Hot ductility recovery determines the alloy’s susceptibility to heat-affected zone (HAZ) liquation cracking during welding. In this work, the hot ductility recovery of a next-generation Ni-based superalloy, VDM Alloy 780, was physically simulated using a Gleeble thermomechanical simulator and compared with that of the widely used Alloy 718 to evaluate their relative susceptibility to HAZ liquation cracking. VDM Alloy 780 exhibits a significantly superior hot ductility recovery, resulting in a 25 °C narrower brittle temperature range despite a six-fold larger grain size and more extensive supersolidus melting. Enhanced hot ductility recovery in VDM Alloy 780 is attributable to the combined effects of a higher resolidification finish temperature, associated with substantial replacement of Fe by Co and reduced Ti content, and disrupted susceptible (random) grain boundary connectivity due to a larger proportion of coincident-site lattice (CSL) boundaries compared to Alloy 718. These findings highlight the greater HAZ liquation cracking resistance in VDM Alloy 780 compared to Alloy 718, making the new alloy a promising candidate for aero-engine fabrication. 

The weld solidification cracking susceptibility of VDM Alloy 780, a new superalloy designed to possess superior thermal stability up to 750 ℃ in comparison to Alloy 718, was evaluated under different pre-weld solution annealing conditions that produce different base material grain sizes, and compared to that of Alloy 718, using the Varestraint weldability test. Solidification cracking in VDM Alloy 780 is unaffected by pre-weld solution annealing, and the cracks are associated with the NbC and γ/Laves eutectic constituents. In actual fabrication, where restraint is low to moderate, VDM Alloy 780 is expected to possess comparable susceptibility to solidification cracking to Alloy 718.

Nickel-based superalloys are indispensable for hot structural components in aeroengines due to their exceptional high-temperature strength and oxidation resistance. Alloy 718 has been the industry standard for decades; however, its application is limited to temperatures below approximately 650°C. The continued drive toward higher engine operating temperatures to improve fuel efficiency and reduce emissions has motivated the development of next-generation superalloys with greater temperature capability. VDM Alloy 780 and G27 are recently introduced wrought Ni-based superalloys designed for service temperatures approaching 750°C and are promising candidates for aeroengine components fabricated by fusion welding. The successful implementation of these new superalloys in fabrication depends critically on weldment integrity, which is influenced by factors such as the fusion zone (FZ) microstructure, base material (BM) grain size, and susceptibility to weld hot cracking.

In the first part of the thesis, microstructural characterization reveals that weld solidification in both superalloys results in Nb-rich MC carbides and irregularly shaped Laves phase as the major interdendritic constituents in the as-welded FZ microstructure, mainly due to Nb microsegregation. Because the Laves phase is inherently brittle and generally detrimental to the mechanical performance of superalloys, its removal through post-weld solution heat treatment (PWSHT) is essential. A PWSHT at 1060°C for 1h is identified as optimal, achieving complete dissolution of Laves phases without promoting η-phase formation.

Importantly, this heat treatment results in only relatively moderate BM grain growth in both alloys, with slower grain coarsening kinetics relative to Alloy 718. This retarded grain growth behavior is beneficial for preserving a relatively fine-grained BM microstructure while enabling complete removal of the detrimental Laves phases inthe FZ through PWSHT.

In the second part of the thesis, hot cracking susceptibility was systematically investigated using Varestraint and Gleeble hot-ductility tests. Solidification cracking in both alloys is found to be largely insensitive to pre-weld solution heat treatments (SHTs). In contrast, heat-affected zone (HAZ) liquation cracking is strongly influenced by pre-weld SHTs, through altering the grain size and the magnitude of boron segregation at grain boundaries. When optimized, both alloys exhibit hot-cracking susceptibility comparable to, and in some cases superior to, that of Alloy 718. Overall, the results demonstrate that VDM Alloy 780 and G27 possess viable weldability for aeroengine fabrication, provided that pre-weld SHTs are carefully tailored to control microstructure, particularly grain size and grain boundary boron segregation, to minimize HAZ liquation cracking susceptibility.

