Technology of full density powder materials and products Professor Jakob Kübarsepp InstituteofMaterialsEngineering Tallinn UniversityofTechnology
Introduction: fulldensityprocessing • Hot pressing • Hotisostatic pressing • Sinter HIP • Powderextrusion • Powderforging • Alternativehotconsolidationprocesses
1. Introduction: full density processing • Powder metallurgy (P/M) is the most diverse manufacturing approach among various metalworking technologies. • There are three main reasons (see Fig.) for using powder metallurgy - the art and science of producing powders and of utilizing powders for the production of massive materials and shaped products
Introduction: full density processing (cntd. 1) Influence of porosity (density) on impact strength and tensile strength of P/M materials.
Introduction: full density processing (cntd. 2) The effect of porosity on fatique strength
Introduction: full density processing (cntd. 3) • Permissible porosity depends on application field • Higher loads call for higher density • Conventional P/M technology (press- and -sinter technology) in most cases doesn`t enable to achieve full density. • Hot consolidation processes (hot pressing, HIP, extrusion etc) enable to produce full-density or near full-density or near full-density powder materials/products
Introduction: full density processing (cntd. 4) High performance applications require higher densities The symbols are: P/M = press and sinter reP = press, sinter and repress P/S+F = press, sinter and forge CIP+S= cold isostatically press and sinter HIP = hot isostatically press HIP+F = hot isostatically press and forge Three variables influencing P/M processing methods – component size, density and performance (as a percentage of wrought
Introduction: full density processing (cntd. 5) • Challenge of P/M technology – production of net-shape (with final dimensions) and full density materials/products • Utilizing full density processing the performance levels can exceed (see Fig.) those associated with wrought materials (if control is gained over defects, impurities, microstructure and product homogeneity) The density effect on the strength and ductility of a forged 4640 steel
Introduction: full density processing (cntd. 6) Liquid phase sintering (LPS) is widely employed to obtain full density products without application of simultaneus pressure during sintering. Applications of LPS: W-Ni-Fe heavy alloys, WC-Co cemented carbides, TiC-Fe cermets, Co5Sm-base magnets etc. Infiltration – other technique for obtaining full density (see section 7.9). Typical microstructure of a liquid-phase sintering and infiltration system Phase diagram of an ideal system for liquid-phase sintering
Introduction: full density processing (cntd. 7) • The traditional P/M cycle involves the sequential application of pressure (to compact powder) and temperature (to sinter the powder). Many full density processes involve simultaneous heating and pressurization. • The simultaneous heating and pressurization add cost and complexity that are best justified by increased performance.
Introduction: full density processing (cntd. 8) Thefourconcernsdiscussinghotconsolidationofpowders are: • Temperature:fulldensityprocessingisnormallyeffective at T>0.5Ts (Ts–melting temp.) • Stress:onlyfulldensitydoestheeffective stress at theinterparticlecontactsequaltoapplied stress (becauseporesactas stress concentrators) • Strain:largeshearstrainsdisruptsurfacefilms on theparticles, butcontributeto tool wear. • Strainrate: • highstrainratereducesductility (fractureismorelikely) • lowstrainrategivesmoreplasticdeformationwith a higherfinaldensity
Introduction: full density processing (cntd. 9) Threemechanismsofporeelimination: Plasticyielding– occurswhentheeffective stress exceedsthematerialyieldstrength Powerlawcreep– occurswhenboththe stress and temperature are high and rateofdensificationdepends on thediffusionratefordislocationclimb Lattice and grainboundarydiffusion– occurswhenthediffusivityishighlysensitivetotemperature and thetemperaturehasthemostimportantinfluence on thedensificationrate Initial and intermediate stages of densification for spherical powders
Introduction: full density processing (cntd. 10) Application fields of full density or near-full density powder materials • Structural ceramics • High-temperature composites • Metal-bonded diamond tools • P/M high speed steel tools • Products from refractory metals • Products from Ni-base superalloys and corrosion resistant alloys • Products from low-alloy steels etc.
2. Hot pressing2.1 Process • Uniaxial hot pressing = pressing on sintering temperature • Uniaxial pressing in graphite die at pressures up to 50 MPa and temperatures up to 2200C in controlled atmosphere (Ar, N2, vacuum)
Hot pressing (cntd. 1) Hot pressing is the simplest hot consolition technique: Temperature: up to 2200oC Time: 1…3 hours Pressure: up to 50 MPa Die material: graphite, refractory metals, ceramics
Hot pressing (cntd. 2) Mechanism of pore elimination (compaction): • particle rearrangement and plastic yielding/plastic flow (at point contacts) • grain boundary and lattice diffusion (as densification progresses)
Hot pressing (cntd. 3) Hot pressing can be used for consolidation of different material combinations: • single powders or powder mix • alternating layers of powders (for production of laminated composites) • mix of powder and whiskers • powders + thin sheets of metal/ceramics • sheets of continous fibers with powders (plasma sprayed on fibers) • powder cloth + fiber mats etc. Depending on the material the lay-up sequence will vary significantly.
