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Technology of full density powder materials and products Professor Jakob Kübarsepp Institute of Materials Engineering Tallinn University of Technology.

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Presentation on theme: "Technology of full density powder materials and products Professor Jakob Kübarsepp Institute of Materials Engineering Tallinn University of Technology."— Presentation transcript:

1 Technology of full density powder materials and products Professor Jakob Kübarsepp Institute of Materials Engineering Tallinn University of Technology

2 1.Introduction: full density processing 2.Hot pressing 3.Hot isostatic pressing 4.Sinter HIP 5.Powder extrusion 6.Powder forging 7.Alternative hot consolidation processes

3 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

4 Introduction: full density processing (cntd. 1) Influence of porosity (density) on impact strength and tensile strength of P/M materials.

5 Introduction: full density processing (cntd. 2) The effect of porosity on fatique strength

6 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

7 Introduction: full density processing (cntd. 4) Three variables influencing P/M processing methods – component size, density and performance (as a percentage of wrought 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

8 Introduction: full density processing (cntd. 5) The density effect on the strength and ductility of a forged 4640 steel 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)

9 Introduction: full density processing (cntd. 6) Phase diagram of an ideal system for liquid-phase sintering Typical microstructure of a liquid-phase sintering and infiltration system 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, Co 5 Sm-base magnets etc. Infiltration – other technique for obtaining full density (see section 7.9).

10 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.

11 Introduction: full density processing (cntd. 8) The four concerns discussing hot consolidation of powders are: 1)Temperature: full density processing is normally effective at T>0.5T s (T s – melting temp.) 2)Stress: only full density does the effective stress at the interparticle contacts equal to applied stress (because pores act as stress concentrators) 3)Strain: large shear strains disrupt surface films on the particles, but contribute to tool wear. 4)Strain rate:  high strain rate reduces ductility (fracture is more likely)  low strain rate gives more plastic deformation with a higher final density

12 Introduction: full density processing (cntd. 9) Initial and intermediate stages of densification for spherical powders Three mechanisms of pore elimination: 1)Plastic yielding – occurs when the effective stress exceeds the material yield strength 2)Power law creep – occurs when both the stress and temperature are high and rate of densification depends on the diffusion rate for dislocation climb 3)Lattice and grain boundary diffusion – occurs when the diffusivity is highly sensitive to temperature and the temperature has the most important influence on the densification rate

13 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.

14 2. Hot pressing 2.1 Process Uniaxial hot pressing = pressing on sintering temperature Uniaxial pressing in graphite die at pressures up to 50 MPa and temperatures up to 2200  C in controlled atmosphere (Ar, N 2, vacuum)

15 Hot pressing (cntd. 1) Temperature: up to 2200 o C Time: 1…3 hours Pressure: up to 50 MPa Die material: graphite, refractory metals, ceramics Hot pressing is the simplest hot consolition technique:

16 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)

17 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.

18 Hot pressing (cntd. 4) 2.2 Applications 1.Large parts/products from WC-Co hardmetals 2.Products from Be alloys in nuclear reactors, missile and aerospace applications 3.Diamond-metal composite cutting tools (e.g diamond+Co) 4.Micro Infiltrated Macro Laminated Composites – MIMLC 5.Fiber-reinforced materials 6.Sputtering targets for deposition of thin films etc.

19 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

20 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)

21 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

22 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.

23 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

24 Hot pressing (cntd. 10) Example of MIMLC:W-Ni-Fe grandient material

25 Hot pressing (cntd. 11) MIMLC: Al 2 O 3 -Ni grandient material One of the prime uses is material against:  ballistic penetration  hyper-velocity impacts

26 Hot pressing (cntd. 12) Hot pressing of Al 2 O 3 -Ni gradient material MIMLC composite lay up assembly (for production of Al 2 O 3 -Ni gradient material)

27 Hot pressing (cntd. 13) Schematic processing sequence for producing the fiber reinforced glass and glass-ceramic matrix composite by hot pressing 1000 o C, 300 bar

28 Hot pressing (cntd. 14) A typical stress-strain curve for fiber reinforced glass matrix composite

29 Hot pressing (cntd. 15) Hot pressing of composites by utilizing in-situ displacement reaction synthesis: Examples of in-situ synthesized (high-temperature) composites: 1)Mo 2 C + 5Si = 2MoSi 2 +SiC (MoSi 2 – based composite reinforced by SiC particles) 2) 4.5NiAl + 3NiO = 2.5Ni 3 Al + Al 2 O 3, or 12NiAl + 3NiO = 7.5NiAl + 2.5Ni 3 Al + Al 2 O 3 (Al 2 O 3 reinforced Ni-Al composite)

