or fused (often using a laser) to produce the final shape.

The main benefit of additive prototyping is its capability to create almost any shape, except trapped negative spaces.

Substractive Prototyping
Substractive prototyping is a conventional method of rapid tooling that is used create a die tool. In place of machning the steel in the tempered condition and afterwards heat treating it, pre-hardened steel used. The process helps in shortening lead time. Sophisticated cutting tools provide for machining of steels up to 40-42HRC. Die life can increase from a few thousand shots to tens of thousands, which depends on configuration of the part.

The primary advantage of subtractive process is its ability to create very accurate tools with superior surface finish.

Advantages of Rapid Tooling

Some of the important advantages of rapid tooling are -

• Variety of tooling methods for unique and specific requirements
• Designs can be quickly transformed from prototype to production
• Rapid tooling cuts production time
• Possible to create complex designs
• Economical

Tool to Determine Casting Alloys & Processes

Casting buyers generally seek professional advice and help to make a decision between different types of casting alloys or processes. Experts cannot make choices for them but can only provide information, which will help the buyers to make an informed decision. However, the information required to make an informed decision is huge and with the variety of ways available to design a casting, the decision process can be very difficult.

In collaboration with American Foundry Society, Product Development & Analysis LLC has developed a tool to help casting buyers and designers determine which particular alloy and casting process suits best to their specific metal casting designs. This tool is known as Casting Alloy & Process Selector (CAPS) Tool . The casting buyer is just required to enter the basic data about the metal (if available - weight, alloy, wall thickness, surface finish, production volume), and the tool will present the different casting process.

The tool gives the results on the basis of data, facts and information stored in it and provides industry averages that cannot be considered absolute. Though the tool helps the casting buyers and designers to narrow down their choices, the user should contact the manufacturer for specific as different manufacturers have different capacities

New Manufacturing Methods : Expected to Cut Down the Cost of Titanium Alloys

Ever since the introduction of titanium and its alloys in the mid of 20th century, these metals have become critical materials for uses in aerospace, energy and chemical industries. Titanium and its alloys combine properties, such as high strength-to-weight ratio, superior mechanical properties and corrosion resistance that makes them an ideal choice for a multitude of critical applications.

Today, the titanium alloys are used in highly demanding applications, including the static and rotating gas turbine engine parts. Titanium alloys are also used to design some of the most critical and highly stressed civilian and military airframe components. Though highly preferred metal choice for such critical applications titanium alloys tend to be too expensive and this limits their widespread use in industrial applications.

However, improved manufacturing techniques and processes are in development stage that are expected to reduce the prohibitively high cost of titanium alloys. Oak Ridge National Laboratory and industry partners are working progressively to formulate a new and innovative non-melt consolidation process. This new technique is expected to cut down the amount of energy and cost required to produce titanium alloy parts and components by upto 50%. The process will make it viable to use titanium alloys for a multitude of engineering applications, including armor for military vehicles.

Recently, the Oak Ridge National Laboratory used new low cost titanium alloys to design a door for the Joint Light Tactical Vehicle, which is a next-generation combat vehicle. While the main purpose of the use of titanium alloy door was to reduce the weight of the vehicle, it also provided the benefit of decreasing the threat of armor – piercing rounds. Light in weight, the titanium alloy door also enhanced the functioning of door and cut down the overall weight and the fuel efficiency of vehicle, thereby making it even more beneficial for the military and industrial applications.

According to experts, the non-melt approach that comprises roll compaction for directly fabricating sheets from powder, press and sinter techniques to create exact near net shape components and extrusion, provides several benefits over conventional melt processing.

While titanium powder is used to design products in the traditional melt processing method, the powder remains in its solid form during the entire process. This saves a vast amount of energy required for processing, cuts down the amount of scrap by a great amount and allows for new alloys and engineered composites.

Though powder metallurgy has been used to design parts and components for several years, the process is generally not used to fabricate titanium products because of the high cost of traditional titanium powders. New low cost titanium powders are now allowing the researchers to formulate these technologies for titanium.

In coming years, researchers expect the introduction of lightweight corrosion-resistant titanium alloys into several other products, including automobiles that will benefit from the reduced weight and will be able to provide better fuel efficiency.

Reducing Zinc Contaminants is Critical to the Success of Die Castings

Manufacturers prefer zinc alloys for a variety of applications that range from small electrical connectors to automotive parts and decorative components. According to a research study, the global consumption of zinc casting alloys is more than one million metric tons.

Zinc alloys are used in casting applications because of their mechanical characteristics, electrical conductivity, dimensional accuracy and machinability. Functional components are manufactured keeping in mind the stable mechanical properties over the design life of part, along with good dimensional precision. Metals for decorative components are chosen depending on their ability to maintain good corrosion resistance over a variety of exposure conditions. Many castings are designed with both functional as well as decorative attributes, however these characteristics may be vitiated if the zinc alloy is contaminated. For instance, high level of lead may cause intergrannular corrosion during plating works.

The presence of tin, lead, cadmium and antimony at a level higher than the permissible limits can adversely affect the mechanical properties and corrosion resistance of zinc castings.

