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Metal Powder Bed Fusion
PRESENTATION BY PUSHKAR DESHPANDEMEET BHATIPRATIK DESHMUKHAMAN TAKSHAK
What Metal Powder Bed Fusion based AM
Metal: Basic building blocks of todays life. Most abundantly used class of materials
Powder: Metal Powder in the Range of typically 0.1mm but various sizes are used depending on the application
Bed: It refers to a operation or work table on which the part will be manufactured
Fusion: Combining 2 or more Particles Thus Metal Powder Bed Fusion refer to the system where Metal Powder
is fused together by various methods in layers to achieve the Final Product
How Does It work?
Schematic diagram of the Selective Laser Melting (SLM) powder-bed process (Source VDI 3404)
Classification of Metal Powder Bed Processes
Based on Method of Sintering/ Heating
Direct Metal Laser Sintering (DMLS)
Electron Beam Melting (EBM)
Selective Laser Melting (SLM)
Based on Fusion Mechanism
Solid State Sintering
Liquid Phase Sintering or Partial Melting
Full Melting Chemical Induced
Binding
Powder Scattering
Blade based Roller based
Classification Based on Heating Method
Direct Metal Laser Sintering (DMLS)• *Details yet to be added
Electron Beam Melting (EBM)
Selective Laser Melting (SLM)
Example Of Application of DMLS
SpaceX ‘s Regenaratively Cooled Super Draco Engine 1st fully printed rocket
engine Made of Inconel Alloy Capable of handling 6.9
mPA
Engine Emerging from Metal Powder Bed
Various Mechanisms Of Fusion in Metal Beds
Sources: Laser Powder Bed Fusion by Kruth, Katholieke Universiteit Leuven
Click icon to add picture
COMMON METAL POWDERS
Stainless steels Austenitic steels Duplex and special stainless steels Ferritic stainless steels Martensitic stainless steels Precipitation hardening stainless
steels High-speed steels Low-alloy steels
Other alloys Binary alloys Cobalt alloys Copper alloys Diamond catalyst alloys Master alloys MCrAlY alloys Nickel alloys Super alloys Tool steels
QUERIES PERTAINING TO METAL POWDERS
Will the part operate under a pressure? Must it be leak tight? Must the part be protected from corrosion—how severe? Will the part be machined—which surfaces, what tolerances? Will the part require heat treatment—what type? Will the part be used in a high-impact-loaded application? Will the part be used in a wear application—which surfaces? Is surface finish an
important design feature, where, how to measure? Will the part be used in a thermally demanding application? Will the metal have appropriate flow properties for manufacturing?
COMMON RAW MATERIALS TABLE
Workpiece material
Density (grams/cc)
Yield strength (psi)
Tensile strength (psi) Hardness
Iron 5.2 to 7.0 5.1*103 to 2.3*104
7.3*103 to 2.9*104 40 to 70
Low alloy steel 6.3 to 7.4 1.5*104 to 2.9*104
2.00*104 to 4.4*104 60 to 100
Alloyed steel 6.8 to 7.4 2.6*104 to 8.4*104
2.9*104 to 9.4*104 60 and up
Stainless steel 6.3 to 7.6 3.6*104 to 7.3*104
4.4*104 to 8.7*104 60 and up
Bronze 5.5 to 7.5 1.1*104 to 2.9*104
1.5*104 to 4.4*104 50 to 70
Brass 7.0 to 7.9 1.1*104 to 2.9*104
1.6*104 to 3.5*104 60
PHYSICAL IMPROVED PROPERTIES COMPARISON AFTER
SINTERING• Ferrous materialsYield strength-650 N/mm2 Ultimate tensile strength-900 N/mm²Elongation-2%• Stainless steelsYield strength-640 N/mm2Ultimate tensile strength-720 N/mm2Elongation-1%(reduced ductility)
• Copper alloys-Yield strength-140N/mm2Ultimate tensile strength-240N/mm2Elongation-10-20%• Aluminum AlloysYield strength-170-320 N/mm2Ultimate tensile strength-200-320N/mm2Elongation-.2-5%
COMMON METAL POWDER PROPERTIES
DENSITY Powder fusion process can yield metal densities in excess of 99 percent of the reference density. Some materials are reported to have been fabricated at full density and some have been reported with a
spread of densities (e.g., 99.2 to 99.5 percent). Density is influenced by development of pores or entrapment of un-melted powders during processing. Hot isostatic pressing is occasionally used to improve as-fabricated densities. MECHANICAL BEHAVIOR Powder metal fusion increase in strength and decrease in ductility is expected compared with conventional
wrought alloys. Differences in fracture toughness and behavior under dynamic conditions are unknown.