30 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: MPa Time: hours HIP-low strain rate process because stress rise in slow

31 Hot isostatic pressing (HIP) (cntd. 1) Preparative steps of HIP: 1.Encapsulation: container production leak testing filling 2. Degassing (evacuation of absorbed gases and moisture) and crimping of the can

32 Hot isostatic pressing (HIP) (cntd. 2) Typical HIP cycles: I HIP in unit with internal heating zone II HIP of externally heated encapsulated part

33 Hot isostatic pressing (HIP) (cntd. 3) A cross section view of the HIP vessel

34 Hot isostatic pressing (HIP) (cntd. 4) Recent developments of HIP equipment: 1.Increased Cooling HIP systems (providing productivity and decrease in cost) 2.Ultra High Pressure and Temperature Systems pressures up to 1000 MPa Temperatures up to 3000  C 3. Duplex systems for HIP, WIP, CIP (hot isostatic pressing, warm isostatic pressing, cold isostatic pressing)

35 Hot isostatic pressing (HIP) (cntd. 5) Advantages of HIP: 1.Materials/products of higher performance 2.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

36 Hot isostatic pressing (HIP) (cntd. 6) Disadvantages of HIP: little shear on the particle surfaces (because of consolidation by hydrostatic stress). Therefore: 1)it is necessary (in some applications) to subject HIP-compact to post-consolidation deformation or 2)the microstructural defects can be minimized by use of powders atomized with rapid solidification rate and clean handling in inert gas

37 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 Mostly pre-alloyed powders are used

38 Hot isostatic pressing (HIP) (cntd. 8) Two models of the HIP process 1.Microscopic model: calculates density as a function of the process variables: time, pressure, temperature, initial packing density, material properties. Model predicts density. 2.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).

39 Hot isostatic pressing (HIP) (cntd. 9) Microscopic model: three stages of consolidation process 0) stage of packing density 1)stage of connected porosity 2)stage of closed porosity

40 HIP maps (density vs temperature maps) at constant pressure and initial packing density Two mechanisms of HIP densification: (1)Plastic yielding (2)Power-law creep (the dominant mechanism of densification) Microscopic model: Hot isostatic pressing (HIP) (cntd. 10)

41 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

42 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.

43 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 tons worldwide

44 Hot isostatic pressing (HIP) (cntd. 14) Primary carbides in heat treated P/M T15 (left) and conventional T15 (right) high-speed steels 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

45 Hot isostatic pressing (HIP) (cntd. 15) HIP fabrication of metal matrix composites Ti/SiC

46 4. Sinter HIP Disadvantages of HIP: 1)Encapsulation prior HIP-ing is as expensive and time consuming step 2)Canning in most cases precludes the fabrication of compex shapes Sinter HIP techniques (Sinter+HIP, Sinter/HIP or Sinter-HIP etc.) enable to produce complex and net shape full density products

47 Sinter HIP (cntd. 1) 4.1 Sinter + HIP Both Sinter + HIP and Sinter/HIP = containerless pressure- assisted consolidation technique of sintered parts (with porosity below 8%)

48 Sinter HIP (cntd. 2) Different technological schemes of Sinter+HIP: 1.Sinter + HIP in different cycles 2.Sinter + HIP in the same cycle eliminating extra sintering and cooling step 3.CHIP = CIP/Sinter + HIP 4.PIM/Sinter + HIP where: CIP – cold isostatic pressing PIM – powder injection molding

49 Sinter HIP (cntd. 3) Sinter + HIP of Ti-alloys

50 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 : Sinter HIP (cntd. 4) 1.CHIP = CIP + Sinter + HIP 2.PIM/Sinter + HIP = PIM + Sinter + HIP

51 The first stage of PIM/Sinter + HIP process, where: PIM – powder injection molding Sinter HIP (cntd. 5)

52 Sinter HIP (cntd. 6) 4.2 Sinter/HIP or Sinter-HIP 1.Sinter / HIP = pressure assisted sintering/overpressure sintering 2.The main difference between”Sinter+HIP in the same cycle” and Sinter/HIP: –use of substainally lower (almost an order of magnitude) gas pressure –the requierments of the equipment are less demanding