Nickelbaserade superlegeringar är oumbärliga för lastbärande strukturella komponenter i flygmotorer tack vare deras exceptionella hållfasthet vid högatemperaturer och goda oxidationsbeständighet. Alloy 718 har varit industristandard i flera decennier; dess användning är dock begränsad till temperaturer under cirka 650 °C. Den fortsatta strävan efter högre drifttemperaturer i motorer för förbättrad bränsleeffektivitet och minskade utsläpp har motiverat utvecklingen av nästa generations superlegeringar med utökad temperaturkapacitet. VDM Alloy 780 och G27 är nyligen introducerade smidda nickelbaserade superlegeringar, konstruerade för drifttemperaturer upp mot cirka 750 °C, och utgör lovande kandidater för flygmotorkomponenter som tillverkas genom smältsvetsning. En framgångsrik implementering av dessa legeringar i tillverkning är i hög grad beroende av svetsens integritet, vilken styrs av faktorer såsom mikrostrukturen i smältzonen (FZ), grundmaterialets (BM) kornstorlek samt benägenhet för varm sprickbildningvid svetsning.

Mikrostrukturell karakterisering visar att stelnandet i svetsen i båda legeringarna leder till bildning av Nb-rika MC-karbider och oregelbundet formad Laves-fas som de dominerande interdendritiska strukturbeståndsdelarna i den ovärmebehandlade FZ-mikrostrukturen. Eftersom Laves-fasen är inneboende spröd och generellt skadlig för den mekaniska prestandan hos svetsade superlegeringar är dess avlägsnande genom efterföljande upplösningsvärmebehandling efter svetsning (PWSHT) nödvändig. En PWSHT vid 1060 °C under 1 timme identifierades som optimal och resulterade i fullständig upplösning av Laves-fasen utan att främja bildning av η-fas. Viktigt är att denna värmebehandling endast gav upphov till relativt måttlig korntillväxt i grundmaterialet, med långsammare korntillväxt jämfört med Alloy 718. Detta beteende för korntillväxt är fördelaktigt för att bevara en relativt finkornig mikrostruktur i grundmaterialet samtidigt som fullständig eliminering av den skadliga Laves-fasen i smältzonen möjliggörs.

Benägenheten för varmsprickbildning undersöktes systematiskt med hjälp av Varestraint- och Gleeble-tester för varmduktilitet. Stelningssprickbildning i smältzonen visade sig i stort sett vara okänslig för upplösningsvärmebehandling före svetsning (SHT). Däremot påverkades sprickbildning i den värmepåverkade zonen (HAZ) starkt av SHT:er före svetsning, genom förändringar i kornstorlek och omfattningen av borsegregation vid korngränser. När värmebehandlingarna optimerades uppvisade båda legeringarna en benägenhet för varmsprickbildning som var jämförbar med, och i vissa fall bättre än, Alloy 718. Sammantaget visar resultaten att VDM Alloy 780 och G27 uppvisar god svetsbarhet för tillverkning av flygmotorkomponenter, förutsatt att upplösningsvärmebehandlingar före svetsning anpassas noggrant för att styra mikrostrukturen, särskilt kornstorlek och borsegregation vid korngränser, i syfte att minimera benägenheten för att bilda uppsmältningssprickor i HAZ.

The hot cracking susceptibility of G27, a new superalloy designed for superior mechanical performance relative to Alloy 718, is compared to that of Alloy 718 by Varestraint testing. G27 is found to be more prone to solidification cracking than Alloy 718. Thermodynamic calculations show that G27 has a wider solidification temperature range (STR) and a higher crack susceptibility index (CSI) based on the steepest slope of the temperature vs. square root of the fraction solid (fs) curve at fs = 0.98. The higher B and Zr contents in G27, compared to Alloy 718, are likely to be the primary factor contributing to the higher cracking susceptibility, as these elements and their interactions show the strongest influence on the calculated STR and CSI compared to the other elements based on the Scheil simulation. Regarding heat-affected zone (HAZ) liquation cracking, G27 also shows greater susceptibility, likely due to its larger resolidification temperature range of HAZ intergranular liquid and a lower fraction of special boundaries compared to Alloy 718. Although G27 is more prone to hot cracking than Alloy 718, the difference in cracking response between the two is relatively modest, making G27 a promising alternative to Alloy 718 due to its superior mechanical properties.  

