FE Update: Simulation software helps boost foundries’ bottom line

In general, foundries effectively recycle their materials. More than 90% of all cast parts in Germany are made from remelted scrap metal. And the reuse of molding materials such as sand and water means almost no waste.

However, foundries spend a lot on energy and materials — on average 40% of all costs. Physical laws dictate that an average energy input of 2,000 kW-hr per metric ton of final casting product is needed. This adds up to a total energy consumption of 11 billion kW-hr in the German foundry industry per year. Over 50% of this energy is used just to fill gates and risers.

Fortunately, casting-simulation software helps foundry engineers optimize casting parameters, often before the first part is poured. Simulation helps to minimize the amount of material cast and, thereby, the amount of energy needed for the melting process.

Simulation also plays a role in cutting CO2 emissions by helping users slash process and cycle times for high-production castings. Engineers can use simulation to optimize heat-up and temperature distribution in permanent molds, plan layouts that maximize the number of parts molded at one time, and reduce or eliminate preproduction trial-and-error runs.

Here are a few example of how simulation software such as Magmasoft can help optimize casting operations.

Outside-of-the-box casting techniques
New ways of doing things usually pose potential risks and rewards. Simulation lets engineers take more risks because they can predict the results of changes they make. Engineers are free to make unusual design changes and see what will happen virtually without waiting until multiple different castings are poured.

For example, one company detected a shrinkage defect in its complex ductile-iron carriers late in the machining process. Simulation showed the root cause: The pass feeding molten metal to the critical area was getting cut off prematurely. Engineers changed the riser layout to eliminate the defect.

They also took design chances by making unusual changes to the gating system. This slashed the pouring weight by 13 kg, a savings of 13 metric tons of melt and 12,272 kW-hr energy used to melt the raw material per year. The redesign also reduced the riser neck cross section by 25%, resulting in lower riser-removal costs. The modified layout shortened pouring time by 2.5 sec and slashed solidification time by 11 min, increasing productivity by 15%. The original job was to eliminate the defect. The final design, based on simulation, resulted in significantly lower production costs.

In another example, simulation results encouraged pump manufacturer Otto Junker in Germany to cast a steel pump housing that had direct-pour top risers instead of the typical side risers. This lowered the amount of liquid metal needed by 81%, reduced molding time by 79%, and minimized the time needed to burn-off the risers by 87%. The company reduced its total production costs for the part by 12%.

Additionally, a South American iron foundry increased the casting yield for a ductile-iron differential-case housing from 62 to 67% by using simulation to develop a nontraditional gating system. The design lowered the overall scrap rate from 17 to 7%, saved 700,000 kW-hr/yr to produce 24,000 parts and slashed total costs by $500,000.

Simulation boosts quality
Equipment manufacturer John Deere, Moline, Ill., cut the scrap rate of a gray-iron part from 10.3 to 1.4% and saved $66,936/yr by modifying the part and gating system. The company also boosted its casting yield from 58 to 64% for an additional savings of $66,600/yr. The foundry claimed that if it had used simulation at an earlier stage, it could have potentially saved $140,000 more in the first year of production and would have avoided casting design and pattern changes that cost $120,000.

In another case, mechanical-engineering company Heidelberger Druck AG in Germany relocated a mold gate based on simulation results and thereby significantly reduced the amount of repair welding it had to perform on a cover. Temperature losses in the original part had led to incomplete filling of a rib. Simulation let engineers see how material flow was affected by moving the gate to different locations.

Energy savings in heat treatment
Many castings obtain their final mechanical properties after the casting process during heat treatment. The optimal layout and energy input during heat treatment strongly relates to when a necessary microstructure develops. Magmasoft lets users model the entire heat-treatment process and the resulting microstructures.

The software also lets users simulate residual stresses. Designers previously added large safety margins to each heat-treatment step because the way heat-treatment furnaces transmit energy to parts was not well understood. Simulation does away with these safety margins.

