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HPAM - Moataz - AM of Ni Superalloys - Coggle Diagram
HPAM - Moataz - AM of Ni Superalloys
Fundamentals
Ni mainly formed of gamma and gamma prime
Both phases are FCC
Gamma prime has chemical composition of Ni3Al, and is formed of two superimposed primitive cubic lattices that form an FCC structure - forms coherent strengthening precipitates with surrounding gamma matrix
Gamma prime is highly ordered phase, maintains its order up to 1385 C
As the fraction of gamma prime of gamma double prime increases, so does the stress rupture strength of the Ni superalloy
Ppt strengthening
Coherent - joined with surrounding matrix, impart stress on surrounding matrix, have strengthening effect
Incoherent - no bonds to surrounding matrix - foreign particles - no strain on surrounding lattice, do not impede flow of dislocations, no strengthening effect, act as stress concentrators.
Solution treating = dissolve all phases, even chemical distribution (homogenisation)
Ageing - Holding a solution treated material at a given temperature so that ppt.s form
Underaged - small and underdeveloped ppt.s
Peak aged, optimum size and distribution of precipitates for strengthening
Overaged - coarsening of precipitates
Welding
Weldability decreases with increasing Ti and Al contents - increased strain age cracking with increase gamma prime levels
Solidification cracking (hot tearing) - Occurs during solidification when the surrounding cooling material exerts a tensile stress. The high solid fraction prevents the backfilling of interdendritic regions which act as crack initiation points.
Liquation cracking - occurs during rapid heating of solid material away from melt pool below the liquidus temperature. Grain boundary gamma and gamma prime (glassy grain boundary phases) eutectic material melts. The liquid films can act as a crack initiation point under the tensile stress. Rapid heating does not allow time for precipitates to dissolve - they melt instead
Strain age cracking- results from reheating material to the ageing region. Existing residual stress combines with stress induced from shrinkage with gamma prime precipitation and reduced ductility to form cracks
Mechanism not known - occurs due to dip in ductility in temperature range 0.4 - 0.9 of Tm. The reduction in material coupled with the thermal stresses induced by the could be responsible for some of the cracking in the material fabrication process. Can be imaged using laser scanning microscopy. Want to have bendy grain boundaries with precipitates in them - precipitates prevent accumulations of strain concentrations
Welding map
Top left (high power, low tool speed) - solidification cracking
Low tool power and speed - not fully penetrating
Top right - high tool power and tool speed - liquation cracking
Defects
Hot tearing are essentially just solidification cracks that are viewable on a macroscopic scale
Carbides form at grain boundaries (Co, Cr) - ideally want short, numerous carbides rather than long, continuous ones - makes crack propagation much more difficult
Voids formed from incomplete powder melting/ poor wetting of previous build layer
Solidification cracks do not show any particular directionality - form due to large accumulation of heat (high tool power, low tool speed)
SLM will result in pores of 10-50 um size, similar to the size of the pores in powder. Post fabrication HIPing has completely closed the pores in the microstructure.
HIPing results in coarsening the alpha / alpha prime microstructure, slightly reducing the tensile strength. However, improves ductility. HIPing can also close cracks
Energy density model correlates well with solidification behaviour (closure of pores) but it does not correlate with the formation of cracks. The general trend shown by the data obtained from the IN625 is that porosity decreases with increased energy density
Structural integrity
Absence of structural defects
Low residual stresses
Microstructual homogeneity (isotropy)
Mechanical properties isotropy
Inherent material susceptibility to cracking
All of the 4 different types of cracking
Solubility of gases (e.g. hydrogen in Al - gas pore formation)
Susceptibility to residual stress formation
Powder quality or inherent porosity in powders
Insufficient thermal conduction - lack of consolidation
Evaporation of liquid metal when tool power too high - formation of key holes
Insufficient remelting
HIPing
Closing of micropores, enhancing properties, as in casting
Adds cost, increases lead time, disturbs the supply chain
May not help too much with high temperature performance due to thermally induced pores
Limitations
Surface connected cracking remains following the treatment.
The amount of surface cracking remaining depends on the component geometry and orientation with respect to the build direction. The slow cooling necessary within the HIP vessel leaves the material in an overaged state showing coarse gamma - not the ideal cuboidal structure