The weld heat-affected zone liquation cracking susceptibility of new 718-type superalloy VDM Alloy 780 was studied under various pre-weld solution heat treatment (SHT) conditions using the Varestraint test and compared with Alloy 718. Similar to Alloy 718, liquation cracking in VDM Alloy 780 occurs by constitutional liquation of NbC and supersolidus melting. Among the conditions studied, the 1050 °C/0.5 h SHT exhibits the lowest cracking propensity, followed by 1090 °C/2h, 1150 °C/4h, and 1150 °C/4h + 1050 °C/0.5 h. Within this SHT range, grain coarsening and equilibrium segregation of boron at the grain boundaries are identified as major factors deteriorating the cracking resistance in VDM Alloy 780, with grain coarsening being the more dominant factor. When both factors coexist, the cracking susceptibility further increases. The most significant grain coarsening takes place at 1150 °C, while the strongest boron segregation occurs after the 1050 °C SHT. In its optimal SHT condition that resists cracking (1050 °C/0.5 h), VDM Alloy 780 exhibits superior liquation cracking resistance to Alloy 718 despite having a six times coarser grain size, which appears to be due to its substantially higher fraction of coincident site lattice boundaries (25 % higher).  

The hot ductility behavior of new Ni-based superalloy G27 was investigated using Gleeble™ 3800 under various standard solution-annealing conditions to study how solution annealing treatments, which result in different grain sizes, affect the alloy’s liquation cracking susceptibility. Hot ductility loss is primarily attributable to subsolidus grain boundary (GB) liquation caused by boron (B) segregation. The 1010 ◦C/1h condition, which produces the finest grain size due to GB pinning by γ′, shows minor B segregation, the shallowest ductility drop, and the narrowest brittle temperature range (BTR).

In contrast, the 1110 ◦C/1h condition, which results in the coarsest grains, exhibits pronounced B segregation, the steepest ductility drop, and the widest BTR. The extent of B segregation is suggested to be influenced by both grain size and the presence of Mo–B-rich precipitates. In the 1010 ◦C/1h condition, fine-grain size and Mo–B-rich particles—which bind significant amounts of B—minimize B segregation. Substantial B segregation at 1110 ◦C/1h results from the combined effects of coarse grains and thed issolution of Mo–B-rich precipitates, releasing all bound B atoms into the matrix that subsequently segregate to the GBs. The formation of a thicker intergranular liquid film enriched in B due to increased B segregation is believed to extend the resolidification temperature range during cooling, thereby widening the BTR in G27.

Effective thermal management is essential in metal additive manufacturing to ensure process stability and desirable material properties. Directed energy deposition using a laserbeam and wire (DED-LB/w) enables the production of large, high-performance components but remains sensitive to adverse thermal effects during multilayer deposition due to heat accumulation. While prior studies have investigated interlayer temperature control and substrate preheating in DED modalities, including laser-powder and arc-based systems, the influence of substrate preheating in DED-LB/w has not been thoroughly examined.

This study employs substrate preheating to simulate heat accumulation and assess its effects on melt pool geometry, wire–melt pool interaction, and the microstructural evolution of Alloy 718. Experimental results demonstrate that increased substrate temperatures lead to a gradual expansion of the melt pool, with a notable transition occurring beyond 400 ◦C. Microstructural analysis reveals that elevated preheat temperatures promote coarser secondary dendrite arm spacing and the development of wider columnar grains. Moreover, Nb-rich secondary phases, including the Laves phase, exhibit increased size but relatively unchanged area fractions. Observations from electrical conductance measurements and coaxial visual imaging show that preheat temperature significantly affects the process dynamics and microstructural evolution, providing a basis for advanced process control strategies.

VDM Alloy 780 and G27 are new γ'-hardening polycrystalline Ni-based superalloys that are promising material candidates for aerospace applications, especially in the hot sections of an aero engine where welding is extensively used to fabricate themassive hot structural components by joining small pieces of superalloys. VDM Alloy 780 was developed to surpass the service temperature capability of Alloy 718 (up to 750°C) while maintaining the excellent workability of Alloy 718, whereas G27 was designed to match, if not exceed, the mechanical properties of Waspaloy, especially in terms of crack growth resistance under dwell fatigue.

In a superalloy weldment, the weld fusion zone (FZ) microstructure that typically consists of γ dendrite and secondary phase constituents that form due to elemental microsegregation during rapid weld solidification is one of the critical factors that invariably control the properties of the welded superalloys. In the present work, the observed elemental microsegregation pattern observed in the weld FZ of VDM Alloy 780 and G27 during the solidification of the weld results in the formation of Nb-rich MC carbides and coarse irregular-shaped Laves phases as the major secondary phase constituents in the interdendritic region. 

