Investment casting for the aerospace industry 

Why investment casting is the aerospace industry standard

Aerospace investment casting is not just a manufacturing technology, but a critical process that determines the safety and efficiency of flight operations. In our practice, we see that more than 70% of the hot sections of modern gas turbine engines are manufactured using this method. The reason is simple: only this technology makes it possible to create parts from heat-resistant superalloys with complex internal geometry of cooling channels, which cannot be obtained by machining or sand casting.

When the temperature in the combustion chamber exceeds 1400°C, and the melting point of the alloy itself is 1350°C, the only thing that saves the blade from destruction is the internal air cooling system. The manufacturing accuracy of such channels should be within ±0.05 mm. Any deviation leads to local overheating and catastrophic engine failure. We have encountered situations where savings on the quality of the wax model led to the rejection of an entire batch of castings worth millions of rubles, so control at the modeling stage is more important here than the melting process itself.

This article is based on real-life experience in supplying components to turbine and rocket engine manufacturers. We'll break down the technical nuances that differentiate aerospace casting from industrial casting, explain certification requirements, and show you how to avoid common mistakes when choosing a contractor.

Technological features of the process for aviation components

The process begins not with the metal, but with the creation of a master model. In the aerospace segment, the surface requirements for the wax-up model are extreme. If a roughness of Ra 3.2 is acceptable for a pump wheel, then for a turbine blade of the first stage this parameter should often not exceed Ra 0.8 even before the metallization stage. We use high pressure injection molding to ensure repeatable geometry. The error at this stage is scaled: a 10-micron defect on the model turns into a critical discrepancy on the finished part after the metal shrinks.

Assembling clusters (trees) requires pinpoint precision. Unlike art casting, where the trees can be chaotic, in aerospace the layout of the models is calculated algorithmically. This is necessary to ensure uniform heat removal during crystallization. An incorrect angle of inclination of the feeder leads to the formation of shrinkage cavities inside the body of the part. Such defects are invisible to the eye and are detected only during X-ray inspection, when the part is already ready. One of our clients lost three months of work because they ignored the mold filling simulation and relied on the foundry's expertise.

Applying the ceramic shell is the most labor-intensive step. Aerospace alloys require 9 to 12 layers of zirconia or alumina ceramics. Each layer is dried in a controlled environment to within 1% moisture content. Violation of the drying regime leads to delamination of the shell (“shell”) during pouring. Molten metal under a pressure of 5-10 atmospheres breaks through weak points, and the melt flows into the furnace. This is not just a defect, it is a risk of damaging expensive equipment.

Wax burning occurs in autoclaves under steam pressure. Complete carbon removal is critical here. Wax residues in the micropores of ceramics upon contact with molten titanium or nickel superalloy cause gas porosity. We introduced a procedure for spectral analysis of the atmosphere in the furnace before pouring, which reduced the percentage of defective gas porosity from 4% to 0.5%. This is the case when technological control directly affects the marginality of the project.

The metal is poured in vacuum induction furnaces. For alloys such as Inconel 718 or VZHL-12U, the melting temperature reaches 1550-1600°C. Vacuum is necessary not only to prevent oxidation, but also to degas the melt. After pouring, the mold cools at a controlled rate. Directional crystallization makes it possible to grow single-crystalline grains along the axis of the blade, which increases the heat resistance significantly. Under such conditions, ordinary equiaxed grains quickly lose strength due to creep.

The final stage is ceramic removal and mechanical processing. The ceramic shell is removed by water hammer or chemical etching. Mechanical processing is minimal, since the allowances are only 0.3-0.5 mm. However, removing the sprues and cleaning the surface requires the use of EDM machines, as the hardness of the annealed superalloy makes traditional milling extremely expensive. Each batch undergoes mandatory incoming inspection according to ASTM or GOST RV standards.

Materials and alloys: the choice for extreme conditions

The aerospace industry dictates stringent requirements for materials. Aerospace investment casting works primarily with three groups of alloys: high-temperature nickel alloys, titanium alloys, and specialty steels. The choice of material determines the entire technology chain: from the type of ceramic to the melting method.

Nickel superalloys (Inconel, Hastelloy, ZhS, VZhL) occupy a leading position. They retain strength at temperatures up to 90% of the melting point. The main problem when casting these alloys is the tendency to form carbides and intermetallic compounds at grain boundaries, which reduces the life of the part. We solve this problem by strictly controlling the chemical composition of the charge and using vacuum arc remelting (VAR) of the starting materials. Customers often ask why we require certificates for each raw material melt. The answer is simple: sulfur content even at a level of 0.005% can provoke hot cracks during the crystallization of thin-walled elements.