New models even let users predict the amount of local carbon saturation in cast iron and steel. Say the total austenitization time for a wind-energy part was 6 hr. Reducing this time by 1.5 hr saves 128 kW-hr/metric ton of product without sacrificing final properties or microstructure. For 500 heat-treated parts, savings add up to 100,000 KW-hr/yr.

Aluminum molds
Energy savings in mass-produced castings that use metal molds rather than sand molds are comparatively high because metal molds can be used for more parts. The number of degrees of freedom in permanent molds is much lower than in sand casting, but it is still possible to cut costs using simulation.

In one case, the original gating system for a motorcycle fork produced using the tilt-pour casting process resulted in several quality issues. Worse yet, casting yield was only 49%. Simulation helped engineers eliminate filling turbulence, and a hotspot and its related defect. They used smaller gates, which boosted the casting yield by 18.5%. In addition, the faster filling of thin walls shortened solidification time to cut cycle times by 10%.

Savings in high-pressure die casting
In high-pressure die casting, 40 to 60% of process energy goes to melting metal. The remainder is used for the actual casting process. The energy input needed for melting depends on the amount of scrap (typically 5 to 7%), melting losses (2 to 5%), and casting yield, the ratio between casting weight and total pouring weight (30 to 70%).

Raw metal is usually melted with natural gas, but the amount needed can vary by a factor of seven, depending on the equipment and environmental policies of different foundries. And the amount of electricity used can vary by a factor of two, for an average value of 5,603 kW-hr per metric ton of final castings. With these uncertainties, simulation can help designers better design and place gating systems, which can significantly reduce the amount of energy needed.

Optimizing gating systems and remelt
Using a gearbox housing as an example, a research project evaluated the energy savings possible by switching from an oil-based die-cooling technique to a water-based technology without affecting casting quality. A comprehensive design of experiments study (DOE) was conducted using casting simulation to evaluate the effect of several process parameters and gating designs. The software compared all of the calculated trial runs and showed the best solutions.

Here, simulation netted a 25% reduction in runner volume, which meant that 12% less material was needed per shot. The better design, in combination with the lower pouring weight, slashed cycle time by 8%.

Buckling Analysis with FEA

Linear finite-element analysis does not provide enough information about buckling to make correct design decisions, especially when designing lightweight components.

In many design projects, engineers must calculate the factor of safety (FOS) to ensure the design will withstand the expected loadings. Calculations require correctly recognizing the mechanisms of failure, and this is a difficult task. All too often we associate structural failure only with yielding and are satisfied when design analysis shows a sufficient FOS related to yield.

 

However, yielding is not the only mode of failure. For example, it is necessary to consider displacements to ensure the part or assembly does not deform too much. Also important is buckling, which is all-too-often forgotten and yet poses a dangerous mode of design failure. Buckling happens suddenly, without little if any prior warning, so there is almost no chance for corrective action.

Certain problems tend to arise in buckling analysis performed with finite-element analysis (FEA). These problems are best presented in the context of two other failure modes: excessive displacements and yielding, as summarized in the Failure modes table.

Linear-buckling analysis
First, consider a linear-buckling analysis (also called eigenvalue-based buckling analysis), which is in many ways similar to modal analysis. Linear buckling is the most common type of analysis and is easy to execute, but it is limited in the results it can provide.

Linear-buckling analysis calculates buckling load magnitudes that cause buckling and associated buckling modes. FEA programs provide calculations of a large number of buckling modes and the associated buckling-load factors (BLF). The BLF is expressed by a number which the applied load must be multiplied by (or divided — depending on the particular FEA package) to obtain the buckling-load magnitude.

The buckling mode presents the shape the structure assumes when it buckles in a particular mode, but says nothing about the numerical values of the displacements or stresses. The numerical values can be displayed, but are merely relative. This is in close analogy to modal analysis, which calculates the natural frequency and provides qualitative information on the modes of vibration (modal shapes), but not on the actual magnitude of displacements.