After welding, post-weld solution heat treatment (PWSHT) is typically carried outon superalloy weldment to relieve residual stresses and modify the as-welded microstructure. Considering large-sized irregular Laves phases that are generally known to be potentially detrimental to the mechanical properties form in the aswelded FZ of VDM Alloy 780 and G27, suitable PWSHT needs to be developed to remove the Laves phases to achieve optimum microstructures and reliable properties of the welded superalloys. WSHT for an hour performed at 954°C only partially eliminates the Laves phases in both alloys with substantial η phase formation along the interdendritic regions. The excessive amount of η phase also needs careful consideration when developing suitable PWSHT since it may further potentially deteriorate the mechanical properties of the weldment. The Laves phases are completely dissolved without forming η phase after implementing PWSHT at 1060°C for 1h. Thus, with regard to the secondary phase constituents in the FZ,PWSHT at 1060°C/1h is suitable for VDM Alloy 780 and G27 since it completely removes Laves phases with no η phase formation.

Apart from removing Laves phase, it is also essential to understand the effect of PWSHT at 1060°C/1h on the new superalloys' base metal (BM) grain sizes. For the widely used Alloy 718, PWSHT at 1060°C/1h is considered unfeasible since itresults in an excessive grain coarsening in the BM, reducing its tensile and fatigue performance. Based on the results in the present study, PWSHT at 1060°C/1h increased BM grain sizes from 50 to 63 µm in VDM Alloy 780 and 25 to 57 µm in G27. However, the extent of grain growth in VDM Alloy 780 and G27 is proven to be less severe than that in the widely used Alloy 718, showing that the grain growth kinetics in new superalloys are slower than in Alloy 718 at 1060°C, which is believed to be due to pinning effect of Mo-rich Laves phase in G27, and likely to be due to solute segregation at grain boundaries in VDM Alloy 780. The slower kinetics of grain growth at 1060°C in VDM Alloy 780 and G27 relative to Alloy 718 is beneficial in preserving a finer grain structure in the BMs of VDM Alloy 780 and G27 relative to Alloy 718 during PWSHT that aims to remove the Laves phases in the weld FZ

VDM Alloy 780 och G27 är nya γ'-härdande polykristallina Ni-baserade superlegeringar som är lovande materialkandidater för rymdtillämpningar, särskilt i de varma sektionerna av en flygmotor där svetsning används flitigt för att tillverka de massiva heta strukturella komponenterna genom att sammanfoga små bitar av superlegeringar. VDM Alloy 780 utvecklades för att överträffa driftstemperaturkapaciteten hos Alloy 718 (upp till 750°C) samtidigt som den utmärkta bearbetbarheten hos Alloy 718 bibehölls, medan G27 utformades för att matcha, om inte överträffa, de mekaniska egenskaperna hos Waspaloy, särskilt när det gäller spricktillväxtmotstånd under uppehållströtthet.

I en superlegeringssvetsning är mikrostrukturen i svetsfusionszonen (FZ) som vanligtvis består av γ dendrit och sekundärfasbeståndsdelar som bildas på grund av elementär mikrosegregation under snabb svetsstelning en av de kritiska faktorerna som alltid styr egenskaperna hos de svetsade superlegeringarna. I det aktuella arbetet resulterar det observerade elementära mikrosegregationsmönstret som observerats i svetsen FZ av VDM Alloy 780 och G27 under stelningen av svetsen i bildandet av Nb-rika MC-karbider och grova oregelbundet formade Laves-faser som de viktigaste sekundära fasbeståndsdelarna i den interdendritiska regionen.

Efter svetsning utförs vanligtvis värmebehandling efter svetsning (PWSHT) på superlegeringssvetsning för att lindra kvarvarande spänningar och modifiera den svetsade mikrostrukturen. Med tanke på stora oregelbundna Laves-faser som är allmänt kända för att vara potentiellt skadliga för de mekaniska egenskaperna i den svetsade FZ av VDM Alloy 780 och G27, måste lämplig PWSHT utvecklas för att ta bort Laves-faserna för att uppnå optimala mikrostrukturer och tillförlitlig egenskaper hos de svetsade superlegeringarna. WSHT under en timme utförd vid 954°C eliminerar endast delvis Laves-faserna i båda legeringarna med betydande η-fasbildning längs de interdendritiska regionerna. Den överdrivna mängden η-fas behöver också noggrant övervägasnär man utvecklar lämplig PWSHT eftersom det ytterligare potentiellt kan försämra de mekaniska egenskaperna hos svetsen.