53 Sinter HIP (cntd. 7) Advantages of Sinter/HIP (Sinter-HIP) as against conventional HIP: 1)Elimination of both containerization and separate sintering cycle 2)Pressure (5-15 MPa) is generally an order of magnitude lower 3)The requirements of the equipment are less demanding and therefore the cost of low- pressure equipment is lower

54 Sinter HIP (cntd. 8) Sinter/HIP process is preferable for production of low-cobalt alloys of medium grain size (  2  m)

55 Sinter HIP (cntd. 9) Sinter + HIP process is preferable for low-cobalt alloys of fine grain size (<1  m)

56 Sinter HIP (cntd. 10) Industrial Sinter/HIP cycle

57 Sinter HIP (cntd. 11) Applications of Sinter/HIP: 1.Hardmetals (cemented carbides) 2.Composites (MoSi 2 +Al 2 O 3 ) 3.P/M tool steels 4.P/M stainless steels 5.Si 3 N 4 based structural ceramics 6.Complex shapes utilizing CIP+Sinter/HIP and PIM+Sinter/HIP

58 Sinter HIP (cntd. 12) 4.3 Pulsed Sinter/HIP Time-temperature- pressure profile of the Pulsed Sinter/HIP: 1. Thermal Spike HIP 2. Pressure Spike HIP

59 5. Powder extrusion Powder extrusion mechanics Powder extrusion: high-temperature consolidation at high strain rates and high stresses. Three main groups of processes: 1 – loose powder extrusion 2 – extrusion of pre- sintered powder compacts 3 – extrusion of encapsulated powders

60 Conventional processes of extrusion are: -extrusion of pre-compacted and pre-sintered compacts (e.g Fe-based materials – high speed steels) -extrusion of encapsulated powders (refractory and reactive powders) Specific feature of extrusion: powder partides are subjected to high shear and deformation helping braking up oxide skin and promting nascent particle-particle contact (the major adventage over HIP) Powder extrusion (cntd. 1)

61 Extrusion pressure (for wrought and particulate material): where K – extrusion constant for the material (combines flow stress, friction, temperature, material properties etc into one parameter) decreases with increasing temperature - extrusion ratio (reduction ratio) must exceed 10 for adequate densification Powder extrusion (cntd. 2)

62 Powder extrusion (cntd. 3) Special processes of powder extrusion:  duplex extrusion (co-extrusion)  multi-temperature extrusion  extrusion of composites in semi-solid (mushy) state  extrusion with changing direction of powder flow  extrusion using low extrusion ratios

63 Powder extrusion (cntd. 4) Special process of extrusion – duplex extrusion (co-extrusion) is a process which results on one material forming a layer on another over the entire length of the final product (e.g tube on rod). Special process of extrusion – multi-temperature extrusion for good proportional co-reduction. Examples are:  co-extrusion of Cr (1300  C) and steel (1050  C)  co-extrusion of UO 2 (2000  C) and steel (700  C) Multi-temperature co-extrusion of chromium and steel

64 Special process of powder extrusion: extrusion of composites in the semi-solid state (mushy state extrusion). Pecularity: liquid phase fraction 10…30 vol % decrases extrusion pressure (flow stress) markedly. Example: metal matrix composites (MMC-s) Powder extrusion (cntd. 5)

65 Powder extrusion (cntd. 6) Special process of powder extrusion: extrusion with changing direction of powder flow 1 – conventional extrusion 2 – single shear extrusion 3 – double shear extrusion

66 Powder extrusion (cntd. 7) Extrusion with changing direction of powder flow aims to eliminate the problem of in homogeneous shear in the conventional extrusion The total shear deformation in the double shear extrusion process can be expressed (see Fig.) T total = V 2 /V 1 + V 3 /V 2 = S1 / S2 + S2 / S3 where S1 / S2 and S2 / S3 represent the extrusion ratio for zone 1 and zone 2, respectively Schematic of the deformation in double shear extrusion

67 Powder extrusion (cntd. 8) Special process of powder extrusion: extrusion using low extrusion ratios

68 Powder extrusion (cntd. 9) Applications: 1.Intermetallic compounds (Ti 3 Al, Ni 3 Al etc.) 2.Cu-based alloys 3.Ni-based alloys 4.W-based alloys 5.P/M high speed steels 6.Oxide dispersion hardened (strengthened) alloys etc.

