Seetharaman S. Fundamentals of metallurgy

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Hot isostatic pressing

In hot isostatic pressing, a hydrostatic pressure is applied to the powder by the

action of a gas (Argon, nitroge n) with a pressure up to 300 MPa simultaneously

with a temperature up to 2000ëC.

Before HIPing, the powder is placed into a

metallic or a glass container (`can') which is out-gassed and sealed. Spherical

powders are the best suited for hot isostatic pressing, which is a long time

process (around two cycles per day). The cycle duration can be reduced to a few

hours for temperatures lower than 1250ëC, which allow the opening of the

isostatic press without requiring the cooling down of the furnace and so

reloading with a preheated can.

In spite of the high capital cost, hot isostactic pressing is used to produce

large semi-finished components in hardmetals,

high speed steels, superalloys

(most common techniq ue),

titanium alloys and beryllium. Moreover a can with

a complex form allows the production of near-net-shape components. This

ability has been used for superalloys and titanium alloys.

Hot extrusion

There are three basic methods for the hot extrusion of powders. The first method

consists in filling a heated extrusion container with a loose powder (magnesium

alloys), which is heated during the 15 to 30 seconds before starting its extr usion.

In the second method the powder has been compacted and sintered before

extrusion (aluminium alloys)

and molybdenum, superalloys and high-speed

steels. The third method, which is the mostly used (beryllium, stainless steel,

aluminium, copper and nickel dispersion strengthened alloys), consi sts in filling

a metallic capsule (`can') with the powder, to protect the powder from

contamination by the atmosphere. The can is out-gassed and sealed before

heating and extrusi on. The material of the can (e.g. copper, low carbon steel)

should have a stiffness similar to that of the powder at the extrusion temperature,

should not react with the powder and should be removable by etching or

mechanical stripping.

Circular, elliptical or rectangular cross-section parts can be obtained by hot

extrusion. For a more complex cross-section shape the `filled billet' technique

was developed.

The mild steel filler contains a cavity with the shape of the

desired cross-sect ion, which is filled by the powder, the whole is placed in a

carbon steel can and then extruded. Seamless tubing of stainless steel has been

produced with this technique.

Hot forging

Metal powders that have been previously compacted and sometimes also

sintered (preform) can be hot forged into parts of high density (elimination of

496 Fundamentals of metallurgy

the residual porosity) and closely controlled dimensions.

A height strain of

50% is at least needed for pore elimination and good interparticle bonding.

During hot forging the preform, which has a simpler shape than the final

product, undergoes a lateral flow especially at the beginning of the deformation.

This process allows subsequent quenching that makes it attractive for hardened

steel. A production rate of 20 to 40 strokes per minute can be reached.

The main applications of this process concern the automotive industry (gears,

connecting rods).

Electric field assisted sintering ± `spark' sintering

Electrical field assisted sintering is an emerging techno logy to densify powders

wi th nanosize or nanostructure while avoiding t he coarsening which

accompanies standard densification routes thanks to its very short sintering

time. Howeve r, it can also be used for coarser powders. This process is

applicable to metals (Al, Fe, W, Mo, Be, Ti), metallic alloys (Al-Si-X, Fe-Co),

intermetallics (Al-Ti, Al-Fe) and ceramics. It could be used to produce cutting

tools, metal forming dies, gears, pump components, electric motors, household

equipment.

Electrical field assisted sintering (FAST) consists in applying simultaneously

a pressure with a punch and an electrical field to the powder that fills a

conductive die. The electrical discharge induced by the electrical field at the

contact of the particles and in the gaps between them is responsible for the

physical activation of the powd er particle surface (melting and vaporisation,

inducing the cleaning of the particle surface) that favours sintering.

The main pa rameters ar e related to the powder characteristics, the

compaction (e.g., uniformity of the packing of the die) and the characteristics

linked to the electrical field (intensity, voltage, pulse pattern that induces the

electrical discharge). The reached density is typically 98% to 99% of the

theoretical density in very short time (10 min).

Infiltration

Infiltration consists in filling the pores of a sintered material with a liquid metal

or alloy, having a (much) lower melting point.

The driving force of infiltration

is the reduction of surface free energy as in sintering. Infiltration is promoted by

a low wetting angle of the liquid with the solid and by establishing a pressure

gradient, e.g. putting the solid (porous skeleton) in a vacuum and applying

pressure to the infiltrating liquid.