Titanium alloys (Ti-6Al-4V, VT6, VT20) are in demand for airframe and compressor parts due to their high strength-to-weight ratio. Casting titanium is more difficult than nickel due to its high chemical reactivity. At temperatures above 800°C, titanium reacts with oxygen, nitrogen and hydrogen, becoming brittle. Therefore, melting is carried out in a vacuum not lower than 10^-3 Pa, and sometimes in an atmosphere of pure argon. Ceramic molds for titanium should be neutral; standard silicate binders are not suitable; yttrium oxide or special graphite forms are used. An error in the selection of mold material leads to the formation of an alpha layer on the surface, which has to be removed by deep etching, changing the dimensions of the part.

A deep understanding of the behavior of exotic materials such as titanium and nickel alloys (eg N06625) under extreme conditions is a key competency for high-tech equipment manufacturers. It is this experience that underlies the work of companies specializing in creating critical units for the energy and oil and gas industries, such asWuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd.. While their primary focus is on the production of heat transfer equipment (titanium shell-and-tubes, air coolers, waste heat boilers) and components made from marine brass and copper-nickel alloys, their engineering expertise in working with corrosion and heat resistant materials to ASME and PED standards directly intersects with aerospace casting requirements. Understanding the metallurgy of these alloys to create reliable tube sheets and bundles in harsh environments ensures the same high level of quality control and material selection as used in aircraft components.

Heat-resistant steels (15Х5М, 20Х3МВФ) are used for less loaded components, such as bearing housings or exhaust system elements. The key factor here is resistance to oxidation and creep at moderate temperatures (up to 600°C). Steel casting technology is more mature and cheaper, but the requirements for structure uniformity remain high. We recommend the use of rare earth element modification of the melt for grain refinement, which is confirmed by impact tests.

It is important to understand the difference between wrought and cast versions of alloys. An attempt to cast a part from a wrought alloy (for example, conventional VT6 instead of cast VT6L) will lead to segregation and low tightness. Casting alloys have an adjusted composition to improve fluidity and reduce the tendency to crack. When ordering, be sure to indicate the brand with the index “L” or an equivalent according to the AMS specification.

Quality control and compliance with international standards

In the aerospace industry, the concept of “marriage” has absolute meaning. The part either meets 100% specification or it is destroyed. Compromises are unacceptable. Our quality control system is built on the principles of AS9100 and includes multi-step verification at every stage.

Primary control is carried out at the stage of wax models. 3D optical scanning is used to compare the model geometry with the CAD model. The tolerances here are ±0.1 mm. We identified a case where wax casting mold wear was only 0.05mm, but this resulted in the wall thickness of the finished metal part being outside the lower tolerance limit after shrinkage. Regular scanning can prevent such situations.

Radiographic inspection (RT) is required for 100% of critical parts. It reveals internal defects: pores, cavities, slag inclusions. ASTM E192 or GOST RV 0015-002 standards classify defects by type and size. The permissible pore size depends on the wall thickness and the area of the part. For example, in the root of the blade, pores with a diameter of more than 0.3 mm are unacceptable, while in the feather part, single pores up to 0.5 mm are allowed, provided they are isolated. Image interpretation is carried out by level II or III operators according to the ISO 9712 standard.

Penetrant testing (PT) and magnetic particle testing (MT) detect surface cracks. For aerospace parts, high contrast fluorescent penetrant flaw detection is used. The sensitivity of the method makes it possible to detect cracks with an opening width of less than 1 micron. It is important to note that the surface must be perfectly clean before testing. Abrasive residue from sandblasting can close the crack entrance and give a false negative result.

Coordinate Measuring Machines (CMMs) are used to final check the geometry. The part is scanned at hundreds of points. Particular attention is paid to the profile of the back and trough of the blade, since aerodynamics depends on the accuracy of the profile. A profile deviation of more than 0.1 mm can reduce the efficiency of a turbine stage by several percent, which on an engine scale means a loss of thrust and excessive fuel consumption.

Production certification is required. ISO 9001 certification is a basic requirement, but for aviation applications AS9100 Rev D is required. This standard includes additional requirements for risk management, material traceability and configuration management. Without a valid AS9100 certificate, no major aircraft manufacturer will sign a supply contract. Also, shipments to certain regions require compliance with NADCAP (National Aerospace and Defense Contractors Accreditation Program) requirements for special processes such as heat treatment and non-destructive testing.