69 6. Powder forging The role of final density of pure iron on the mechanical properties (tensile strength, elongation, impact resistance). Powder forging offers a process for producing parts with very high densities.

70 Powder forging (cntd. 1) Different hot powder forging techniques: 1.Powder forging (powder upsetting) 2.Powder repressing 3.Superplastic forming of powders The powder forging is very similar to conventional hot forging. The major difference – preform is particulate material.

71 Powder forging (cntd. 2) Differentiation between two schemes: Powder forging: Powder repressing:  low strains  more defects, not optimal mechanical properties  low die wear, more complex shapes  high strains  better structure, good mechanical properties  high die wear

72 Powder forging (cntd. 3) The main mechanism of pore elimination (compaction) – plastic yielding (plastic flow) There take place changes during powder forging in:  density (porosity)  rate of work hardening (increasing with the porosity (see Fig.))  Poisson`s ratio (  0.3  0.5) Compressive deformation of porous iron with various initial porosities

73 Powder forging (cntd. 4) The pore collapse in forging is significantly different from that encountered under the hydrostatic conditions (HIP) The difference in pore collapse contributes to more shear and bonding in powder forging (as against HIP) Pore collapse in powder hot forging and hot isostatic pressing (HIP)

74 Powder forging (cntd. 5) Height strains >50% ensure:  pore elimination  good interparticle bonding (due to disintegration of oxide films during shear) The preform and forged height to diameter ratio (H/D) showing the region of high densification without fracture

75 Powder forging (cntd. 6) All mechanical characteristics except ductility (hardness, yield strength, dynamic properties) of powder forged materials are often superior to wrought products Inclusions seriosusly degrade properties in spite of full density (see Fig.) The effect of oxigen inclusions on the impact toughness of full-density 4340 steel processed by powder forging

76 Powder forging (cntd. 7) Features of powder forging: a) powder forging in trap die (to net shape) – tight control of preform mass is important b) conventional forging in an impression die with flash – large variations of the mass of preform is permitted a) b)

77 Powder forging (cntd. 8) Stages of powder forging process: (a)powder fill (b)pressing (c)ejection of preform (d)preheating (in controlled atmosphere) (e)powder hot forging (in controlled atmosphere)

78 Powder forging (cntd. 9) Superplastic forming: Superplasticity – ability of particular material under certain conditions to undergo extraordinary high tensile elongations. This phenomenon occurs in:  metals/alloys  intermetallic compounds  glass ceramics  ceramics

79 Powder forging (cntd. 10) Powder superplastic forging (superplastic forming) One of preconditions of superplastic forming – structural superplasticity (relies on microstructure) Requirements of structural superplasticity: 1.Ultra fine (stable during the process) grain size (metals < 10  m, ceramics < 1  m) 2.Deformation temperature T > 0,5 T S 3.Low strain rates ( ); high strain rate sensitivity

80 Powder forging (cntd. 11) Superplastic forming: Important factor for superplastic forming – strain rate sensitivity (= resistance to “necking”): Flow stress:   =K  m where m – strain rate sensitivity exponent (in superplastic condition m>0.3)  – strain rate K – material and structure constant Strain rate sensitivity exponent m vs temperature for the superplastic forming of ultra high carbon steel (UHC)

81 Powder forging (cntd. 12) Changes on grain structure before and after superplastic forming and conventional powder forging: Dominating mechanisms of deformation of metals and intermetallic compounds: grain boundary sliding diffusion intergranular dislocation slip

82 Powder forging (cntd. 13) Difference between a conventional and isothermal die forging (nearly identical to superplastic forming)

83 Powder forging (cntd. 14) Another precondition of superplastic forming – transformation superplasticity (internal stress superplasticity) Requirements: for superplastic forming (of ceramics): 1.Ultra fine (sub-micron) grain size (<1  m) 2.Stress-induced transformation toughening – internal stresses generation by phase transformations Example: Yttria-stabilized tetragonal zirconia (ZrO 2 +5,2 wt%Y 2 O 3 ), elongation to failure up to 800% at 1550  C NB! The finer grain size makes superplastic ceramics prone to concurrent grain growth

84 Powder forging (cntd. 15) Advantages of powder forging over conventional forging: 1)near-net shape capability 2)full-density products/materials 3)high degree of material utilization and energy savings 4)enhanced mechanical, particularly dynamic properties (fatique strength, fracture toughness)

85 Powder forging (cntd. 16) Applications of powder forging: Components for automotive transmission (Fe-based alloys) Applications of powder superplastic forming: 1.Ultra high carbon steels (UHC –steels) 2.Mechanically alloyed materials ( e.g TiAl – Ti 3 Al intermetallic compounds) 3.Superplastic ceramics (e.g ZrO 2 -Y 2 O 3, Al 2 O 3 +ZrO 2 -Y 2 O 3 ) etc.

