beryllium processing by powder metallurgy

BERYLLIUM PROCESSING BY POWDER METALLURGY T_his pa�er describes the p owder metaUurgical processing

o f beryllium into ft,ne-grained forms from which finished shapes are made. Emphasis i.! git-en to the v�um-hot-press-machine process which is used for the majority . of beryllium parts being made today.

by K. eryllium

G.

Wikle and V.

is often thought of as a new metal im­

Atomic and Space Ages; however, B portantalsoto thethought

it should

be of as the metal used in the powder metallurgical operation-with res­ pect to size of parts produced-being carried out to­ day on a commercial basis. It is usually necessary for the beryllium metallur­ gist to explain to those not familiar with the metal just why beryllium is fabricated commercially f r o m powders ra ther than b y casting ingots and working these into forged, rolled, or extruded shapes. The chief reasons are as follows: 1) To be of practical use in nuclear, missile, and other appli cations, beryllium metal must have a fine grain structure, usually o f random orientation, uniform density and composition, and must be sound and Cabricable by machining and other processes; 2) For bery l lium, foundry technology has n o t � n developed which can c o nsistently and competi­ !Ively produce sound and crack- and porosity-free castings that are either u niformly fine-grained as­ cast or can be reliably fabricated by hot- and cold­ working processes into fine-grained, machinable shapes having satisfactory mechanical properties; 3) A major breakthrough in beryllium technol­ ogy occurred in 1946 when C. B. Sawyer and co­ workers at the Brush Bery1lium Co. developed powder metallurgical techniques (Process Q), which produced berylliu m metal having a uniformly sound, den se, and fine -grained structure. Such vacuum hol­ P�essed metal (QMV) could be machined and other­ 'N!se successfull y fabricated into shapes having ex­ ellent mech anical properties, dimensional stability, � nd corrosion r esistance· �) By careful control ver grain size, grain orien­ tation, a n d BeO content the mechanical properties of powder met llurgical bodies can usually o ptim ized for given applications. (See Figs. 1 and largest

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:rylb_um

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2) ; and

acuum-hot-press-and-machine is the process b S) . Y which most fabricated

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beryllium is commercially today. This process can be reliably ex­ �ted to produce beryllium bodies over 5 tons in ( e quivalent in volume to 20 tons and more Wei r tee l). and 75-in. in diam and with an excellent ni orm1ty and level of properties. The technology

mto shape s

��

·

C.

Potter

of machining vacuum hot-pre�ed beryllium metal is highly advanced. Contrasted with the commercially available and proven vacuum-hot-press-and-machine process, the technology of making large bodies by rolling, forg­ ing, drawing, spinning, etc., from either sintered blocks or powder itself has not yet been well de­ veloped. Fabricating large bodies by such conven­ tional processes requires large capacity equipment, complicated jacketing, heating, handling, and cool­ ing techniques, i nvolves considerable risk, and leav something to be desired in final properties, yields, re-utilization of scrap, and overall costs. The art of powder forging, however, appears to be advancing at an encouraging rate. Briefly summarizing, because of the inability to produce sound, fine-grained castings commercially and to fabricate castings successfully into shapes o! good quality, the vacuum-hot-press-and-machine process has produced the bulk of all fabricated bery­ llium used in the US to date.

Quality standards To realize the need of many of the special handl­ ing techniques used in processing of beryllium, it is best to consider here the quality standards estab­ lished by the beryllium industry itself and required by most customer, government, and society specifl­ cations. The goals that must be achieved quality­ wise in the final berylliu m metal parts are describro below for the most popular grade of structural QMV beryllium (S-200) blocks, vacuum hot-pressed Crom blend powder (P-200):

1) Soundness: density-greater than 99.5 pct of

theoretical-and porosity-unconnected and less by than 1h pct with n o porosity or cracks revealed . dye penetrant or b y radi�graphy;. no 2) Relatively free of h1gh-dens1ty mclus!ons. mclu ed n�J ( diam in. 0.050 than r large ions inclus l 1um rad �o­ foreign matter is more dense than bery high density as up s show ly readi and ically graph spots); ght 3) Uniform properties; ble; 4) Readily machina . mic ron avg grain 5) Uniform, fine-grain size (40 diam or less); any direct'10�) . 40 6) Mechanical pro per ties (in . 1eld min , �o.ooo psi 000 psi tensile strength _ and ., min in. 2 m on gati strength min., and 1 pct elon •

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K. G. Wikle and V. C. Potter are with Brush Beryllium Co., Elmore, Ohio. This paper was presented at the 1961 AIME Annual Louis. _ Meeting in St

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METALS-SJ7 AUGUST 1961, JOURNAL OF

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20

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(1.5

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GRAIN I

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CONTENT

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SIZE - M,ICRONS. I 2-5 JO ZJ) Pt'R

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Fig. 1-left, Effect of groin

I

JS

rolling plane for QMV Beryllium sh

97 pct metallic beryllium in the pebbles.