Cost-effectiveness and lead times

Many customers consider lost wax casting to be an expensive pleasure. Yes, the cost per casting is higher than sand casting or machining from rolled stock. However, when calculating total cost of ownership (TCO), the picture changes. The material utilization factor (KIM) for casting is 0.6-0.8, while for forging it drops to 0.1-0.2. For expensive alloys like monocrystalline nickel, the savings on material are enormous.

Reduced machining is the second saving factor. Complex internal cavities, which during machining would require dozens of operations, special milling cutters and multi-axis machines, are formed immediately during casting. This reduces production time from weeks to days. We carried out an analysis for a client producing fuel injectors: switching to casting reduced unit costs by 35% despite the high cost of tooling.

Production time depends on the complexity of the part and the volume of the batch. Development of new equipment (molds for wax) takes from 3 to 5 weeks. The first sample (prototype) can be received 6-8 weeks after the drawings are approved. Serial production of a batch of 100 pieces takes another 4-6 weeks. It is important to plan these deadlines in advance. An attempt to speed up the process of drying ceramic forms or heat treatment inevitably leads to defects.

The minimum order quantity (MOQ) is usually between 10 and 50 pieces depending on the part size. For small parts (up to 100g), the MOQ may be higher due to the labor involved in assembling the trees. However, we offer services for consolidating orders from different clients into one melt, which allows us to lower the entry threshold for small projects and development work (R&D).

Logistics and packaging also affect the cost. Aerospace parts require individual packaging in vacuum bags with desiccant and tightly secured in containers to prevent damage during transportation. We use MIL-STD or equivalent civil packaging standards. Delivery is carried out by air with full cargo insurance coverage.

Typical mistakes when ordering and how to avoid them

Working with dozens of clients, we have identified a number of recurring mistakes that lead to missed deadlines and increased costs of projects. The first and most common mistake is providing incomplete design documentation. The drawing must contain not only views and sections, but also technical requirements for surface quality, tolerances for unspecified dimensions, labeling and packaging requirements. Failure to specify an acceptance standard (for example, “ASTM E192 Level 2 inspection”) gives the foundry the right to interpret defects at its own discretion, which often does not coincide with customer expectations.

The second mistake is ignoring material shrinkage. Different alloys have different linear shrinkage coefficients. For stainless steel it is about 1.5-2.0%, for titanium - 1.0-1.2%, for aluminum alloys - up to 1.3%. If the designer provides nominal dimensions without adjustment for shrinkage, the finished part will be smaller than required. Correcting this defect by mechanical processing is not always possible, especially when it comes to thin walls.

The third mistake is unrealistic allowance requirements. The desire to get a part “to zero” immediately from the mold leads to a sharp increase in the percentage of defects. Casting is a statistical process. There should always be a technological allowance for subsequent processing of the base surfaces. We recommend placing an allowance of at least 0.5 mm on critical surfaces in order to be able to correct possible warping after heat treatment.

The fourth mistake is choosing a material without taking into account casting properties. As mentioned earlier, not all alloy grades are suitable for casting. An attempt to save money by ordering a casting from a cheap wrought alloy results in a part with low mechanical properties and internal porosity. Always consult with foundry technologists when selecting a material grade.

The fifth mistake is the lack of a non-destructive testing (NDT) plan. The customer often indicates “100% control”, but does not specify the methods and sensitivity. This results in the manufacturer performing a visual inspection and deeming the task completed. The contract must clearly specify NDT methods (RT, UT, PV, MP), control zones and rejection criteria. Only this approach guarantees a product suitable for use in aviation.

Prospects for technology development until 2026

The aerospace casting market is undergoing active transformation. Forecasts from analytical agencies indicate an increase in demand for components for new generations of engines with increased bypass ratios and reduced CO2 emissions. This requires the introduction of new materials and technologies.

Одним из главных трендов является внедрение аддитивных технологий в процесс изготовления восковых моделей. 3D-печать воском позволяет создавать модели сложнейшей геометрии без дорогостоящей металлической оснастки. Это сокращает срок вывода нового изделия на рынок с месяцев до недель. Мы уже используем гибридный подход: критические внутренние каналы печатаются на 3D-принтере, а внешняя геометрия формируется традиционным литьем в пресс-форму. Это снижает стоимость прототипирования на 40%.