 

Theoretically, it is possible to calculate as many buckling modes as the number of degrees of freedom in the FEA model. Most often, though, only the first positive buckling mode and its associated BLF need be found. This is because higher buckling modes have no chance of taking place — buckling most often causes catastrophic failure or renders the structure unusable.

The nomenclature is “the first positive buckling mode” because buckling modes are reported in the ascending order according to their numerical values. A buckling mode with a negative BLF means the load direction must be reversed (in addition to multiplying by the BLF magnitude) for buckling to happen.

As a consequence of discretization error, linear buckling analysis overestimates the buckling load and provides nonconservative results. However, BLFs are also overestimated because of modeling errors. FE models most often represent geometry with no imperfections and loads and supports are applied with perfect accuracy with no offsets. In reality though, loads are always applied with offsets, faces are never perfectly flat, and supports are never perfectly rigid. Even if supports are modeled as flexible, their stiffness is never evenly distributed. Imperfections are always present in the real world. Considering the combined effect of discretization error (a minor effect) and modeling error (a major effect), designers should interpret the results of linear buckling analysis with caution.

Wind turbine blades that change pitch boost wind power efficiency

Blade-pitch systems avoid wind-turbine catastrophes in high winds.

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Everyone’s owned a pinwheel at one time or another. I remember riding in the back of my parents’ car, sticking my pinwheel out the window as we traveled down the street. The speed at which the pinwheel turned matched the velocity of the car. On backstreets with low-speed limits the pinwheel did great. But high-speed interstates were another matter: My pinwheel soon flew apart as the car accelerated to cruise speed.

Wind turbines can suffer a similar fate. Not by sticking them out the window of a car, of course, but by subjecting them to wind speeds found in thunderstorms and other weather events that exceed turbine design limits. Those conditions force turbine designers to somehow prevent the rotor speed of the turbine from exceeding its design range. Ignoring such precautions may produce a scene similar to the wind-turbine explosion videos that now populate YouTube. (One example: http://bit.ly/4FEHbZ.)

Brakes could be used to keep the rotor from turning, but they’re subject to failure or overload by the wind force on the turbine. The turbine in the example video went into a runaway condition when its brakes failed, letting the rotor freewheel.

A better technique used to combat runaway conditions like this is blade pitch control. The pitch of a wind-turbine blade describes the angle of the blade chord to the plane of rotation. It is analogous to the way the pitch of a leadscrew determines how far its load moves with each rotation of the screw. Similarly, the pitch of a turbine blade is the distance the blade would travel through the air in one rotation if it were 100% efficient. Obviously turbine blades don’t move forward. Rather, the wind pushes air past the blades as though the blades had moved forward.

Wind speed versus the desired turbine rpm determines blade pitch. There is a specific pitch angle for any given wind speed to optimize output power. Pitch angles greater or less than this value reduce power output, even to the point of zero rotation with high winds.

The hub of a wind-turbine propeller houses the pitch-control system which, according to the European Wind Energy Association, accounts for about 3% of a wind turbine’s total price. That small investment makes a difference when conditions deteriorate. In fact, many turbine makers now consider pitch control a good “insurance” policy.

When wind speed reaches 25 m/sec (50 mph) or higher, the pitch-control system fail-safes the blades in a manner that reduces wind loading and stops the turbine rotor from turning. These systems also monitor wind speed and load to set the turbine blades at the best angle needed for power output. Changes in blade pitch typically start when wind speed reaches 12 to 13 m/sec (27 to 29 mph), the point where the turbine reaches peak performance. If wind starts to exceed that level, the pitch-control systems kick in to reduce the blade angle of attack, taking a lower percentage of energy from the wind to keep the generator near 100% output without overspeeding.