86 Powder forging (cntd. 17) Powder forged automotive transmission components: Powder forged connecting rods Powder forged internal ring gear

87 7. Alternative hot consolidation processes 7.1 Combustion synthesis (self-propagating high temperature synthesis; SHS; SPS; reactive/ reaction synthesis) Combustion synthesis (SHS, SPS) = formation of chemical compounds (usually refractory compounds) involving self-propagating exothermic reactions between the reactants. Preconditions: exothermic reaction proper granulometry of powders external source of heat for initiation

88 Combustion synthesis (cntd 1) Combustion synthesis reaction is as follows:  a i X +  b j Y j = Z + Q where: X – V, Mo, W, Hf, Ti etc metals Y – C, B, N, Si, O, S – non-metals or metals (production of intermetallic compounds) Z – chemical compound Q – exothermic release of energy Variations of combustion synthesis:  Propagation of combustion wave (SHS)  Simultaneus combustion (SC)

89 Combustion synthesis (cntd 2) Three different reaction methods: -solid phase system -solid-liquid system -solid-gas system Advantages of combustion synthesis (in comparison with conventional P/M technology) 1)Exothermic process 2)Short processing time 3)A wide variety of different materials can be produced 4)Production of chemical compounds as well as composites 5)High purity of the end product

90 Combustion synthesis (cntd 3) Examples of reactions of combustion synthesis: Ti + C = TiC (all solid state) Ni + 3Al = Ni 3 Al (disappearing liquid phase) 2Al + N 2 = 2AlN (solid-gas system) 3SiO 2 + 6C + 2N 2 = Si 3 N 4 + 6CO 2 (gas-phase present in both reactant and product)

91 Exothermic combustion synthesis can be take place during sintering of multi-component powder materials = reactive sintering. Reactive sintering – sintering process where an exothermic reaction is initiated in a mixture of dissimilar (elemental) powders. The reaction produces a compound and the heat from the reaction is used to simultaneously sinter the product. Combustion synthesis (cntd 4)

92 7.2 Pressure-assisted combustion synthesis Pressure assisted combustion synthesis is used in conditions where conventional process results in porosity of product. The porosity could be due to: porosity of reaction mixture, changes in molar volume, volatile impurities etc. Pressure can be applied by various means: 1)Uniaxial pressure 2)Isostatic pressure 3)Pseudo-isostatic pressure 4)Shock wave 5)Centrifugal force

93 Pressure-assisted combustion synthesis (cntd. 1) Different schemes of pressure-assisted combustion synthesis: 1)Reactive hot pressing (RHP) 2)Reactive hot isostatic pressing (RHIP) 3)Pseudo-HIP (P-HIP) 4)High pressure-assisted combustion synthesis (HPCS)

94 Pressure-assisted combustion synthesis (cntd. 2) Reactive hot pressing (RHP) (1): Uniaxial hot pressing and simultaneus combustion synthesis

95 Pressure-assisted combustion synthesis (cntd. 3) Reactive hot pressing (RHP) (2): Time-temperature- pressure profile for a typical reactive hot pressing operation: Pressure: up to 50 MPa Time:  600 sek (  10 minutes)

96 Pressure-assisted combustion synthesis (cntd. 4) Reactive hot pressing (RHP) (3): Applications: Ceramics (TiB 2, TiC etc), intermetallics (NiAl, Ni 3 Al etc) and their based composites The effect of pressure on the residual porosity for pressure-assisted combustion synthesis of Ni 3 A and NiAl compositions

97 Pressure-assisted combustion synthesis (cntd. 5) Reactive hot isostatic pressing (RHIP): Time-temperature- pressure profile for a typical reactive hot isostatic pressing operation Pressure: 100…200 MPa Time: 2…4 hours Application: intermetallic compounds (NiAl, FeAl, Ni 3 Al, Al 3 Ta etc) and their based composites (Al 3 Ta – Al 2 O 3 fibers, NiAl – Al 2 O 3 fibers, NiAl – TiB 2 etc.)

