What You Can Do withExplosion We l d i n gWhat You Can Do withExplosion We l d i n g

DAVID CUTTER (dcutter@pacaero.com) is vice president of R&D at Pacific Aerospace and Electronics, Inc., Wenatchee, Wash.

Lead — Arc welding of a forged steel cup-and-cross tiedown assembly into an aluminum aircraft carrier deck plate. The welding process ismade possible by the application of explosion welded materials. The all-welded final assembly is extremely rugged and free of any possible entrypoints for galvanic corrosion.

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BY DAVID CUTTER

esign engineers are often facedwith a dilemma of material selec-tion, where the material that

would work the best for one specific ele-ment of a design is lacking in propertiesrequired by other elements of the design.For example, a material may exhibit goodcorrosion resistance, electrical conductiv-ity, or thermal conductivity, yet be lack-ing in the strength, hardness, weldability,or wear resistance that may be requiredin the final design. A powerful tool to helpresolve this dilemma is provided by theutilization of explosion welded materials.

Explosively welded materials allow de-sign engineers to specifically place a cer-tain material exactly where they need itin the design, without compromising othercritical elements. The correct applicationof explosion welded materials can yieldsignificant gains in strength, reliability,and cost-effectiveness throughout theproduct’s lifetime.

The explosion welding phenomenonwas first observed during the early Wo r l dWar II years. It was noted that the forceof explosions had welded bomb fragmentsto impacted metal objects. It was not untilthe early 1960s that DuPont developed apractical explosion welding process, cul-minating with the issuance of U.S. Pa t e n tNo. 3,140,539, Process for Bonding Metalsby Explosive Means.

Since then, the process has been con-tinuously refined and applied to an ever-growing number of applications in manymanufacturing industries. A search ofU.S. patents in Classification 228.107,Metal Fusion Bonding Using ExplosiveEnergy, lists 148 patents issued since 1976.Many of these are for refinements of thebasic process, while others cover applica-tions of explosion welded materials, forindustries as diverse as hermetic elec-tronic packaging, golf clubs, sputter tar-gets, and cooking griddles. Whether weld-ing lightweight materials to strong mate-rials or noble materials to support mate-

rials, the explosion welding process hashelped to achieve the “impossible” formore than 30 years.

The explosion welding process is, atfirst inspection, quite straightforward.The metals to be joined are fixtured closeto each other with a small separation. Alayer of explosives is spread evenly overthe top plate. The “sandwich” is deto-nated, and Bingo! Out of two separate andoften totally dissimilar materials, we nowhave one metallically welded stack. Thisbimetal plate can then be machined forincorporation into a variety of products.

In reality, however, the process is muchmore complex. A successful weld, free ofnonwelded areas or delaminations re-quires careful control of a number of pa-rameters, and considerable experience isrequired to produce consistently high-quality welds.

The most common explosive materialused is ANFO (ammonium nitrate-fueloil), but other explosives are used for spe-cialty applications where a particular det-onation velocity and yield are required.

The quantity of explosive varies widely,but most welding jobs can be completed

with charges ranging anywhere from 20to more than 2000 lb. Clearly, this weld-ing process cannot be carried out at yourlocal welding shop. Explosion weldingmust be performed by licensed and expe-rienced explosives engineers in a veryremote location, where all the usual safetyprecautions associated with any blastingoperation are enforced. There are numer-ous codes covering the storage and han-dling of explosives that must be rigorouslyobserved.

During the welding process, very highforces at the impact area (estimated at

39WELDING JOURNAL

Fig. 1 — Artist’s rendition of the dynamics of the explosion welding process.

Fig. 2 — The wavy weld line on this elec-tronic component reveals that it was ma-chined from an explosively welded block(6061 aluminum to 4047 aluminum).

The process has been continuously refined

and is being applied to an ever-growing

number of applications

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Table 1 — Metals Compatibility for Explosion Welding

41WELDING JOURNAL

several hundred thousand lb/in.2) causethe first few atomic layers of each mate-rial to form a plasma jet that in turn prop-agates out of the impact zone, taking withit any contamination and unwanted oxidelayers — Fig. 1. The impact causes themetals to form a true metallic bond, wherethe metals share valence electrons.

Due to the extreme cooling rate at thebond transition, the formation of inter-metallics is largely suppressed. On a moremacroscopic level, an explosion weldedstack usually displays a characteristicwaviness along the weld line — Figs. 2, 3.This waviness is a function of the differ-ent material properties and the weldingparameters used. The amplitude of thewaviness can be influenced by the impactangle and the detonation velocity. In ex-treme cases, it can be so high that un-wanted voids or inclusions form undercrests of the waves.

Generally speaking, an explosionwelded joint exhibits weld strength at leastas great as the weaker of the two base met-als. The welded product is routinely testedusing pull tests for ultimate tensilestrength, and ultrasonic inspection to re-veal interior non-bonded areas — Figs.4–6. The U.S. Navy has adopted a specifi-cation MIL-J-24445 for joining aluminumto steel. In addition, ASME publishesspecifications detailing clad products.

The explosion welding process hasbeen proven to work on a wide range ofmaterials, and new combinations arebeing tried. In certain cases, the weld qual-ity can be improved by inserting a thinlayer of a third material between the two

dissimilar layers. Stacks of four or morelayers are not uncommon — Fig. 7.

Table 1 displays a chart of the variouscombinations of materials that Pa c i f i cAerospace has successfully welded. Othersuppliers will have encountered additionalmaterial combinations.