Other considerations

purity

beryllium must be kept out of the air during its processing. By Atomic Energy Commission health

standards, persons shall not be exposed to air con­ taining more than 25 micrograms of Be per cu m without a respirator or fresh air mask. Weighted exposures based on a 3-month period and calculated on a stead y 8 hr-a-day work schedule shall not ex­ ceed an average of 2 micrograms per cu m. Such

exposure limitations impose quite a rigorous need to isolate and ventilate all processe s involved with

beryllium powder. In additio n to concern about quality and exposure the berylliu m industry is faced with the proble of working with powder presently worth over $60 per lb. Thus, processing must be carried out with a maximum yield so that minimum material is Jost complet ely and minimum scrap must be reproces sed through earlier steps.

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Powder processing A flow sheet describing beryllium powder manu­ facture and processing at the Brush Beryllium Co. is shown in Fig. 3. The steps involved are discusse d below with preliminary reduction and vacuum cast­ ing operati ons also briefly reviewe d.

Extraction and Refining of Metallic Beryllium

The .first operation in the extractive process for obtain ing metallic beryllium is in the reducti on furnace where pebbles are formed. The reaction '

+

MgF.,

yields metallic pebbles in a mixed slag of MgF, and BeF, called a reduction melt. The reduction melt is crushed, ball-milled, and leached to yield cleaned pebbles of metallic beryllium. The size of the peb­

bles range from approximately

BeO, and other impuritie.

¥s

to

l lh

in. in diam. The chemic a l assay of these pebbles is in excess of 538-JOURNAL OF METALS, AUGUST 1961

by vacuum melti

charge generally compost 50 pct material retu r n e d f

i s melted down and pou1e

mately 500 microns). Mel

molds and vacuum c a s t in approximately 70 lb and 9-in. in diam x 20 in.

lor

greater than 98 pct met .. 11

Ingots are meticulou s l y and

surface

sand blasting.

dross

-Above, Effect of reduchoa

p one orientation porollel to

t

rolled ot 1400.F.

h BeF,, MgF., MgO,

cring

to or entrapped

er refined to higher In this operation, a O pct Be pebbles and nachining operations er vacuum ( approxi­

Beryllium pebbles are

In addition to the need of keeping foreign ma­ terial out of the beryllium powder which causes ob­ servable high density (radiographic) inclusions,

0 content on the strength

s1zc

of vacuum hot.pressed beryllium F ratio on strength and degree of b

7) Chemical composition: 2.00 pct max BeO, 0.18 pct max Fe, 0.15 pct max C, 0.16 pct max Al, 0.12 pct max Si, and 0.08 pct max Mg.

BeF, +Mg� Be

P

·

entrap

The ingot prod uced in up so it can be fed to the a'

poured into graphite 1s produced weighing ·uring approximately or \ chemical assay ed. E'ryllium is achiev . on £;d of mold reacti and ts by chipping broken um casting is corn­ for ·1 oning mills o wder. This is acc m­

minution into -200 mesh 1 plished by a chipping opPro. ion . are chucked The top ends of v a c u un1-en& afld

tod

111 ...........

and

bar, 1''4 tubltiC 1lmple lhaPH

!'lat&-. 11\,,..I 1trlJ1 Aftd foll rn1tll 1•rlf

.implr Iha�

lllllfl hAPH

Y1•nn!'blnl

Thin or brr• •ll'IK1

Jllltl

...

IUl't'9

'Tllln Ir

""'". tJltt....•••••., • •ttttlt11 ,.,.... Potp

lllPtn1 hlt\Jr

- '""" r.x lloll rr

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i

�l llP UT

t 2

•y In

atr

tntttmtdla1!1' t.o ct and ._u .... . .....,,, lll'!ll'lt tJiliJKI

Jll.:rctnJ>

I p0wdri �I 3.1·1D TSI, r lo �II# \AtutllJ\ al 1l7S'• •c. (;otn U n•·c' t)•, r vllllff, llfflorm• I

l111pl

turtt1tt 111

klnf

1.r- All•I ht>I forctnt 111 If�·-

htol

\ . .....,...,..

.t.UCUSl 1

After

density

determination

pressed blocks are leached

8

the

vacuum

hot­

hr in sulfuric acid.

Leaching removes the outer surface in which beryl­

lium i s alloyed with iron, and cleans the surface

of oxidized areas and other inclusions adhering to the surface of the block. The advantage of this leaching operation is a clean surface which allows chips obtained during scalpi n g to be attritioned directly into powder with a m inimum of beneficia­

tion.