Both die casting manufacturers and alloy producers are agreeing on the need to improve purity levels by adopting advance processes and sophisticated technology. In recent years, however, several instances of out-of-specification imported castings have been reported. According to experts, these instances are related to the hand sorting of scrap used to produce secondary alloy. Presence of little tin and lead can have significant impact on the purity of zinc alloys.

Presence of tin and lead in zinc alloys can be attributed to several factors, including - solder leftovers and bronze bushings, which are not separated from the secondary feed. Latest technology separation, alloying and alloy analysis methods can be used to rule out the risk of contamination of ingots, however, these must be carefully followed to ensure the supply of in-specification product.

Affect of Impurities
Though tin is considered as the most harmful impurity for zinc alloys, the presence of lead, cadmium and antimony at levels higher than permissible limits can also badly affect the mechanical properties and corrosion resistance of zinc alloys. In addition, when these impurities coexist in the contaminated alloy, their effects can get compounded. These impurities act as microscopic-size cathodes, finely diffused throughout the alloy along its grain boundaries and reduce the properties of zinc alloys. The zinc alloys generally corrodes at these areas.

Quality Control Procedures
To avoid zinc contaminants and to ensure the production of successful die castings, the zinc alloy producers needs to take several steps, including -

• Use only certified special high grade zinc in their manufacturing. This eliminates the risk of lead, cadmium and tin contamination.
• Each lot of zinc and die casting alloy must be produced as per the ISO 9001:2002 documented norms and procedures, which include different assays of the molten material during pouring off of the heats.
• Track and identify the likely harmful sources of contaminant, including customer returned scrap for reclaim.

Die casters can also ensure the quality of alloys that they use by procuring their raw materials from reliable suppliers and receiving certification for the purity of alloy purchased. It is also important for the buyers, designers and specifiers of zinc die castings to look up to a recognized standard, such as ASTM B86 or EN 12844, on all drawings and purchase orders.

5 Ways to Improve Die Performance

While selecting a die casting, designers generally think that tooling is not reliable or that short die life will affect production. Research & developments over the last decade, however, have proved that a variety of methods are available that can be used to improve die life, enhance the performance of die and reduce the overall cost-per-part.

The 5 key factors, which can extend die life are -

• Die Design
• Die Material
• Heat Treatment
• Coatings
• Operation / Maintenance

These factors can be adjusted and modified as per the requirement to improve the die performance anywhere from 50 % to nearly 100 %. This is a significant improvement in performance that typically outsets any further increase in cost for producing superior-quality, long-lasting tooling, making it a better choice than cheaper low quality tooling.

Judging Die Performance
In general, improved die performance results in more parts at a lesser cost. The 3 factors, which affect the tooling and limit the die performance are -

• Soldering and chemical attack between the die material and liquid metal.
• Wear & erosion.
• Heat checking or thermal fatigue occurring as a result of the thermal cycling, which takes place with each shot or casting cycle.

These factors can be eliminated or reduced to improve the die performance and reduce the overall costs.

Performance Improvement Ways -
The 5 key factors given here for the improvement of die performance are a beginning point for determining what combination of elements will have the highest impact on performance in a specific situation. As like there are several nuances within each performance improvement element, the particular steps, which will produce the best results will depend upon various factors, such as the die casting type (hot or cold chamber), the cast alloy, size of shot, or the wall of casting (thin wall or thick wall).

Die Design
For the improvement of tool life, the parts should always be designed with round edges or generous radii. Gate location is also a critical factor, as the control of metal flow in die cavity is a major factor in producing sound die castings. The metal must necessarily flow quickly and uniformly into the die, as it minimizes the sharp direction changes, which can result in premature tool wear.

Die Material
For majority of tool construction, casters generally prefer superior grade H13 tool steel as the starting point. However, many die casters nowadays, are realizing that the die life can be significantly improved by using modified versions of H11 steel, heat treated to a hardness level of 48 HRC. H11 has greater molybdenum content, with lower vanadium and silicon, facilitating greater toughness than H13, superior temper resistance, which improves the die life in demanding applications, such as - thick-walled castings.

Heat Treatment
Research have proved that heat treatment process is as critical in impacting the die life as the selection of the proper die steel. The thermal fatigue cracking can be greatly reduced by using an austenitizing temperature, which will place the optimum amount of carbide forming elements in solid solution in the austenite. When combined with a speedy quench rate, the toughness remains unaffected.

Coatings
Coating and/or surface treatment helps in improving the tool life by preventing the soldering and erosion between the tool steel and the liquid alloy being cast. Though surface modification techniques like ferritic nitrocarburizing and plasma ion nitriding also improve the die life, coatings generally provide better improvement.

Operations/Maintenance
As good design is the starting point in ensuring the long die life, proper operation and maintenance are critical to the overall success in improving the die performance. These include the die set up procedure, pre-heating tools and verifying clamping pressures. In case, a hard coating has been applied, the cleaning pressure needs to be changed to avoid damaging the coating.