Hot pressing (cntd. 4)2.2 Applications • Large parts/products from WC-Co hardmetals • Products from Be alloys in nuclear reactors, missile and aerospace applications • Diamond-metal composite cutting tools (e.g diamond+Co) • Micro Infiltrated Macro Laminated Composites – MIMLC • Fiber-reinforced materials • Sputtering targets for deposition of thin films etc.
Hot pressing (cntd. 5) Ceramic materials are characterized by extremely low fracture toughness. Efforts to improve the toughness include: • Incorporation of particulates • Incorporation of whiskers or fibers • Cermet (ceramic/metal composite) technology in which the tough metallic component absorbs energy • Inclusion of a phase which undergoes tranformation due to the stress field associated with a crack (“transformation toughening”). Transformation leads to volume expansion producing compressive stress on the crack tip • Utilization of the concept of compositional gradient – MIMLC-type materials
Hot pressing (cntd. 6) Improved toughness of ceramic materials and composites can be attributed to: • crack branching and crack deflection • crack bridging (e.g in cermets where metal matrix ligaments effectively bridge the macroscopic crack plane) • fiber of whiscers matrix decohesion and pullout (e.g in fiber/whiscers reinforced ceramic matrix composites) • plastic deformation = one of the best mathods of improving fracture toughess of a material (e.g in MIMLC-type materials)
Hot pressing (cntd. 7)Conceptual MIMLC structure Soft, ductile, low modulus high toughness phase Hard, brittle, high modulus low toughness phase Schematic view of a conceptual MIMLC structure
Hot pressing (cntd. 8)MIMLC – gradient material: • On a macro scale is laminated with alternating layers of hard and brittle ceramic-like material and ductile and tough metal-like layer • On a micro scale the ceramic-like material is inter-penetrated with the ductile and tough metal like material.
Hot pressing (cntd. 9) Improved toughness of MIMLC: can be attributed to: • Increased resistance (higher than that of ceramic-metal composites) to fracture due to energy absorbtion via plastic deformation • Crack bridging in plastic zone of a composite
Hot pressing (cntd. 11) MIMLC: Al2O3-Ni grandient material One of the prime uses is material against: • ballistic penetration • hyper-velocity impacts
Hot pressing (cntd. 12) Hot pressing of Al2O3-Ni gradient material MIMLC composite lay up assembly (for production of Al2O3-Ni gradient material)
Hot pressing (cntd. 13) Schematic processing sequence for producing the fiber reinforced glass and glass-ceramic matrix composite by hot pressing 1000oC, 300 bar
Hot pressing (cntd. 14) A typical stress-strain curve for fiber reinforced glass matrix composite
Hot pressing (cntd. 15) Hot pressing ofcompositesbyutilizingin-situdisplacementreactionsynthesis: Examplesofin-situsynthesized (high-temperature) composites: • Mo2C + 5Si = 2MoSi2+SiC (MoSi2 – basedcompositereinforcedbySiCparticles) 2) 4.5NiAl + 3NiO = 2.5Ni3Al + Al2O3, or 12NiAl + 3NiO = 7.5NiAl + 2.5Ni3Al + Al2O3 (Al2O3reinforcedNi-Alcomposite)
3. Hot isostatic pressing (HIP)3.1 Process • HIP = high-temperature pressing (hot consolidation) of encapsulated powders or sintered high-density (density >92% of theoretical) compacts. • Pressure: 100...300 MPa • Time: 2...6 hours • HIP-low strain rate process because stress rise in slow
Hot isostatic pressing (HIP) (cntd. 1)Preparative steps of HIP: • Encapsulation: • containerproduction • leaktesting • filling 2. Degassing (evacuationofabsorbedgases and moisture) and crimpingofthecan
Hot isostatic pressing (HIP) (cntd. 2)Typical HIP cycles: I HIP in unit with internal heating zone II HIP of externally heated encapsulated part
Hot isostatic pressing (HIP) (cntd. 3) A cross section view of the HIP vessel
Hot isostatic pressing (HIP) (cntd. 4)Recent developments of HIP equipment: • Increased Cooling HIP systems (providing productivity and decrease in cost) • Ultra High Pressure and Temperature Systems • pressures up to 1000 MPa • Temperatures up to 3000C 3. Duplex systems for HIP, WIP, CIP (hot isostatic pressing, warm isostatic pressing, cold isostatic pressing)