98 Pressure-assisted combustion synthesis (cntd. 6) Pseudo-HIP (P-HIP) (1): Pseudo-HIP is similar to conventional HIP, but pressure is applied through a granular particulate-based medium instead of gas

99 Pressure-assisted combustion synthesis (cntd. 7) Pseudo-HIP (P-HIP) (2): Technological pecularities: Ceramical particulate-based medium (e.g SiO 2 ) as the pressure-transmitting medium Pressure: 100…700 MPa Time:  0,5 hours Application: ceramics and ceramic-based composites (TiAl-TiB 2 etc). NB! Pseudo-HIP is not isostatic pressing

100 7.3 Ceramic consolidation process (CERACON-process) The CERACON- process is similar to pseudo-HIP, but the pressure is not isostatic (and results in unequal deformation of the preform) The CERACON- process = pseudo- isostatic forming process

101 Ceramic consolidation process (cntd.) Technological pecularities: Preheating of both preform and ceramic granules Pressure: up to 1500 MPa Time: 30…60 sek Unequality (  0.5%) of deformation of the preform Application: near-net shape components from: -metallic alloys (Ti, Ta, Al) -intermetallic compounds (TiAl, NiAl etc) -high-temperatures superconductors etc

102 7.4 Ceramic mold process Alternatives to produce full density net shapes: Step 1: production of complex net shapes by: 1)loose powder packing on a complex shaped mold 2)cold isostatic pressing (CIP) 3)powder injection molding (PIM) Step 2: Densification to full density by utilizing HIP (or pseudo-HIP) The ceramic mold process is a variation of conventional HIP intended to drastically reduce machining

103 Ceramic mold process (cntd. 1) Shematic of the ceramic mold process: production of ceramic mold is similar to that in investment casting

104 Ceramic mold process (cntd. 2) The ceramic mold process is also suitable for producing: 1)Deep cavities (see Fig.) 2)Dual-property components by bonding different materials together in one cycle

105 Ceramic mold process (cntd. 3) Technological pecularities: an oversized ceramic mold to compensate for the shrinkage during HIP application of pseudo-isostatic pressure through ceramic particulate medium Applications:  net-shape / near-net shape products from metallic alloys (e.g Ti-alloys)

106 7.5 Rapid omnidirectional compaction (fluid die process) Technological pecularities: similar to hot isostatic pressing where pressure transmitting medium is a “fluid die” (omnidirectional nature of applied pressure) higher productivity in comparison with conventional HIP cycle (rapid HIP)

107 Technological pecularities (cntd): pressure much higher than in HIP Time 1…15 sek Temperature: lower than in HIP Applications: WC-Co hardmetals, intermetallic compounds etc. Powder filled fluid die (made from low carbon steels, Cu+10 Ni alloy, etc) Rapid omnidirectional compaction (cntd. 1)

108 Rapid omnidirectional compaction (cntd. 2) Drawback of utilizing metallic “fluid” die – extensive machining of fluid die Solution: use of cast composite fluid die (see Fig.) enabling mechanical stripping of consolidated part and eliminating expensive canning sequence ROC process using a cast composite fluid die

109 7.6 Plasma activated sintering (PAS) (field activated sintering technique (FAST), spark-plasma sintering) Plasma activated sintering = pressure- assisted sintering activated by electrical discharges between powder partides

110 Plasma activated sintering (PAS) (cntd. 1) Time-temperature-pressure cycle for the PAS-process: 1)application of pressure 2)initiation of pulsed discharges resulting in formation of pulse plasma between particles (to activate powder surface) 3)resistance heating (for densification) 4)lowering of pressure and temperature

111 Plasma activated sintering (PAS) (cntd. 2) Technological parameters: Pressure: MPa Total time:  10 minutes Pulse on-state duration: 30…60 sek Application: ceramic materials metallic materials ceramic-metallic composites (hardmetals, cermets) etc.