The infiltration parameters are the infiltration temperature (reduct ion of the

viscosity of the infiltrating liquid) and the dwell time above the liquidus of the

infiltrating liquid. Short times are preferred to limit eventually extensive

reaction of the liquid with the skeleton (Fe-Cu). Infiltration can be advantageous

Understanding and improving powder metallurgical processes 497

compared to liquid phase sintering when the liquid is insoluble in the skeleton

material (W-Cu).

Rapid prototyping

Rapid prototyping (RP) concerns techniques that produce a complex part or a

prototype to net or near-net shape by building the object layer by layer (addit ive

processing) from a computer aided design (CAD) file without the need of

specific expensive tooling (direct processes).

In many cases, a post-processing

step such as, for example, co-firing, sintering or infiltration, etc., is needed to

ensure sufficient strength of the part. Rapid prototyping techniques can be

classified in direct, also referred as solid free-form fabrication (SFF), and

indirect processes, the latter being the most widely used. Indirect processes use a

pattern (model) or a mould made by a rapid prototyping technique to produce

the final object, which can be metallic. The following concerns only the direct

processes.

RP techniques mainly used for plastic products have been adapted for metals,

ceramics or composites. The main advantages are the ability to produce complex

parts and the absence of specific tooling that reduces the production time.

However, the low dimensional accuracy and the high roughness of the part are

the main drawbacks. The layer formation procedure is specific to each RP

technique. The most common RP techniques are stereolithography (SL or SLA),

fused deposition modelling (FDM or FDC and FDMet, respectively for ceramic

and metal), ink jet printing (IJP) or three-dimensional printing (3DP) and

selective laser sintering (SLS) or direct metal laser sintering (DMLS) or

selective laser melting (SLM).

Laser technology is involved in more than a half of all the RP techniques:

laser photo-polymerisation (stereo-lithography, . . .), laser fusion or sintering

(SLS), laser cladding (laser generating, controlled metal build-up (CMB), laser-

aided powder solidification with powder jet (LAPS-J), LENS, . . .), laser cutting

(LOM, . . .), laser-induced CVD (SALD, LCVD).

The applications of the metallic products are injection moulds, tools, dies,

implants, etc.

Stereolithography (SL or SLA) consists in photo-polymerisation of a liquid

monomer by an UV laser. The laser is scanned in sel ected areas of a liquid

monomer layer, defined by a CAD file, to cure them.

After one layer has been

processed another layer of resin is coated on the top of it (called re-coating) until

the part is finished. This process has been adapted to produce metals.

Fused deposition modelling (FDM) consists in depositing a continuous hot

extruded (low viscosity) thermoplastic filament with an elliptical cross section

on a fixed substrate, following a path defined by a CAD-file.

In this case the

not heated part of the filament acts as a punch. When one layer is completely

cooled down the build platform indexes down and the deposition of the next

498 Fundamentals of metallurgy

layer is performed. Adaptation of this process by using a particle-loaded

thermoplastic filament followed by debinding and sintering allows the

fabrication of ceramic (FDC) and metallic (stainless steel) components (FDMet).

Multiphase jet solidification (MJS) is similar to the fused deposition process

of metals and ceramics.

The low melting metal or the powder-binder mixture

is introduced in a patented extruder, melted (viscosity < 200 Pa s) and

subsequently deposited on a substrate. Part of stainless steels, high speed steels,

FeNi, Ti and SiC have been produced.

Three-dimensional printing (3DP or IJP) consists in printing with an ink jet

printer a low viscosity binder, which forms a droplet, on a layer of deposited

ceramic or metallic powder.

After drying of the binder (e.g. latex, wax

emulsions, homogeneous solution phase binders), the next powder layer is

deposited and so on until the part is finished. The part is then ultrasonically

washed and its unprinted regions are removed through redispe rsion in the

ultrasonic bath. The 3DP processes can be divided into dry powder processes

and wet slurry processes, where the slurry of the powder is sprayed onto a

substrate, dried, followed by the printing of the binder. The dry powder process

has been used to produce injection moulds in stainless steel and tool steel parts

(latex binder), which are subsequently sintered and infiltrated with Cu or Cu-

alloys. The dimensional accuracy and the removal of the loose powder in the

narrow regions are the main problems. The advantages of 3DP are its ability to

produce very complex shapes with fine features (100 m), its flexibility (many

materials) and its well-controlled process.

Selective laser sintering (SLS) consists in scanning a thin layer (100±

200 m) of powder with a laser, that induces local sintering of powder particle s

corresponding to a path defined by a CAD-file.