COMMON METAL POWDER PROPERTIES
CRYSTALLOGRAPHIC TEXTUREDue to rapid cooling rates and directional solidification, significant crystallographic texture exists in metals made using additive manufacturing processes. The texture and its effects can be somewhat controlled by varying the scan direction during deposition. NONEQUILIBRIUM MICROSTRUCTUREMaterials produced using powder bed fusion methods have cooling rates (~103-108 K/s). At these cooling rates, several effects on metals happen including Suppression of diffusion-controlled solid-state phase transformations Formation of supersaturated solutions and non-equilibrium phases Formation of extremely fine refined microstructures with little elemental segregation Formation of very fine second-phase particles such as inclusions and carbides."
PROCESS COMPARISON IN POWDER BED FUSION
ELECTRON BEAM MELTING BUILD TEMPERATURE OF THE 200DEGREES HIGHLY POROUS STRUCTURES OBTAINED METAL POWDER IS PRE-SINTERED SO TOUGH
TO REMOVE CARRIED OUT UNDER VACUUM METAL CONDUCTIVITY RELATED MODERATE SURFACE FINISH
Laser sintering INITIAL TEMPERATURE OF THE ORDER OF 680-
720DEGREES MICROSTRUCTURE VERY RIGID AND FREE
FROM INCLUSIONS OR POROSITY POWDER REMOVAL IS EASY CARRIED OUT UNDER NOBLE GAS METAL ABSORPTIVITY RELATED GOOD SURFACE FINISH
ELECTRON BEAM EXPOSURE TO METAL POWDERS
Pre-sintering of metal powders leads to high temperatures leading to powder sticking
Pressure drop due to velocity variations leads to porosity
Density of the material will increase with increase in line energy(estimate of laser power)
High Thermal conductivity can lead to difficulty in manufacturing
Highly porous structures are highly promising for the design and development of novel structured supports.
LASER POWER EXPOSURE ON METAL POWDER
LASER SINTERING SUCH AS SPOT SIZE ,DIVERGENCE ANGLE AND LASER POWER VARY METAL POWDER PROPERTIES
SPOT SIZE=BEAM DIAMETER DIFFERENCE/GAP BETWEEN LAYERS
SMALLER SPOT SIZE RESULTS IN BETTER SINTERING DIVERGENCE ANGLE=WAVELENGTH OF BEAM/3.14*BEAM
RADIUSHIGHER POWDER LAYER THICKNESS LEADS TO LARGER DIVERGENCE ANGLE PROPERTIES SUCH AS HIGH THERMAL CONDUCTIVITY, HIGH
SURFACE TENSION AND REFLECTIITY REQUIRES HIGHER BEAM POWER
METAL ABSORTIVITY TUNED WITH LASER WAVELENGTH TO SELECT BEAM INTENSITY FOR METAL POWDER
POWDERBED FUSION FACTORS AFFECTING METAL POWDERS
DIFFERENT PBF FACTORS IN THE PROCESS MIGHT AFFECT DIFFERENT METAL POWDER PROPERTIES
POWDER SPREADING LASER POWER SCANNING SPEED POWDER LAYER THICKNESS PREHEATING
POWDER SPREADING EFFECT ON METAL PROPERTIES