Weld TransitionsExplosively welded bimetal sheets can

be used as weldable transitions betweentwo dissimilar metals. For example, an ex-plosively bonded strip of aluminum andstainless steel can be inserted between analuminum structure and a stainless steelstructure. This allows for direct conven-tional welding of the stainless steel struc-ture to the stainless steel side of thebimetallic strip, and then direct conven-tional welding of the aluminum structureto the aluminum side of the bimetallicstrip. The resulting joint is a fully weldedaluminum-to-stainless steel structure.This application allows designers and fab-ricators to apply specific materials to thelocation or function that they are bestsuited for without having to make the en-tire structure out of one of the metals,thereby limiting the design’s potential.

Precious Metal Conservation Thin layers of precious metals, refrac-

tory metals, and other expensive alloys canbe explosively welded to the specific areaof a part where it is needed. This not onlysignificantly reduces the cost of the man-ufactured part, but allows for the use ofmore structurally sound materials to beused where strength is required.

Galvanic Corrosion PreventionExplosively welded dissimilar metals

act as a significant inhibitor to galvanic

activity that would occur between me-chanically fastened dissimilar metals.Maritime applications benefit from theuse of explosively welded metals by allow-ing for weldable transitions between dis-similar metals that significantly reduce oreliminate galvanic corrosion.

Corrosion-Resistant Linings A common application for explosively

welded (or clad) metals is as corrosion- orerosion-resistant linings for pressure ves-sels, chemical process tanks, heat ex-changers, and tube sheets — Fig. 8.

In addition, there is a significant costreduction afforded by the opportunity toutilize structural steels to improve wallstrength without having to manufacturethe entire structure from the typicallycostly corrosion-resistant metals.

Bearing Surfaces Similar to corrosion-resistant linings,

bearing materials can be explosivelybonded to sheets of metal. This allows astructurally sound component to have abearing surface clad directly to it, signifi-cantly reducing the wear of the part andimproving its operation.

Radiation Shielding Thin layers of shielding materials can

be bonded to other structural metals orcomponents. This has been a cost-effec-tive method of providing radiation shield-ing to satellites.

Examples

Marine Shipbuilding ComponentsBecause of the galvanic corrosion-

prevention characteristics referred to pre-viously, explosion welded transition ma-terials have become prevalent throughoutthe shipbuilding industry. Cylindricallybonded aluminum-to-steel transition

Fig. 5 — The chisel test showed no separa-tion at the weld in this stainless steel-copper specimen.

Fig. 4 — Tensile testing of this Al-stainlesssteel-Al specimen displays the strength ofthe explosion welds exceeded that of thealuminum.

Fig. 3 — The characteristic wavy explosiveweld appearance is clearly visible in thiscross section.

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rings enable shipbuilders to securely weldforged steel cup-and-cross tie-down ele-ments into the aluminum deck of aircraftcarriers — Lead photo and Fig. 9.

Hermetic Electronic PackagingElectronics packages used for modern

military and aerospace applications in-variably require the use of lightweight ma-terials while simultaneously providing theutmost in reliability. In the past, connec-tors made from steel or Kovar™ were goldplated then soldered into a gold-platedaluminum housing. With prolonged usageand temperature swings, the solder jointswere prone to fail. This allowed moistureto enter the housing and cause the entireunit to fail. By utilizing precision-machined aluminum-to-steel explosivelywelded transitions, the assembly can belaser beam welded, eliminating solderjoints entirely — Fig. 10. As an added ben-efit, should a connector fail, the damagedpart can be machined out and a new con-

nector welded in its place, without affect-ing the rest of the package. With soldertechnology, connector removal requiresheating the entire package, which coulddestroy the housing.

Chemical Process VesselsOn the other dimensional scale, explo-

sion welding can be used to create somerather large structures. Dynamic Materi-als Corp. explosively clads large plates ofhot-rolled carbon steel with corrosion-resistant materials such as titanium orstainless steel. The plates are rolled intocylinders then welded together to formlarge process and pressure vessels. Plates40 ft long and ten ft wide are routinelybonded — Fig. 11.

Sputter TargetsCathodic sputtering targets used in

semiconductor manufacturing are madefrom expensive, high-purity materials tominimize contamination during the sput-

tering process. In order to reduce thequantity consumed of these materials, thetarget plate is usually welded to a lower-cost backing plate, which provides me-chanical support and enables coolingthrough the back side. This bond must pro-vide very good thermal and electrical con-ductivity between the carrier and backerplates, as well as maintain these proper-ties as the target erodes through normaloperation and becomes thinner. Duringoperation, thermal stresses develop thatcould lead to a debonding of soldered orbrazed assemblies. Explosion welding pro-vides superior weld strength comparedwith these other technologies, resulting ina more reliable interface that will extendthe effective lifetime of the target.

Explosion welded materials offer de-signers unique possibilities to design and

Fig. 9 — Explosion welded aircraft tie-downfittings welded into aluminum aircraft car-rier decks never need replacing. See leadphoto.

Fig. 10 — This aluminum electronics pack-age features steel connectors in an all-welded construction.

Fig. 11 — This large high-pressure autoclaveis used to leach laterite nickel ore in hot sul-furic acid. The vessel is constructed fromcorrosion-resistant titanium clad to 4-in.-thick steel for strength. (Courtesy of Dy-namic Materials Corp., Boulder, Colo.)

Fig. 6 — This stainless steel-aluminumsample was subjected to a 90-deg bend test.The part yielded in the aluminum awayfrom the weld line.

Fig. 7 — Eight metallic layers were explo-sively welded in multiple operations to pro-duce this colorful ornamental stack.

Fig. 8 — This shell and tube heat exchangerwas manufactured from zirconium-cladsteel for use in the chemical industry. (Cour-tesy of Dynamic Materials Corp., Boulder,Colo.)