Powder Metallurgy

Powder metallurgy may be define as the art of producing metal powders and using

them to make serviceable objects.

Powder metallurgy principles were used far back in 3000 B.C. by the Egyptians to

make iron parts.

The use of gold, silver, copper, brass & in powders for ornaments was common

during the middle age.

Recently, materials with mechanical properties far better than those of conventional

materials have been developed by improving heat-treatment, powder composition

and processing methods to achieve higher densities.

So powder metallurgy has become a manufacturing technique to produce

considerably complex shaped components to exact dimensions at high rates of

production with extremely low costs.

• Instructor : Prof. Soon-Chul Ur of MSE (x5385) • Grade Policy 1. Mid term exam: 40% 2. Term paper and presentation: 40% 3. Lecture attendance : 20%

• TEXT : 분말재료 및 공정, 한국분말야금학회, 2010.3 (Powder Metallurgy & Particulate Materials Processing), • Sub-Text : Powder Metallurgy Science, R. M. German, Metal Powder Industry, 1997.

Powder Metallurgy

• Introduction

• Characterization of powders

• Synthesis and Fabrication of powders

• Microstructure control of PM materials

• Pre-treatment of PM materials

• Compaction of PM materials

• Consolidation and sintering

• Full density processing

• Finishing and property measurement

• Application of PM materials

• Nano technology in PM materials

Powder Metallurgy

P/M net-shape gears are common, save machining time

Powder Metallurgy (P/M)

Method: • Make fine metal powders and sort • Mix powders to get “alloy”

• Iron alloys most common, also Bronze • Compaction

• Powder is pressed into a “green compact” • 30-1400 MPa • Still very porous, ~70% density • May be done cold or warm (higher density)

• Sintering • Controlled atmosphere: no oxygen • Heat to 0.75 of Tm • Particles bind together • Part shrinks in size • Density increases, up to 95% • Strength ~ Density

P/M (vs. Casting): • Mass produce small steel parts, net-shape • Less waste • Unusual “alloys” • Range of densities, porosity (adv or disadv) • Less energy use • But: Smaller parts and less complexity (2.5D)

마이크로 크기 분말 재료로부터 부품을 제조하는 기술 (분말 야금/PM )

통상적인 분말야금법: 원하는 형태로 분말성형(pressing)→ 단단하고, 견고한 성

형체(compact)를 얻기 위하여 가열에 의하여 입자를 접합(Sintering)

- 원하는 부품형상을 얻기 위하여 제작된 punch와 die를 사용하여 유압식 혹은

기계식 Press에 의하여 수행되는 작업

- 소결: 금속의 용융온도 이하 온도에서 가열 ; non-melting process

Press Sintering furnace

마이크로 분말소재 공정(PM process)

Metals

Ceramics

나노 ∙마이크로 소재 공학

마이크로 소재 공정

경제성 (economic): 정밀도와 비용절감, 소 품종 다량생산 (예: automotive parts (bearing, valve seat, plugs, injector sensor, connecting rods, sprockets and etc) 독특성(unique) : 차별화된 특성 및 미세구조(예: 다공질 필터, 산화물 분산강화 터빈합금(ODS superalloys), Cermet, 경사기능합금(Functional Gradiented Materials; Ti-hydroxyapatite), 전기접점함금(Cu-Cr), WC 등 유일성 (captive): 다른 공정으로는 만들기 어렵거나 불가능한 공정, 분말성형+(저온)소결 (예:초경재료, 고온재료, 폴리머 등의 난용해성 소재, MoSi2, TiB2, MgB2, amorphous alloy, and etc)

PM 소재를 사용하는 이유

마이크로 분말소재 공정의 차별점 주조공정(casting)을 적용할 수 없는 경우

마이크로 분말소재 공정의 유일성, 경제성

Unique points

Merits in economical view

마이크로 분말 야금(powder metallurgy : PM)의 장점

•사용된 원료 분말 중 97% 이상 높은 수율을 갖는 부품 제조 (chip-less).

• 원하는 기공을 갖는 다공질 제품의 제조 가능.

- 예: 윤활제가 함유된 bearing 또는 gear

• 미세조직, 화학적 성분, 물리적 성질이 상대적으로 균일한 재질을 얻을 수 있음

• 주조법(casting)에 비하여 낮은 온도의 공정 : non melting process

PM에 의해서만 제조가 가능한 재료-예: W (for filaments), Cermets(내열합금) 등.

• 최종제품 형상에 가깝게 제조 가능(near net shape) →후 가공 공정 감소.

• 미려한 부품 표면.