The non-s intered particles act

as support and so allow complex forms such as hollow sections, overhangs or

undercuts, as for 3DP. After completion of the sintered layer, a new thin layer

of powder is spread on top of it and then laser-scanned. The non-sintered

particles are removed when the part is finished. SLS processes are mainly used

for thermoplastics materials and less for metals and ceramics, because they

require a higher energy input. Metals and ceramics can be produced by indirect

or direct sintering approach. In the indirect sintering approach, a binder is used

with the powder to be laser-sintered and a post-processing step is needed to

obtain full density. For example, a polymer coated steel powder was processed

by SLS and subsequently infiltrated with bronze or copper for tool making

applications.

In the direct sintering approach, a high power laser (e.g. 1 kW,

laser) is used to sinter a preheated powder feed (reduction of thermal

stresses) to reach the high temperature required for sintering. Liquid phase laser

sintering was successfully performed for powder mixture such as Fe-Cu,

stainless steel-Cu, WC-Co, TiB

-Ni and Fe

C-Fe. The SLS technique has a

relatively fast building rate com pared to the SLA or 3DP techniques for

ceramics.

Understanding and improving powder metallurgical processes 499

Direct metal laser sintering (DMLS) and selective laser melting (SLM) are

similar to selective laser sintering (SLS). But in the case of DMLS most of the

powder layer is melted whereas in SLM it is completely melted. As a

consequence, there is a good metallurgical bonding between the layers and

densities up to 100% can be reached.

Laser cladding (LC) includes processes such as laser engineered net shaping

(LENS) and a variant of shape deposition manufacturing processes (SDM).

Laser engineered net shaping (LENS) and direct metal deposition (DMD)

processes belong to laser powder deposition processes. They comprise focusing

a laser beam (Nd:YAG) on a metallic substrate to create a weld pool while

simultaneously injecting the metal powder directly into the laser beam, where it

melts. The moving substrate along a defined path produces the component line-

by-line and layer-by-layer. The high cooling rate allows the deposition of non-

equilibrium phases (e.g. extended solubility al loys) and/or fine-grained

microstructure, as full density and high mechanical properties can be achieved.

The fabrication of multi-material graded and layered structures is possible.

12.4 Conclusions

Powder metallurgy is a mature industrial activity used to produce metallic and

alloy components. There is a wide variety of powder production techniques,

ranging from high output atomisation processes to high-value nano-powder

production facilities. A broad spectrum of consolidation routes leads to simple

or complex components. The high number of powder production techniques and

consolidation methods allows enhancement of the relation between processing,

microstructure and properties. Thanks to their high flexibility, PM processes

find applications in the transport, energy, medical, machinery, and many other

sectors.

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502 Fundamentals of metallurgy

13.1 Introduction

Steel is the most popular metallic material and nearly 1,000 Mt of it was

produced in 2003. This amount accounts for over 90% of annual total metals

production in the world. Human life is greatly supported by steel due to its

inherent advantages in production and properties compared with other

materials. The production advant ages include: (1) plentiful availability of raw

materials (iron ore and steel scrap), reducing agents (coke and pulverized coal)

and refining fluxes (mostly lime and dolomite), (2) high output rate per plant

(10 Mt/year), and (3) relative ly low total material demand (5 t/t-steel) and ore

reduction energy (22 GJ/t-steel). The advantages in properties encompass (1)

high Young's modulus, (2) outstanding combination of high strength and

ductility, and hence good formability, (3) good corrosion resistance on alloying,

and (4) excellent magnetic properties. These properties have been significantly

improved at competitive cost by alloying and controlling structure and texture,

yet leaving more room for further improvement.

The production process and properties of steels have developed quite

interactively as triggered by market demand. The key driver of the development

in recent years has been, in many cases, market. In fact, assembly industries

have required advanced steels with superior properties at shorter delivery times

to produce better products at less cost for end users. The requirement has driven

the steel industry to develop steels with improved or new properties. Such steels

have prompted the emergence of new production processes in the steel industry.

Conversely, the new steel production processes have sometimes unveiled

avenues to produce more advanced steels with better properties at lower cost

and/or better productivity. The advanced steels have prompted the assembly

industry to refine their design and assembling process. Examples are plentiful

and some representative ones will be discussed in this chapter.

Improving steelmaking and steel properties

T E M I , Royal Institute of Technology, Sweden

13.1 Stee l manufacturing processes f rom raw material

s to semis (top) and semis t o products (bottom) (cou rte sy

of AISE S tee l

Foundation).