Другой важный тренд — развитие монокристаллического литья третьего и четвертого поколения. Новые сплавы с добавлением рения и рутения позволяют повысить рабочую температуру еще на 50-70°C. Однако литье таких сплавов требует сверхточного контроля градиента температур в печи. Ошибка в 10 градусов может привести к образованию дефектных зерен. Инвестиции в модернизацию печного парка становятся обязательным условием выживания для литейных заводов.

Цифровизация процессов контроля также набирае т обороты. Внедрение искусственного интеллекта для анализа рентгеновских снимков позволяет автоматизировать поиск дефектов и исключить человеческий фактор. Системы машинного зрения обучаются на тысячах снимков брака и выявляют аномалии быстрее и точнее оператора. К 2026 году ожидается, что более 50% крупных литейных производств будут использовать AI-ассистентов для контроля качества.

Environmental requirements are becoming more stringent. Процессы удаления керамики и регенерации материалов должны соответствовать новым экологическим нормам ЕС и США. Переход на водорастворимые связки и замкнутые циклы водооборота становится стандартом отрасли. Производители, игнорирующие “зеленую” повестку, рискуют потерять доступ к рынкам развитых стран.

Frequently Asked Questions

Какой минимальный вес детали возможен при литье по выплавляемым моделям?

Технологически возможно литье деталей весом от 1 грамма. Однако в аэрокосмической отрасли экономически целесообразно производить детали весом от 10-20 граммов. Слишком мелкие детали требуют сложной сборки в большие деревья для эффективности плавки, что увеличивает трудозатраты. Для микро-деталей чаще применяется прецизионное литье в особых условиях, но для стандартных авиационных компонентов оптимальный диапазон начинается от 50 грамм.

Можно ли сваривать отливки из жаропрочных сплавов?

Да, сварка возможна, но она требует специальной технологии и последующей термообработки. Сварка литых жаропрочных сплавов (особенно монокристаллических) крайне сложна из-за риска образования трещин в зоне термического влияния. Обычно используется аргоно-дуговая сварка (TIG) или электронно-лучевая сварка (EBW) в вакууме. Перед сваркой необходима тщательная подготовка кромок и подогрев детали. После сварки обязательно проводится диффузионный отжиг для снятия напряжений. В ряде случаев проще отлить деталь целиком, чем варить её из нескольких частей.

Какой срок службы пресс-форм для восковых моделей?

Срок службы алюминиевых пресс-форм составляет в среднем 5 000 – 10 000 циклов впрыска. Стальные закаленные формы служат дольше — до 50 000 циклов и более. Износ формы проявляется в изменении размеров модели и ухудшении качества поверхности. Для аэрокосмических деталей с жесткими допусками мы рекомендуем проводить профилактическое измерение образцов каждые 500 циклов и восстанавливать форму или заменять её при выходе параметров за пределы поля допуска. Экономия на своевременной замене формы приводит к браку металлических отливок, что многократно дороже стоимости новой оснастки.

Предоставляете ли вы отчеты о материале (Mill Test Reports)?

Да, для каждой партии отливок мы предоставляем полный пакет сопроводительной документации, включая химический анализ шихты и готовой отливки, результаты механических испытаний образцов-свидетелей (отлитых вместе с партией), отчеты по неразрушающему контролю и паспорт качества. Все документы соответствуют требованиям AS9100 и могут быть предоставлены в электронном виде с цифровой подписью или на бумажном носителе по запросу. Прослеживаемость каждой плавки сохраняется в архиве не менее 15 лет.

Conclusion and next steps

Литьё по выплавляемым моделям остается безальтернативной технологией для создания высоконагруженных узлов аэрокосмических двигателей. Сочетание точности, возможности работы с экзотическими сплавами и экономической эффективности делает его фундаментом современного авиастроения. Однако успех проекта зависит не только от технологии, но и от компетенции исполнителя, наличия правильной сертификации и строгого соблюдения регламентов.

Если вы планируете запуск нового продукта или ищете надежного партнера для серийного производства компонентов, важно начать с аудита технической документации и оценки технологичности конструкции. Наши инженеры готовы провести бесплатный анализ ваших чертежей и предложить оптимизацию для снижения себестоимости без потери качества.

Мы работаем по стандартам AS9100 и имеем полный цикл производства: от моделирования до финишной обработки и контроля. Свяжитесь с нами сегодня, чтобы обсудить ваш проект и получить коммерческое предложение с расчетом сроков и стоимости.Запросить консультацию инженераили перейдите на страницууслуги литья для аэрокосмической отраслидля изучения подробных кейсов.