• 균질의 마이크로 미세조직 : 고강도, 내마모성 합금 제조에 적합

• 단순 형상 부품의 대량 생산에 적합 → cost effective

• (최근, 복잡한 형상 부품의 대량 생산도 가능해 짐)

마이크로 분말 야금의 한계와 단점 금속분말의 가격이 높다.

(고비용 제조단가, 취급, 보관, 운송 등의 영향)

상대적으로 고가의 제조 장비 필요 : press, 소결로, 혼합기(mixer) 등

금속분말의 경우 보관이나 취급에 어려움.

예: 분말의 열화(oxidation or hydration), fire and explosion

형상 및 크기의 제약: tooling and largest available press

금속분말의 유동성이 열악하여 균질성형에 어려움이 따름.

분말의 형상, 입도 및 입도분포를 제어하는 것은 PM법에서 매우 중요하지

만 항상 용이한 것은 아님.

Powder Metallurgy

Slightly porous appearance is common Range of particle sizes

Powder Metallurgy: Cermet cutting tools (Ceramic-Metal composite)

Microstructure: ceramic particles in metal matrix Cermet-tipped saw blade for long life

Cermet cutting inserts for lathe

Powder Metallurgy: Porous Metals

Oil-impregnated Porous Bronze Bearings

Metal filters

Forged on left; P/M on right

Powder Metallurgy: Connecting Rods

Powdered Metal Transmission Gear

• Warm compaction method with 1650-ton press • Teeth are molded net shape: No machining • UTS = 1,000MPa • 30% cost savings over the original forged part

Powdered Metal Turbine blade-disk (“blisk”): 1 piece!

Conventional Forging vs. Forging of Powdered Metal blank (2nd op)

Basic Steps In Powder Metallurgy (P/M)

• Powder Production

• Blending or Mixing

• Compaction

• Sintering

• Finishing

Powder Production

• Atomization the most common

• Others

o Chemical reduction of oxides

o Electrolytic deposition

• Different shapes produced

o Will affect compaction process significantly

Blending or Mixing

• Can use master alloys, (most commonly) or elemental powders that are used to build up the alloys

o Master alloys are with the normal alloy ingredients

• Elemental or pre-alloyed metal powders are first mixed with lubricants or other alloy additions to produce a homogeneous mixture of ingredients

• The initial mixing may be done by either the metal powder producer or the P/M parts manufacturer

• When the particles are blended:

o Desire to produce a homogenous blend

o Over-mixing will work-harden the particles and produce variability in the sintering process

Compaction

• Usually gravity filled cavity at room temperature

• Pressed at 30-300 MPa

• Produces a “Green” compact

o Size and shape of finished part (almost)

o Not as strong as finished part – handling concern

• Friction between particles is a major factor

Isostatic Pressing

• Because of friction between particles

• Apply pressure uniformly from all

directions (in theory)

• Wet bag (left)

• Dry bag (right)

Sintering

• Parts are heated to ~80% of melting temperature

• Transforms compacted mechanical bonds to much stronger metal bonds

• Many parts are done at this stage. Some will require additional processing

Sintering

• Final part properties drastically affected

• Fully sintered is not always the goal

o Ie. Self lubricated bushings

• Dimensions of part are affected

Die Design for P/M

• Thin walls and projections create fragile tooling.

• Holes in pressing direction can be round, square, D-shaped, keyed, splined or any straight-through shape.

• Draft is generally not required.

• Generous radii and fillets are desirable to extend tool life.

• Chamfers, rather the radii, are necessary on part edges to prevent burring.

• Flats are necessary on chamfers to eliminate feather-edges on tools, which break easily.

Advantages of P/M

• Virtually unlimited choice of alloys, composites, and associated properties

o Refractory materials are popular by this process

• Controlled porosity for self lubrication or filtration uses

• Can be very economical at large run sizes (100,000 parts)

• Long term reliability through close control of dimensions and physical properties

• Wide latitude of shape and design

• Very good material utilization

Disadvantages of P/M

• Limited in size capability due to large forces

• Specialty machines

• Need to control the environment – corrosion concern

• Will not typically produce part as strong as wrought product. (Can repress items to overcome that)

• Cost of die – typical to that of forging, except that design can be more specialty

• Less well known process

Financial Considerations

• Die design – must withstand 700MPa,

requiring specialty designs

• Can be very automated

o 1500 parts per hour not uncommon

for average size part

o 60,000 parts per hour achievable for

small, low complexity parts in a

rolling press

• Typical size part for automation is 1”

cube

o Larger parts may require special

machines (larger surface area,

same pressure equals larger forces

involved)

나노 ∙마이크로 소재 공학

크기 단위 접두어의 의미

CNT: carbon nano tube