INHOMOGENUOUS LAYER SPREADING OVER THE ENTIRE AREA RESULTS IN POROSITY. RELATIVE DENSITY IS THE PRIMARY PARAMATER FOR CHARACTERIZING SPREADING PROCESS HIGHER POWDER LAYER THICKNESS DECREASES THE SINTERING DEPTH AND DENSITY
RELATIVE DENSITY=BULK DENSITY OF COMPACTED POWDER/POWDER MATERIAL DENSITY Higher relative density is reduction in porosity and to tough to machine under A.M. process
SCANNING SPEED EFFECT ON METAL POWDER PROPERTIES
HIGHER SCANNING SPEED LEADS TO INCREASE IN YOUNG’S MODULUS
INCREASING SCANNING SPEED WILL BRING DOWN THE RELATIVE DENSITY AT CONSTANT LASER POWER
HIGHER SCANNING SPEED RESULTS IN FINER MICROSTRUCTURE INCREASING SCANNING SPEED WOULD INCREASE THE COOLING
RATE AND THE THERMAL GRADIENT CONSTANTLY VARYING SCANNING SPEEDS LEAD TO UNEVEN
HEATING OF METAL POWDERS RESULTING IN INTERNAL STRESSES
EFFECT OF LASER POWER ON METAL POWDER PROPERTIES
HIGH LASER POWER AT CONSTANT SCANNING SPEED LEADS TO BETTER FUSION OF POWDERS
INCREASING LASER POWER, DENSITY OF COMPACTED POWDER INCREASES AND HENCE THE RELATIVE DENSITY
EFFECT OF PRE-HEATING ON METAL POWDER PROPERTIES
UNEVEN HEATING DURING THE SINTERING PROCESS, LAYERS LEADS TO INTERNAL STRESS AND WARPAGE OF SINTERING PARTS.
PREHEATING INCREASES INTENSION AND DENSITY OF SINTERING PARTS PREHEATING ECONOMIZES LASER POWER AND ENHANCE THE SCANNING VELOCITY. WHEN THE MATERIALS ARE PREHEATED, THE DENSITY AND THERMAL RESISTANCE INCREASE, IT IS
EASIER TO FORM LAYER ADHESION. THE PREHEATING TEMPERATURE IS DECIDED BY THE MATERIALS, HIGHER PREHEATING
TEMPERATURE MAY HARDEN OR CARBONIZE THE POWDER LAYER
POWDER LAYER THICKNESS EFECT ON POWDER PROPERTIES
THE SINTERING DENSITY DECREASE WITH THE INCREASING OF THE POWDER LAYER THICKNESS.
HIGHER POWDER LAYER THICKNESS CAN OBTAIN GOOD LAYER JUNCTURE.
THE POWDER LAYER THICKNESS IS TOO SMALL TO SPREADING THE POWDER.
HIGHER POWER LAYER THICKNESS REQUIRES HIGHER LASER INPUT AND SCANNING SPEED
COMMONLY USED ALLOYS STRENGTH COMPARISON
PROCESSES FOR DETERMINING METAL POWDER PROPERTIES
SIZE DETERMINATION MORPHOLOGY FLOW PROPERTIES THERMAL PROPERTIES CHEMICAL PROPERTIES
SIZE DETERMINATION METHODS
Laser light diffraction method(size information) Gravitational sedimentation method(determine particle
concentration) Sieving Microscopy method using microscopes such as TEM and SEM Smaller particles
SIZE EFFECT ON PROPERTIES
THE SIZE AND SHAPE DISTRIBUTION OF THE METAL PARTICLES IMPACTS POWDER BEHAVIOR DURING SINTERING
SPHERICAL PARTICLES ARE PREFERRED DUE TO POWDER FLOW AND IMPROVED COMPACTION LEADING TO STRUCTURALLY SOUND PARTS.
MORE SPHERICAL PARTICLES REDUCE INTER-PARTICLE FRICTION – A MAJOR CONCERN WITH POWDER METALLURGISTS.
LARGER PARTICLES MAY OFFER REDUCED PRODUCT PERFORMANCE WITH LITTLE SAVINGS IN RAW MATERIAL COST.
