Improved Aluminum Engine Block and Cylinder Head Heat Treatment
New technology development does not come easily and has forced engineers to redesign engines that introduce enhanced computerization, sensor monitoring and alternate material selections. With vehicle weight being an obvious focus for engineers, aluminum, magnesium, plastics and composites are commonplace in new engine designs. Today, aluminum is considered a material of choice of automobile engineers due to its unique combination of strength, reduced weight, crash-energy absorption, corrosion resistance and thermal- and electrical-conductivity properties. In most cases, to achieve these desirable properties requires the secondary operations following casting or forming of aluminum-intensive automobile components. Traditionally, castings are thermally treated achieving a desired set of mechanical properties, metallurgical structure or residual stress levels. For the most part, light vehicle engines of today almost exclusively integrate cast-aluminum cylinder heads. On the other hand, engine blocks have some ground to cover from a weight-reduction perspective and are generally cast from either aluminum or iron.
Engine development has undergone a significant transformation. As engine designs have evolved, so have their weights, overall size and mechanical performance. Engine blocks and cylinder heads have seen the introduction of several new casting technologies and modified alloy compositions to handle the higher compression levels and elevated operating temperatures. With the introduction of these new technologies, greater demands have been put upon the thermal-treatment methods used to optimize mechanical properties while also minimizing the residual-stress levels. As a result of these demands, auto manufacturers have worked closely with designers of customized thermal-processing systems in the development of a new cylinder head and engine block heat-treatment system. The following will describe how Can-Eng Furnaces International (manufacturer) has provided flexibility in how it responds to the special needs of customers in the development of new, innovative thermal-processing systems.
Current Heat-Treatment TechnologyDue to the long heat-treat-cycle requirements for cast-aluminum engine blocks and cylinder heads, furnace engineers were left with limited design options. These limitations resulted in the design and manufacture of continuously fed, linear-conveyorized heat-treatment systems, which involved the roller-hearth conveying of densely loaded baskets containing aluminum cylinder heads or engine blocks. Figure 1 illustrates an elevation layout of a conventional cylinder head heat-treatment system.
Presently, it is not uncommon for linear roller-hearth heat-treatment systems to span over 200 feet (61 meters) in length. As a result, these systems consume significant amounts of floor space and require additional space for the staging and buffering of baskets. By nature of the repetitive heating and cooling cycles, conveying baskets deteriorate over time, requiring replacement and maintenance. Basket deterioration presents challenges for high-volume heat-treatment systems. These challenges include:
- Less than optimal heating and quenching profiles due to inconsistent product spacing, resulting in inconsistent metallurgical properties.
- Roller-hearth conveyor basket-tracking inconsistencies, leading to basket misalignment, excessive quench delays, lost production time and equipment damage.
- Inability to effectively load baskets, which reduces the optimal output of the heat-treatment system.
Uniform heating and quenching of products is paramount in achieving uniform mechanical properties and minimization of internal stresses. With increased design conditions put upon these engine components and increased fatigue-strength requirements, the acceptable uniformity limits have tightened, forcing a new design criterion for furnace engineers.
New Technology for Heat Treatment and Precision Air QuenchingThrough collaboration with automotive design teams and development testing, the manufacturer has designed and commissioned a new heat-treatment technology used by mass-production manufacturers of cast-aluminum engine blocks and cylinder heads. These systems combine two significant improvements over conventional designs. First, the system eliminates the need to load and unload engine blocks or cylinder heads into densely configured baskets, and secondly, the systems integrate a piece-by-piece handling strategy. Through the combination of these design improvements, the manufacturer has been successful in commissioning a flexible heat-treatment system that provides significant benefits over conventional technology.
This technology, first referred to as Can-Eng’s Basketless Heat-Treatment System (BHTS), integrates a multiple-level, modernized rotary-hearth configuration. Rotary-hearth furnaces have been in use for over 50 years, starting mainly with forging applications where manufacturers took advantage of the system’s common loading and unloading positions. Figure 3 shows a rotary-hearth furnace system.
The basic system design is cylindrical, and product is loaded and unloaded through common openings. The product is positioned on an internal carousel that rotates through a heating and soaking cycle. The carousel can be arranged to handle a wide family of products, including engine blocks and cylinder heads. The carousel is driven from below via a single, high-accuracy drive unit. The modernized rotary-hearth furnace is based on similar design characteristics as its predecessor and integrates multiple carousel levels as a means of increasing the system capacity. Figure 4 provides a layout view of the BHTS that integrates two rotary furnaces for the solution and aging treatment. These furnaces are coupled with a quench system and robotic hardware to provide the individual part handling.
Heat-Treatment System ImprovementsThe BHTS technology provides for the efficient use of a forced recirculation system that utilizes a single recirculating fan versus a linear roller hearth or other chain-conveying, basketless heat-treatment systems that integrate multiple recirculation fans and combustion zones.