Laves-faserna är helt upplösta utan att bilda η-fas efter implementering av PWSHT vid 1060°C under 1 timme. Sålunda, med avseende på sekundärfasbeståndsdelarna i FZ, är PWSHT vid 1060°C/1h lämplig för VDM Alloy 780 och G27 eftersom den helt tar bort Lavesfaser utan η-fasbildning. Förutom att ta bort Laves-fasen är det också viktigt att förstå effekten av PWSHT vid 1060°C/1h på de nya superlegeringarnas basmetall (BM) kornstorlekar. För den allmänt använda Alloy 718 anses PWSHT vid 1060°C/1h omöjligt eftersom det resulterar i en överdriven kornförgrovning i BM, vilket minskar dess drag- och utmattningsprestanda. Baserat på resultaten i denna studie ökade PWSHT vid 1060°C/1h BM-kornstorlekarna från 50 till 63 µm i VDM Alloy 780 och 25 till 57 µm i G27. Omfattningen av korntillväxten i VDM Alloy 780 och G27 har dock visat sig vara mindre allvarlig än den i den allmänt använda Alloy 718, vilket visar att korntillväxtkinetiken i nya superlegeringar är långsammare än i Alloy 718 vid 1060°C, vilket är tros bero på pinningseffekt av Mo-rich Laves-fas i G27, och sannolikt bero på segregering av lösta ämnen vid korngränserna i VDM Alloy 780. Den långsammare kinetiken för korntillväxt vid 1060°C i VDM Alloy 780 och G27 i förhållande till Legering 718 är fördelaktig för att bevara en finare kornstruktur i BMs av VDM Alloy 780 och G27 jämfört med Alloy 718 under PWSHT som syftar till att ta bort Laves-faserna i svetsen FZ.

The influence of pulsed laser beam welding (LBW) parameters on the weld geometry and imperfections of a new nickel-based superalloy called G27 was studied by a statistical design of experiment, and the microstructures of the weld fusion zone (FZ) of the pulsed laser beam-welded G27 were characterized. No evidence of cracks is found in the FZ and heat-affected zone (HAZ). Other weld imperfections, such as undercut and underfill, were also hardly observed.

The pulse factor significantly influenced all the responses, i.e., minimum weld width (Wm), root excess weld metal, and average pore diameter, whereas welding travel speed significantly influenced Wm and root excess weld metal. Power and interaction between pulse frequency*pulse factor were statistically significant in influencing the root excess weld metal and average pore diameter, respectively. The pulse frequency and interactions between power*travel speed, power*pulse factor, power*pulse frequency, travel speed*pulse factor, and travel speed*pulse frequency did not significantly influence any response.

Microsegregation pattern that occurs during weld solidification leads to the formation of Nb-rich MC carbides and Nb-rich Laves phase as the major secondary phase constituents in the FZ of as-welded G27. The presence of brittle Laves phase requires careful consideration when developing suitable post-weld heat treatment of G27.

The influence of pulsed laser beam welding (LBW) parameters on theweld geometry and imperfections of a new nickel-based superalloy called G27 wasstudied by a statistical design of experiment, and the microstructures of the weldfusion zone (FZ) of the pulsed laser beam-welded G27 were characterized. Noevidence of cracks is found in the FZ and heat-affected zone (HAZ). Other weldimperfections, such as undercut and underfill, were also hardly observed. The pulsefactor significantly influenced all the responses, i.e., minimum weld width (Wm),root excess weld metal, and average pore diameter, whereas welding travel speedsignificantly influenced Wm and root excess weld metal. Power and interactionbetween pulse frequency*pulse factor were statistically significant in influencingthe root excess weld metal and average pore diameter, respectively. The pulsefrequency and interactions between power*travel speed, power*pulse factor,power*pulse frequency, travel speed*pulse factor, and travel speed*pulse frequencydid not significantly influence any response. Microsegregation pattern that occursduring weld solidification leads to the formation of Nb-rich MC carbides and Nbrich Laves phase as the major secondary phase constituents in the FZ of as-weldedG27. The presence of brittle Laves phase requires careful consideration whendeveloping suitable post-weld heat treatment of G27.