NANOSCALE POWDERS PROVE TOO COSTLY TO BUY AND USE ALTHOUGH THEY EXHIBIT IMPROVED HARDNESS AND STRENGTH
MORPHOLOGY PROPERTY DETERMINES HOW WELL THE PARTICLES LAY OR PACK
TOGETHER IMPORTANT FACTOR IN REALIZING MINIMUM PART LAYER
THICKNESS AND DENSITY. MORPHOLOGY ASCERTAINS ON THE POWDER BED
PACKING DENSITY AND CONSEQUENTLY ON THE FINAL COMPONENT DENSITY
SPHERICAL, REGULAR OR EQUIAXED PARTICLES ARE LIKELY TO ARRANGE AND PACK MORE EFFICIENTLY THAN IRREGULAR PARTICLES
HIGHLY SPHERICAL PARTICLES TEND TO BE FAVOURED IN THE AM PROCESS LIMITING USE OF CHEAPER POWDER PRODUCTION ROUTES.
FLOW PROPERTIES
MEASURE OF HOW WELL A POWDER FLOWS SPHERICAL PARTICLES ARE GENERALLY MORE FREE FLOWING THAN IRREGULAR OR ANGULAR
PARTICLES PARTICLE SIZE HAS A SIGNIFICANT INFLUENCE ON FLOW – LARGER PARTICLES ARE GENERALLY
MORE FREE FLOWING THAN SMALLER PARTICLES MOISTURE CONTENT IN POWDERS CAN REDUCE FLOW DUE TO CAPILLARY FORCES ACTING
BETWEEN PARTICLES SHORT RANGE ATTRACTIVE FORCES SUCH AS VAN DER WAALS FORCES AND ELECTROSTATIC
FORCES ADVERSELY AFFECT POWDER FLOW AND MAY CAUSE PARTICLE AGGLOMERATION (SHORT RANGE FORCES HAVE A BIGGER IMPACT ON FINER PARTICLES).
PARTICLE SIZE HAS A SIGNIFICANT INFLUENCE ON FLOW – LARGER PARTICLES ARE GENERALLY MORE FREE FLOWING THAN SMALLER PARTICLES
THERMAL PROPERTIES
HIGHER THERMAL CONDUCTIVITY LEADS TO MORE MACHINING TIME UNDER P.B.F. PROCESS. HIGH-POWER LASER IRRADIATION LEADS TO FRAGMENTATION OF METAL POWDER WITH A
SIGNIFICANT INCREASE IN THE EFFECTIVE THERMAL CONDUCTIVITY
CONDUCTIVITY IS DIRECTLY AFFECTED BY POROSITY THE GREATER THE VOID CONTENT, THE LOWER THE CONDUCTIVITY.
SINCE THE CONDUCTIVITY OF A PORE IS ZERO, THE RELATIONSHIP BETWEEN POROSITY AND CONDUCTIVITY IS GIVEN BY THE EQUATION:
K = Ks(1-F)
K = THERMAL CONDUCTIVITY OF THE P/M PART Ks = INTRINSIC THERMAL OR ELECTRICAL CONDUCTIVITY OF THE MASSIVE METAL F = FRACTIONAL POROSITY
CHEMICAL ANALYSIS
THE LASER SINTERING BEHAVIOR OF A METAL POWDER WILL NOT ONLY DEPEND ON THE PHYSICAL PROPERTIES, IT WILL OF COURSE ALSO DEPEND ON THE CHEMICAL PROPERTIES POWDER CHEMICAL COMPOSITION FOR PBF SHOULD IDEALLY BE OPTIMIZED FOR THE
MACHINE . EFFECT OF INTERSTITIAL ELEMENTS, SUCH AS O2 AND N, SINCE COMPONENT PROPERTIES
WILL DEPEND ON THE AMOUNT OF INTERSTITIAL ELEMENTS PRESENT. TENSILE STRENGTH AND DUCTILITY IS INFLUENCED BY OXYGEN CONTENT INCREASE IN OXYGEN RESULTS IN AN INCREASE IN TENSILE STRENGTH AND DECREASE IN
ELONGATION. INTERSTITIAL ELEMENTS CAN INFLUENCE THE MELTING KINETICS OF THE POWDER BY
INTERFERING WITH THE SURFACE TENSION OF THE MELT POOL RESULTING POROSITY.