Scalping removes the outer surface skin and ma­

chines t h e pressed blocks to standard stock sizes. Milling machines, lathes, or vertical boring mills

are used, depending upon the size and shape of the block. A l l machines are equipped with vacuum chip pick-up units which insure low atmospheric dust counts and efficiently collect chips with mini­ mum contamination. After scalping, blocks have smooth and parallel

surfaces and square corners and are then suitable

for effective evaluation by dye-penetrant and ultra­ sonic i n spection. After

I nspection and Final Machining

the scalping operation,

all vacuum hot­

pressed QMV blocks are examined by ultrasonic and

dye-penetrant inspection. These procedures not only

reveal defects that would be objectionable in final parts, b u t also indicate processing difficulties so that

corrective action can be initiated immediately. Lo­

cation of defects minimizes subsequent machining expenses, insures

allows maximum use of material,

maximum metallurgical

parts.

quality in

and

final

The final machining operations are carried out

with a variety of machines which depend on the

size and shape of the QMV block, the number, size

and shape of final parts, and t h e final dimensional

tolerances required. Special machines are not neces­

sarily used but all are equipped with vacuum chip

pick-ups and suitable ventilation. Test bars are re­

moved during final machining for tensile property and grain structure evaluation. Various inspections

are carried out between machining operations.

As required, all finished parts are inspected with

the

standard

techniques :

dye-penetrant

(or Zy­

glo), radiographic, dimensional, and tensile prop­ erties. For special applications additional inspection and evaluation may be conducted .

Advantages The

advantages of the vacuum-hot-press-and­

machinc

follow s :

1) 2)

process for beryllium

can

be listed

Beryllium metal can b e readily ground

powder of good chemical purity;

as

into

Beryllium powder can be readily and consis­ tently compacted in vacuum with low pressures at 1 050°C into uniformly dense bodies of over

3)

99.5 pct of theoretical density; Grain structure produced is fine ( less than 40

microns

4)

with -200

oriented;

mesh powder) and randomly

Mechanical properties are

relatively good in

all d irections and in all parts of pressings;

5) Strength can be

improved

by using recycle

p o w d ers high in BeO content;

6) By use of fine powder (sub-sieve ) , strength,

stabil ity, and other properties can be improved;

544-J OURNAL OF METALS, AUGUST 1961

7 ) Excellent machinability ancl tnHl'hining tech­ nology is highly a d v anced so ti at fabrication

8)

by this method is relative!� eflk" rt;

Chips generated by mach1.1in • returned directly t o the pov;

process

and

are

then

9 Vacuum hot-pressed

1mmc

beryllium

equipment

in weight; and

reusable;

be readily in g standard

m·

ble

is a'

bodies up to 75-in. diam and

ly

'l

rolled, forged, a n d extrude techniques developed for ben

1 0 ) Commercial

(cl ·y) can be r attritioning

t

to

obout

1 1 ) Alloys, if desired, can eas mixed powders with mimmu

press

6 tons

made from egation.

Other fabrication processes

This paper primarily described •

l urgical steps involved

in the

machine process which h a s been

1

\

h v e possible g beryllium nl!nt it would commercial

should be briefly mentioned. At th

appear that all processes having

is a starting

potential involve beryllium po\\

111 c

to guarantee um mechan­ readily ma-

fine-grained, sound bodies having

ical properties and capable of

chined.

Table I lists eight processes \ I or have been employed at one t

e being used another to ,, into more

convert vacuum hot-pressed be

1 six methods �hapes other :Jrocess. Ad­

highly fabricated shapes. Also an'.! of consolidating bery l l i u m powd1•

than the basic vacuum hot-pi

vantages and disadvantages fo·

pointed out.

chnique are

c

Warm and hot extrusion. hot

forging of vacuum hot-pressed be

blocks are processes readily can practices esta blished for beryllium

-hot-press­ being used

being made

for the bulk of beryll i u m metal today. However, other processt:::' \1 commercial application i n fab1

material. Powder must be used

·der metal-

r

·ig, and hot billets and m. provided used. Warm

extruded rod and tubing is further arawn a t ele­ _ vated temperatures into wire ant cold finished

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tubing. Beryllium sheet metal is rcac ily hot be t, _ drawn, spun, and formed into shap s \vh1ch c� e joined together by rivets, bolts, c;oldering, braz ing. welding, and adhesive bonding. r forg­ Direct powder techniq ues· such as powde . sinter­ s 1 es lisure mg, warm press-hot extrude, pre . renti , work, and hot upset forging of sheet, are cur d� pow . cts . being used to fabricate special produ parts ing mak of forging is • by necessity' a method at one · · in the larger hydraulic presses. The Bntish ' . t wav time, found pressureles s-sintering a convenien irgi� _ v of making extrusion billets for tubm� fro Jarge t powder. This process did not require ress­ capital investment necessary for vacuu� I? ­ nter in ing facilities; however, lack of uniformity . ing from powder lot t o powder lo� and 11 r er than parts of the sintered body made y1elds 10 g _ f sh apin desired. Hot-upset-forging is a tech01que .0 ·rnu ..... of · 1n1 '" sheet from powder which results IO a m processcs directionality but a h i g h strength level: period. _ which avoid a lengthy vacuum smt�n 50rne e v fully densify the powder as well as mv loP ve t o. d e degree of plastic deformation a� pe�r odies b good strength and ductility comb mations m

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fabricated from beryllium powder.