Weldability of Cast Superalloys - Effect of homogenization heat treatments on hot cracking susceptibility of cast Alloy 718, ATI® 718Plus®, and Haynes® 282®
Doktorsavhandling, 2020

Precipitation hardened Ni- and Ni–Fe-based superalloys are used in high-temperature sections of aero engines owing to their superior mechanical properties compared to those of the other alloys. However, their better mechanical performance is accompanied by its own challenges during the manufacturing process. For the fabrication of hot structural components, instead of the traditional single piece castings, welding is widely employed by joining wrought parts in sections, where high strength is required, and cast parts, where complex geometrical shapes are needed. This can be challenging, as superalloys are known for their lack of amenability to welding. A weld-cracking phenomenon known as “hot cracking”is of concern during their welding. Especially, the cast materials are known to be more prone to cracking owing to the higher extent of segregating phases that remain from the casting process.The present study investigates the weldability of Alloy 718 and two recently developed Ni-based superalloys ATI® 718Plus® and Haynes® 282® with respect to heat affected zone liquation (HAZ) cracking susceptibility. Pre-weld homogenization treatments were performed at 1120 °C and 1190 °C to study the effect of different microstructures on cracking extent. The testing approach consisted of using Varestraint weldability test to assess the HAZ liquation cracking susceptibility and Gleeble thermomechanical simulator for evaluation of hot ductility behaviour. The results revealed that a lower heat treatment temperature at 1120 °C for 4 h was beneficial in minimizing the influence of liquation, and that the grain growth also contributed to lowering the cracking susceptibility in the HAZ. JMatPro simulations and microstructural evaluation on elements such as Nb as solute and precipitate former in Alloy 718 and ATI® 718Plus®, and Mo in Haynes® 282® were found to be important in the liquation mechanism. Secondary ion mass spectroscopy (SIMS) analysis revealed B, which is a strong melting point depressant, to segregate along the grain boundaries in all the three alloys. In addition, in this thesis, different liquation mechanisms were discussed and an explanation for the overall HAZ liquation cracking mechanism for cast superalloys was proposed.

Boron

Welding

Gleeble

Hot isostatic pressing

Homogenization

Nickel-based superalloys

Segregation

Varestraint

Hot cracking

Niobium

Hot ductility

VDL
Opponent: Associate Prof. Carl Cross, Los Alamos National Laboratory, USA

Författare

Sukhdeep Singh

Chalmers, Industri- och materialvetenskap, Material och tillverkning

Hot cracking in cast alloy 718

Science and Technology of Welding and Joining,; Vol. 23(2018)p. 568-574

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Influence of Hot Isostatic Pressing on the Hot Ductility of Cast Alloy 718: The Effect of Niobium and Minor Elements on the Liquation Mechanism

Metallurgical Transactions A (Physical Metallurgy and Materials Science),; (2020)

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Varestraint weldability testing of cast ATI® 718Plus™—a comparison to cast Alloy 718

Welding in the World, Le Soudage Dans Le Monde,; Vol. 63(2018)p. 389-399

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Influence of homogenisation treatments on the hot ductility of cast ATI® 718Plus®: Effect of niobium and minor elements on liquation characteristics

Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing,; Vol. 799(2020)

Artikel i vetenskaplig tidskrift

Heat-Affected-Zone Liquation Cracking in Welded Cast Haynes® 282®

Metals,; Vol. 10(2020)

Artikel i vetenskaplig tidskrift

Ni- and Ni–Fe-based superalloys are used in high-temperature sections in the rear end of aero engines owing to their superior mechanical properties compared to those of the other alloys. Traditionally, structural components have been manufactured as single piece castings. However, the recent trend for the fabrication of hot structural components has changed, instead of the traditional single piece castings, to welding of wrought parts in sections where high strength is required, and cast parts, where complex geometrical shapes are needed. This can be challenging, as superalloys are prone to weld-cracking phenomenon known as “hot cracking”. Especially, the cast materials are known to be more prone to cracking owing to the higher extent of segregating phases that remain from the casting process. Traditionally, Alloy 718 has been used for manufacturing hot structural components of aero engines. New alloys, such as ATI® 718Plus® and Haynes® 282® have been introduced over the past decade in view of the need for materials that could surpass the maximum operative temperature of Alloy 718 thereby improving the overall aero engine efficiency. The cast versions of ATI® 718Plus® and Haynes® 282® have been developed during the recent years; however, very little is known about their weldability performance. Prior to welding, hot isostatic pressing treatments are performed to homogenize the material and eliminate any porosity that remained from the casting process. Although the standards for HIP treatment of cast Alloy 718 have matured in the aerospace industry, no such standard heat treatments are available for the cast versions of ATI® 718Plus® and Haynes® 282®. In the current study, different heat treatments were performed to investigate the homogenization effect on the segregating phases. Moreover, the effect of homogenization heat treatments on the heat-affected zone liquation cracking was evaluated by means of Varestraint and Gleeble weldability tests. The results contribute to better understand the relationship between microstructural changes and the susceptibility towards cracking. This knowledge can be used in the production to select appropriate homogenization heat treatment and ultimately avoid the cracking problems during welding.

Ämneskategorier

Materialteknik

Styrkeområden

Materialvetenskap

ISBN

978-91-7905-366-6

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4833

Utgivare

Chalmers tekniska högskola

VDL

Online

Opponent: Associate Prof. Carl Cross, Los Alamos National Laboratory, USA

Mer information

Senast uppdaterat

2020-09-23