Pitch-changing systems generally come in two forms: either electric or hydraulic. Rarely do makers of wind turbines use both types. According to research from Intercedent Asia, the choice of pitch-control system depends on the turbine manufacturer. In other words, the type of pitch-control system never becomes a major issue should you find yourself buying a wind turbine.

In a hydraulic system, hydraulic actuators control the pitch of all blades simultaneously. The actuator typically works against a spring that functions as a stop fail-safe upon loss of hydraulic pressure. Hydraulic systems also seem to have a longer life, more driving power for a higher speed response, and a low-maintenance backup system (the spring) in case of failure. However a major drawback is the hydraulic fluid itself. Should a seal leak, it’s possible for the blade to sling hydraulic fluid over a wide geographic area, contaminating the surrounding countryside. Additionally, hydraulic systems tend to use more energy as the hydraulic pump must run continuously to keep pressure high.

Obviously, electric pitch-control systems have no risk of leaking hydraulic fluid. They also consume less power and waste less energy. Once the actuator reaches its desired position, the actuator motor can turn off while still holding the blade at the proper pitch angle. However, electric pitch-control systems need fail-safe batteries or supercapacitors to allow for loss of primary power or control. Fail-safe batteries typically last only two to three years, and then must be replaced — not a simple task as the fail-safe batteries sit in the hub of the rotor, not in the nacelle. (The hub location assures power remains available for the pitch-control system during an emergency such as a grid loss or failure of the slip rings.) Electric systems also work better in colder climes where the oil in hydraulic systems loses viscosity as the temperature drops.

Future developments may bring a third option: a hybrid electrohydraulic system. Hybrid technology uses electricity to control blade pitch for daily operation, but uses hydraulic power to operate the fail-safe that prevents damage to the blades.

Proponents of hybrid solutions say that because pitch control relies mostly on electrical power, it mitigates the risk of leaking oil. They also contend this would lower energy costs. As hybrid pitch-control systems rely on hydraulics for fail-safe power, advocates point out the lack of need for fail-safe batteries and their corresponding maintenance.

Solar-Powered Plane Will Fly Like a Bird

A team led by aerospace researcher Anthony Colozza, with Analex Corp., in Fairfax Va., and NASA Glenn Research Center in Cleveland, Oh., has been working on designs for a plane that will fly like a bird. This aircraft will literally flap its wings in a continuous fluid motion to keep altitude and move. It will carry no jet engines or propellers. The team has completed a feasibility study and worked out the initial design.

"The vehicle would be unmanned, solar-powered, and made of strong, light-weight materials," says Colozza. The plane's body would be made of plastic-like material called ionic polymer-metal composite that deforms when exposed to electric fields. The plane would also use thin sheets of photovoltaic material and a lithium battery.

The plane would fly like an albatross, gliding great distances and circling over the same area for long periods of time, flapping only to regain altitude. To maneuver, the aircraft will adjust its wings into complex shapes, rather than using ailerons or a rudder.

Colozza believes this plane could be flying in just a decade or two, with countless uses such as gathering scientific data, relaying communications, and surveying terrain. He also believes the plane could be used on other planets with atmospheres that would ground air-breathing planes.

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               Test flights will take place this year in Spain.

           The demonstrator uses a Proton Exchange Membrane (PEM) fuel cell/lithium-ion battery hybrid system to power an electric motor, which is coupled to a conventional propeller. During takeoff and climb, when the most power is needed, the system draws on its lightweight batteries. Successful flight tests will demonstrate for the first time that a manned airplane can fly with fuel cells as the only power source.

           The demonstrator aircraft is a Dimona motor glider built by Diamond Aircraft Industries of Austria. With a wingspan of 16.3 m (53.5 ft), the plane should cruise at about 100 km/hr (62 mph) using fuel-cell power.

            "While Boeing does not envision that fuel cells will provide primary power for future commercial airplanes, demonstrations like this could pave the way for using this technology in small manned and unmanned aircraft," says Francisco Escarti, managing director Boeing Research and Technology (Europe).