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