112 7.7 Consolidation by atmospheric pressure (CAP) The CAP-technique utilizes atmospheric pressure to provide pressure during the course of sintering. The goal: production of full dense materials in an inexpensive manner (without HIP)

113 Consolidation by atmospheric pressure (CAP) (cntd. 1) Advantages of CAP technique: 1)The ease of production of complex shape glass molds 2)Assistance of pressure without use of expensive high-pressure equipment (required for hot extrusion or HIP) 3)No costly protective atmospheres are required 4)The ease of glass mold stripping

114 7.8 Thixomolding; (molding of thixotropic alloys) Thixotropic state/structure can be achieved when any material in the mushy state is subjected to higher shearing action (e.g by stirring). Result – formation of nearly spheroidal degenerate dendritic partides and viscosity drops markedly

115 Thixomolding (cntd. 1) Al-Mg phase diagram with a Thixomolding alloy composition and thixomolding temperature range for Al-9% Mg alloy. The goal: production of near-net shape or net-shape parts in a single step

116 Thixomolding (cntd. 2) Process characteristics (as against conventional die casting): 1)Lower molding temperatures – material is in semi-solid state 2)The temperature control at the exit end +/-2  C 3)Elimination of most foundy operations (handling of molten metal, use of fluxes, waste management etc.) 4)Production of complex components directly from particulate material 5)Potential to produce dispersion strengthened/hardened materials

117 Thixomolding (cntd. 3) Application and advantages: Large scale production of complex products from low-temperature alloys (Mg, Al) Significant increase in die life (as against die casting) Low porosity and enhanced mechanical properties of material Schematic of the injection molding machine used for Thixomolding

118 7.9 Infiltration For wetting liquid melt, the capillary pressure:  P = 2  cos(  )/d where d- size of pores  - surface energy of the liquid  - contact angle (=wetting angle formed at the intersection of liquid, solid and vapor phases) Infiltration sequence where capillary forces pull a molten metal into the open pores

119 Infiltration (cntd. 1) Infiltration – the process of filling the pores of compact with open porosity with a lower melting temperature metal/alloy or other material (glass) During infiltration a liquid metal may fill pores by capillary-action (capillary infiltration) or by external force (external pressure infiltration)

120 Infiltration (cntd. 2) 1.Capillary infiltration techniques (see also next Fig.) a)Capillary-dip infiltration b)Full-dip infiltration c)Contact infiltritation Applications: electrical contacts, ferrous structural components for automobiles; metal matrix composites (MMC) etc.

121 Infiltration (cntd. 3)

122 Infiltration (cntd. 4) 2.External pressure infiltration techniques: a)gravity-feed infiltration b)centrifugal pressure infiltration c)vacuum infiltration d)HIP-infiltration (HIP-impregnation) (see also next Fig.) Applications: mainly nonwetting combinations such as: graphite-Cu, Al-SiC, Mg-SiC

123 HIP-infiltration – full density consolidation in conditions when capillary forces are ineffective because of poor wetting, unsuitable pore size distribution or high viscosity of the liquid Infiltration (cntd. 5)

124 7.10 Bonding by hot consolidation For bonding/joining of different materials hot consolidation techniques can be used, e.g.:  Hot isostatic pressing (HIP-diffusion bonding)  Reactive joining (SHS-joining)

125 Bonding by hot consolidation (cntd. 1) 1)HIP diffusion bonding provides bonding between:  two full-dense materials  a full-dense and a porous materials  two porous materials HIP process provides concurrent densification and bonding of different materials.

126 Bonding by hot consolidation (cntd. 2) 1)HIP diffusion bonding (for production of HIP/clad products): enables production of gradient-structured materials NB! large internal stresses maybe built up due to the thermal expansion and elastic modulus mismatch. The stresses increase with an increase in the thickness of joined materials.

127 Bonding by hot consolidation (cntd. 3) Advantages of HIP diffusion bonding (compared with conventional diffusion bonding): 1)Isostatic prssure enables bonding of shaped surfaces 2)Plastic deformation only at microscopic scale enables HIP diffusion bonding at higher temperatures 3)Powder and porous bodies can be simultaneusly densified to a substrate HIP diffusion bonding of powder to solid

128 Bonding by hot consolidation (cntd. 4) 2)Ractive joining (SHS-joining): uses combustion synthesis to join different materials is similar to exothermic welding or exothermic brazing Advantages of SHS joining: 1) minimal heat induced damage to the substrate since generated heat is localized

129 Bonding by hot consolidation (cntd. 5) Advantages (cntd.) 2) Ability to form functionally graded material joints between dissimilar substrates thereby overcoming matching problems. Schematic set-up of two reactive joining processes

130 Thank you for attention!


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