The furnace design engineers utilized computational fluid dynamic (CFD) modeled air-distribution plenums and anti-stratification baffles to ensure the hot gases maintain a uniform flow and velocity throughout the entire heating and soaking phases of the cycle. Improved temperature monitoring and control strategies have been integrated along with existing PID functionality for precise product temperature control. Following commissioning of this system, it was found via Datapaq monitoring that production castings processed using this new heat-treatment technology were capable of achieving improved product temperature uniformity of DT=2°C over conventional technology and a 50% reduction in the time required to heat the product to temperature. As a result of improved process temperature control, the customer developed improved processes that optimized the time and temperature and production capacity.
Quench System ImprovementsFurnace designers have typically struggled with the development of uniform quenching systems for integration with conventional batch or roller-hearth heat-treatment systems. This is largely due to the dense loading practices observed, variations in loading consistency, basket misalignments and limitations in delivering the quench media in a uniform and reliable method. Development engineers understand that quenching of aluminum engine castings is one of the most critical processing steps as well as an opportunity for the creation of residual stresses. This understanding led to the development of an improved quench system that could be integrated with their current BHTS. The objective was to design a system that provides improved quenching uniformity of varying cross sections and flexibility for processing multiple components.
Development of the improved quench design progressed rapidly through the use of CFD modeling and bench testing. Figure 5 provides an illustration of modeling results and time-elapsed infrared-scanning tests conducted during the system development.
Following the development testing, new quench methods were made available to manufacturers of cast engine blocks and cylinder heads. These new methods offer a combination of benefits over traditional technologies. The primary improvements include the individual part quenching of castings for both conventional basket applications and basketless processing methods. As a result of the individual-casting quenching capability, significant quenching uniformity improvements were realized across the casting’s various sections. This is achieved through independently controlled and monitored quench-distribution nozzles. These quenching systems integrate in-situ process monitoring and closed-loop feedback to ensure the uniform distribution of quench media.
Quench systems for integration with the BHTS provided added benefits over conventional basket-type quench systems. The most significant benefit relates to the reduction of system size. This is accomplished through the piece-by-piece handling of castings through a more efficiently sized system. In general, a BHTS-style quench system can be floor mounted, eliminating the need for costly pits and added infrastructure. The recirculating, heating and cooling systems are sized more efficiently for continuous steady-state operation versus the slug loading associated with basket-type quench systems. The quench-media temperature can be controlled within a tighter band, improving the part-to-part uniformity and should eventually contribute to improvements in product quality, Cpk values.
The integration of these new quench developments has created a number of new opportunities for further improvements to the quenching system designs. These new concepts are opening new doors for the development of hybrid quenching technologies that provide gradient quenching, using a combination of quench medias.
ConclusionsManufacturers of mass-produced cast-aluminum engine blocks and cylinder heads can take advantage of the quality and cost-benefit improvements that can be realized through the use of new basketless heat-treating and quench technologies. These improvements include:
- Improved temperature profile and fan life as a result of the unique and compact recirculation design.
- Individual casting handling, which reduces the time required to bring parts to solution and aging temperatures. The previously mentioned process improvements allow manufacturers the ability to better tune their process to provide the optimal combination of cycle time and productivity.
- Minimum reduction of 40% in fuel consumption when compared to similar solution-treated and aged components processed in a conventional roller-hearth system. As a result of reduced fuel consumption, CO2 emissions are reduced.
- Compact design provides a 30% reduction in floor-space requirements as a result of optimal allocation of furnace space.
- Significant reduction in mechanical components when compared to roller-hearth or chain-conveyor basketless heat-treatment systems. This results in simplified maintenance and spares requirements.
- Flexibility to process engine blocks and cylinder heads with the integration of water, precision air quenching and hybrid quenching.
- True lean-manufacturing concept when coupled directly with the prior operations.
- System internals are not affected by the presence of residual foundry sand within furnace. Design eliminates any drive components from furnace internals.
- Through improved quenching uniformity of part geometry, residual stresses of the castings can be reduced.
- Individual part quenching provides improved control of quenching media and reduced variation of properties between castings.
- Quenching system efficiencies are improved through the development of smaller systems that run at steady-state conditions versus batch systems that are cyclic.
For more information: Contact T. D. Donofrio, aluminum equipment product manager for Can-Eng Furnaces International, Ltd., 6800 Montrose Road, Niagara Falls, ON, Canada L2E 6V5; tel: 289-292-2027; e-mail: firstname.lastname@example.org; web: www.can-eng.com or P. Romanin, proposal coordinator, aluminum product group; tel: 289-292-2057; fax: 905-356-1817; e-mail: email@example.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: thermal conductivity, residual stress, cast aluminum, continuous heat-treatment system, roller hearth, rotary hearth, computational fluid dynamics, hybrid quenching, lean manufacturing