Investment casting is also known as lost wax casting. Its products are precise, complex, and close to the part’s final shape. It can be used directly with little or no machining. Therefore, investment casting is an advanced technique for getting close to the net shape.
History of Investment Casting
The history of investment casting can be traced back 4000 years. The earliest countries of origin were Egypt, China, and India, and then spread to other countries in Africa and Europe.
Ancient China had many mold castings, such as Wu Ding Prince and bronze wares in the late Spring and Autumn Period, Zhengzhou Yizun Heping Pan in the Warring States Period, Tongcu Jinboshan Furnace and Changxing Palace Lantern in the Han Dynasty. , and made a gold-plated bronze statue of Amitabha during the Qin and Qin Dynasties of the Sui Dynasty. The armillary sphere of the Ming Dynasty, the statue of Emperor Wudang Zhenwu, and the bronze lion of the Taihe Gate of the Qing Palace.
In West Africa after about the 11th century, castings were cast in large numbers. In the 16th century, artists and sculptors widely used the investment casting process. The bronze statue of the Perseid Cepheids and the Banshee leader by Benvenuto Cellini is one of the most outstanding works.
The world’s earliest written record of lost wax castings is the “East Heaven Love Record” by Zhao Xihu in the Southern Song Dynasty of China (1127-1279 AD). Then there is Cailini’s 1568 treatise, Ming Dynasty Song Yingxing’s “Tiangong Enlightenment” and VaVrinec Krickes’ mid-16th century “Cannon, Ball, Chasing Cannon, Bell Casting and Preparation Guidance”, etc.
At the end of the 19th century, dental castings were used in dental investment casting and combined with centrifugal casting technology to produce dental castings. In the early 20th century, the factors that affect the dimensional stability of wax patterns and shells and the solidification and shrinkage properties of certain metals and alloys began to be investigated to produce thicker dental components. In the early 1930s, the materials used for investment molding were adjusted. From 1900 to 1940, there were more than 400 patents in this field. Investment casting techniques are also widely used in the jewelry industry.
Aeroengine parts operating in harsh environments, such as turbochargers, would not meet performance requirements if conventional alloys were used. In the late 1930s, it was discovered that a drill-based alloy developed by Austin’s laboratory for surgical implants had excellent properties at high temperatures and could be used in turbochargers. However, this alloy is difficult to process, and investment casting has become the processing method to form this alloy. It rapidly developed industrial technology, entered the aerospace and defense industries, and was quickly applied to other industrial fields.
Investment casting has been developing relatively quickly for half a century since it was used in industrial production in the 1940s. According to reports, in 1996, the world investment casting industry (excluding the former Soviet Union) accounted for 50% of North America, 25% of Europe, 20% of Asia, and the remaining 5%. The United States, China, and the United States account for 95%, and Europe is 42% for the UK, 26% for France, 19% for Germany, 7% for Italy, and the remaining 6%. In the United States, sales of investment casting reached 2.61 billion US dollars. In 1970 and 1980, the sales of precision castings in the United States were 250 million US dollars and 1.1 billion US dollars, which were 10.4 times and 2.37 times that of 1970 and 1970, and 1980, respectively. Industry developed rapidly.
Now, investment casting is used in almost all industrial fields except the aviation and weapons industry, especially electronics, petroleum, chemical industry, energy, transportation, light industry, textile, pharmaceutical, medical machinery, pumps, and valves.
Development of Modern Investment Casting
The rapid development of investment castings is achieved through its technological development and technological progress. Great strides have been made in every aspect of the investment casting process. There are also many new materials, new processes, and new equipment that have a greater impact on the development of investment castings, such as water-soluble cores, ceramic cores, improved metal materials, and large-scale investment casting techniques, titanium alloy investment casting, directional solidification and Single crystal casting, filtration technology, hot isostatic pressing, rapid prototyping technology, computer applications in investment casting and mechanized automation.
The development of technology has enabled precision casting to produce not only small castings but also larger castings. The largest investment castings have profile dimensions close to 2m, while the smallest wall thicknesses are less than 2mm. At the same time, investment castings are becoming more precise and parts can achieve higher geometrical tolerances in addition to linear tolerances. The surface roughness value of precision castings is getting smaller and smaller and can reach Ra0.4um.
In addition, due to the improvement of materials and the development of technology, the mechanical properties of castings are getting better and better. Turbine blades are a good example. Due to advances in materials and technology, the performance of turbine blades has been greatly improved. From the 1960s to the 1990s, turbine blade materials (US brands) ranged from IN100, and B1900 to MM200, and MM247, and then progressed to PWA1480. At the same time, due to the development of solidification technology, turbine blades have changed from traditional equiaxed crystals (EQ) to directionally solidified columnar crystals (DS) and then extended to single crystal (SC) blades, making turbine blades more and more important. The working temperature is increased from 980°C to over 1100°C.
The development of titanium alloy investment casting technology has enabled titanium alloys (important structural materials in the modern industry) to use investment casting methods to produce complex and intricate parts, such as aircraft engine intermediate casings, compressor casings, and medical implants. In particular, large-scale monolithic titanium investment castings have emerged, which replace assembly parts, reduce machine weight, prolong service life, and achieve good results. According to reports, the largest titanium investment casting hub produced in 1992 weighed 340kg after welding, with a diameter of 1.918m and a height of 0.591m.
Hot isostatic pressing (HIP) technology has been widely used in turbine blades and other investment casting. It uses high temperature and pressure and relies on metal creep and plastic deformation to repair defects such as looseness and thermal cracks inside castings. After treatment, the density of the casting can reach the theoretical density of the metal, thereby improving performance. Hot isostatic pressing can improve the high-temperature and low-frequency fatigue performance of nickel-based superalloys, titanium alloys, and aluminum alloys by 3 to 10 times; increase the durability of nickel-based superalloys and titanium alloys by more than two times; improve the performance of castings, disperse reduced to one-sixth of the original.
To shorten the production cycle and simplify the process, investment casting is combined with rapid prototyping technology (RPT) that emerged in the 1980s, using RPT’s stereolithography (SLA), selective laser sintering (SLS), and fused deposition manufacturing method (FDM) or layered solid manufacturing (LOM) to make plastic, wax and paper prototypes instead of traditional wax patterns, or use the direct mold production (DSPC) process to produce ceramic shells for investment casting production to enhance market competition force.
The development of mechanization and automation has broken the old concept that “the investment casting process cannot be mechanized”. Japan, the United Kingdom, and the former Soviet Union have successfully produced low-cost automobiles and other civilian parts using the investment casting process.
In conclusion
In short, with the development of casting technology, investment castings have been able to produce more refined, larger, and stronger high-value products. “Precise”, “large” and “thin-walled” are the unique characteristics of modern investment casting. At the same time, investment casting has made breakthroughs in producing and rapidly producing low-cost parts. These have enabled the application of investment casting to expand, so it is in a better position to compete with other processes and has a bright future.
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