POWDER HANDLING CHALLENGES
POWDER SPREADING TO BE SMOOTH TO FORM SMOOTH ,THIN AND REPEATABLE LAYER OF POWDER
CORRECT VOLUME TRANSFER FROM RESERVOIR TO BUILD PLATFORM WITHOUT ANY WASTAGE
POWDER SPREADING SHOULD NOT CREATE ANY SHEAR FORCES THAT MIGHT DISTURB THE PREVIOUSLY PROCESSED LAYERS
METAL POWDER SIZE HOULD BE CALIBRATED PROPERLY ACCORDING TO APPICATION.FOR INSTANCE SMALL SIZE POWDERS AVOID FLOATING AWAY ASS WELL AS ENSURE BETTER SURFACE FINISH.
COMMON DEFECTS WITH METAL POWDERS
POROSITY• UNEVEN HEATING OF SURFACE• TOO MUCH PARTICLE LAYER THICKNESS OR PARTICLE SPACING INTERNAL STRESSES• IN-COHERENT TENSILE OR COMPRESSIVR STRESSES• INCLUSION OF PARTICLES SUCH AS HYDROGEN SULPHIDE CORROSION • CHEMICAL ATTACK FROM GASES, SOLID OR MOLTEN SALTS, OR MOLTEN METALS• LOACLIZED CORROSION ATTACKS IN THE FORM OF SPOTS OR PITS. PITTING CORROSION MAY
OCCUR IN STAINLESS STEELS IN NEUTRAL OR ACID SOLUTIONS
Process parameters
For PBF process parameter is divided in 4 sub categories Scanning related parameters Laser related parameters Powder related parameters Temperature related parameters
Scanning related parameters
Problems with Scanning If object is moved within machine laser path to
build the object structure may change This causes distortion in the part This may cause the part to build properly in one
direction and may distort in other
Laser Related parameters
Laser power is dependent on melting point of material Larger melting point requires greater laser power and vice versa Laser power depends on absorptivity character of powder Laser power, spot size, scan speed and bed temp determine energy
input to fuse material Longer the laser dwelling deeper the fusion Lower laser power requires lower scan speed
Major types of laser systems
PBF uses two types of laser system Continuous Wave(CW) [Mostly used] – produces continuous beam of
laser Pulsed laser – laser appears in form of pulse Pulsed laser has a benefit of overcoming problem of disconnected balls
of molten metal Modern machine will use both
Powder related parameters
Powder size, shape and distribution influence laser absorptivity Finer particle provide greater surface area and absorbs laser more
efficiently Powder bed density typically range from 50-60% Higher the powder packing density and bed conductivity better the
mechanical properties
Temperature related parameters
High bed temperature with high laser power creates dense parts but causes part growth and poor recyclability
Low bed temperature with low laser power gives better dimensional accuracy but causes low density and layer delamination
Powder bed temperature should be uniform and consant to achieve reputable results
Energy Consideration
E = P/(U x SP) – Simplest equation to calculate energy Other characteristics like powder absorptivity, heat of fusion, laser spot
size, and bed temperature are important to calculate energy. Improper combination of these parameters transmits improper amount
of energy This leads to an effect called ‘balling’ in metal pool
Balling Effect
Powder handling
Powder Handling system must follow 4 charcteristics Should have sufficient powder reservoir to continue the process without
stopping machine Correct volume should be transported from powder reservoir to build
platform Powder must be spread properly Powder spreading should not create excessive shear force to disturb previous
layer
Small Powder Particles – Friend or enemy ?
Small particle causes more inter particle friction and electrostatic forces. This causes reduction in flow ability
When particle is small is surface area to volume increases, thus particle become more reactive.
Small particle tends to become airborne and float as cloud of particles thus reducing the sensitivity of sensors
Smaller particle enables better accuracy, good surface finish and thinner layer but carries with it all the above mentioned problems.
Powder Recycling Systems
Unus
ed
Overflow
Build platform
Case Study
Conclusion : To minimize stress optimal process parameters must be
selected Higher densification higher residual stress Lowest deflection was found in squares and stripes