Hot isostatic pressing (HIP) (cntd. 5)Advantages of HIP: • Materials/products of higher performance • Cost savings because of • lower lifecycle cost (of higher performance materials) • the HIP near-net shape process • lower unit costs of large parts and production volumes of small-weight parts
Hot isostatic pressing (HIP) (cntd. 6)Disadvantages of HIP: • littleshear on theparticlesurfaces (becauseofconsolidationbyhydrostatic stress). Therefore: • itisnecessary (insomeapplications) tosubjectHIP-compacttopost-consolidationdeformationor • themicrostructuraldefectscanbeminimizedbyuseofpowdersatomizedwith rapid solidificationrate and cleanhandlingininertgas
Hot isostatic pressing (HIP) (cntd. 7)3.2 Mechanism of consolidation • Full density is achieved more easily under the conditions of higher packing density • Packing density of commercial spherical powders vary from 0.62...0.72 • Mostly pre-alloyed powders are used
Hot isostatic pressing (HIP) (cntd. 8) Two models of the HIP process • Microscopic model: calculates density as a function of the process variables: time, pressure, temperature, initial packing density, material properties. Model predicts density. • Macroscopic model: shape change under pressure is analyzed utilizing plastic theory of porous metals. Macroscopic models are tools to predict the size and shape change (important in production of net-shape parts).
Hot isostatic pressing (HIP) (cntd. 9) Microscopic model: three stages of consolidation process 0) stage of packing density • stage of connected porosity • stage of closed porosity
Hot isostatic pressing (HIP) (cntd. 10) Microscopic model: • HIP maps (density vs temperaturemaps) at constantpressure and initialpackingdensity • Twomechanismsof HIP densification: • Plasticyielding • Power-lawcreep (the dominant mechanismofdensification)
Hot isostatic pressing (HIP) (cntd. 11) Macroscopic model: A comparison of the predicted shape change for axisymmetric product to the actual shape change after HIP
Hot isostatic pressing (HIP) (cntd. 12) 3.3 Application fields: 1. P/M tool steels: • high speed steels • cold work tool steels (0.8…3.9% C) • wear and corrosion resistant steels (1.7…3.7% C) • HIP/clad products (see section 7.10) 2. Ni-base superalloys 3. Corrosion resistant Ni- and Fe-based alloys 4. Titanium and titanium aluminide 5. Metal matrix composites – MMC 6. Hardmetals and cermets etc.
Hot isostatic pressing (HIP) (cntd. 13) Characterization of P/M tool steels: • Microstructure is characterized by fine and evenly distributed carbides which improve toughness, wear resistance, grindability, workability • High-carbon (up to 3.9%) and high-carbide-volume (up to 45 vol%) steels • The largest tonnage application at about 12 000 tons worldwide
Hot isostatic pressing (HIP) (cntd. 14) • All of the P/M tool and high speed steels have: • fine and evenly distributed carbides • improved toughness and wear resistance • improved technological properties such as hot workability, machineability (in annealed condition), more uniform heat treatment response in large section, predictable size change after heat treatment Primary carbides in heat treated P/M T15 (left) and conventional T15 (right) high-speed steels
Hot isostatic pressing (HIP) (cntd. 15) HIP fabrication of metal matrix composites Ti/SiC
4. Sinter HIP • Disadvantagesof HIP: • Encapsulation prior HIP-ingisasexpensive and timeconsumingstep • Canninginmostcasesprecludesthefabricationofcompexshapes • Sinter HIP techniques (Sinter+HIP, Sinter/HIPorSinter-HIPetc.) enabletoproducecomplex and net shapefulldensityproducts
Sinter HIP (cntd. 1)4.1 Sinter + HIP BothSinter + HIP and Sinter/HIP = containerless pressure-assisted consolidation technique of sintered parts (with porosity below 8%)
Sinter HIP (cntd. 2)Different technological schemes of Sinter+HIP: • Sinter + HIP in different cycles • Sinter + HIP in the same cycle eliminating extra sintering and cooling step • CHIP = CIP/Sinter + HIP • PIM/Sinter + HIP where: CIP – cold isostatic pressing PIM – powder injection molding
Sinter HIP (cntd. 4) • The main advantages of Sinter+HIP technology are: • possibility to produce complex and near net shape products • possibility to achieve full density • The most widely used process for consolidation of complex in shape products are : • CHIP = CIP + Sinter + HIP • PIM/Sinter + HIP = PIM + Sinter + HIP