Journal of Materials Processing Technology 85 (1999) 166–170

Microstructure and bonding strength of impact-weldedaluminium–stainless steel joints

Hidefumi Date a,*, Shin Kobayakawa b, Masaaki Naka c

a Department of Mechanical Engineering, Faculty of Engineering, Tohoku Gakuin Uni6ersity, 1-13-1,Tagajo, Miyagi, 985, Japanb Tohoku Gakuin Uni6ersity, 1-13-1,Tagajo, Miyagi, 985 Japan

c Joining and Welding Research Institute, Osaka Uni6ersity, 1-13, Mihogaoka, Ibaraki, Osaka, 678, Japan

Abstract

An aluminium projectile was impact-welded on a stainless steel target using a nitrogen gas gun at an impact velocity over 250m s−1. Effects of surface roughness of the impact face of the target on the bonding area and the strength of the bonding areawere examined. The microstructure and element distribution in the joint were analyzed and the experimental results ofconcentrations of the elements in the compound layer at the interface were compared with the theoretical values. The followingresults were obtained. It was clarified that the bonding strength of the area which increased with the decrease of surface roughnesswas much lower than that of the region which was independent of surface roughness. The thickness of the compound layer formedat the interface increased with the impact velocity. The experimental results of the concentrations of the elements in the layerhardly depended on impact velocity and were close to the theoretical results in which equivalent heat was generated at impactfaces of the projectile and the target. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Impact-welding; Surface roughness; Microstructure of compound layer; Concentration of element; Bonding strength

1. Introduction

It has previously been reported by our group, thatthe projectile was impact-welded to a target at impactvelocity of 250 m s−1 or more using a gas gun [1]. Itwas clarified that the projectile impact-welded to thetarget showed a different melting temperature like ex-plosive welding [2]. Though the compound layer wasobserved at the interface, a periodic wavy interface wasnot formed in impact welding unlike in explosive weld-ing. Accordingly, it was deduced that the bondingmechanism of impact-welding was different from thoseof explosive welding [3].

Here, impact-welding was carried out using an alu-minium projectile and stainless-steel target for examin-ing the bonding mechanism because an acceptablewelding of aluminium projectile to a stainless steeltarget has been obtained over a wide range of impactvelocities and the compound layer formed was firm [4].Since it was observed that the bonding area obtained in

this experiment was much larger than that in the previ-ous one [4], the reason for the increase of bonding areaand the bonding strength of the incremental bondingarea were considered. In addition, the experimentalresults of concentration of the elements was evaluatedby comparison with two theoretical results in which theconcentration depends on the difference of heat con-ductivity between the projectile and the target or heatgeneration at the interface.

2. Materials and experiment

The materials of the target and the projectile werestainless-steel circular plates (SUS304) of diameter 40mm and thickness 5 mm and pure aluminium rods(A1050) of diameter 11 mm and length 20 mm. Thealuminium was annealed at 623 K for 3.6 ks. Theimpact face of the target was polished using polishingpaper after grinding. In the gas-gun apparatus shown inFig. 1, an aluminium projectile collided with a stainlesssteel target at an impact velocity of about 250 m s−1 ormore using highly compressed nitrogen gas. Impact

* Corresponding author. Fax: +81 022 3687070; e-mail:date@tjcc.tohoku-gakin.ac.jp

0924-0136/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved.

PII S0924-0136(98)00284-2

H. Date et al. / Journal of Materials Processing Technology 85 (1999) 166–170 167

welding was carried out in a vacuum chamber becausethe air compressed between the target and impact faceof the projectile prevented welding. The impact velocityof the projectile was evaluated by measuring the time topass a fixed distance using a laser beam system. Thebonding area was observed and measured using a scan-ning acoustic tomograph. The microstructures and ele-ment distribution in A1050/SUS304 joints of thespecimen sliced to a thickness of about 3 mm wereanalyzed by means of SEM and energy dispersive X-rayspectroscopy (EDX). The three point tension test formeasuring the bonding strength of the specimen slicedat a thickness of 3 mm was carried out at a tensilespeed of 2.0 mm min−1 [1].

3. Results and discussion

3.1. Effect of surface roughness of target on bondingarea and strength

The ultrasonic image of the bonding region at animpact velocity of 304 m s−1 obtained by scanningacoustic tomograph is shown in Fig. 2. The large whitecircle and the black area at the center denote the outerdiameter of the target and the bonding region, respec-tively. Lines A–A% and B–B% show slicing lines fortension test. The bonding area is much larger comparedwith that of A1050/SUS304 joint shown in Fig. 3 [4].Fig. 3 is the result reported previously and the impactvelocity was 291 m s−1. The bonding region in thefigure shows the region of high bonding strength at thecenter and an area of weak bonding strength like a ringformed due to the folding of the projectile.

It is obvious that the bonding area depends on thesurface roughness of the impact face of the targetbecause the impact surfaces of the target in Fig. 2 andFig. 3 have been polished using the polishing paper of1000 mesh and 800 mesh, respectively. The average

Fig. 2. Ultrasonic image of bonding area at impact velocity of 304 ms−1 (Ra=0.02 mm).

surface roughness (Ra) polished by the paper of 800mesh was 0.05 mm and that of 1000 mesh was 0.02 mm.The effect of surface roughness on the area is shown inFig. 4. The range of impact velocity was from 340 to360 m s−1. It is clear that the decrease of the surfaceroughness leads to increase of the bonding area.

Three-point tension test was carried out for evaluat-ing bonding strength distribution on the bonding re-gion. A load of about 70% of the ultimate tensilestrength of aluminium was placed on the sliced tensilespecimen. The ultrasonic images of the sliced specimenbefore and after the loading described above are givenin Fig. 5(a and b). It is observed that the outer part ofthe bonding region was torn from the target. The ratioof local fracture area to the bonding region is defined

Fig. 3. Ultrasonic image of bonding area at impact velocity of 291 ms−1 (Ra=0.05 mm).Fig. 1. Experimental apparatus for impact welding test.

H. Date et al. / Journal of Materials Processing Technology 85 (1999) 166–170168

Fig. 4. Effect of surface roughness of target on bonding area.

Fig. 6. Ratio of local fracture to bonding area plotted againstbonding area.

where S is a ratio of the local fracture area to thebonding area, L the bonding length before tension testand L % the bonding length after the test. The ratioplotted against the bonding area is shown in Fig. 6. Thelength of L % measured was 8–10 mm and independentof surface roughness. However, since the bonding areadepended on surface roughness as shown in Fig. 4, it isdeduced from Fig. 6 that the ratio depends on thesurface roughness. Eventually, the bonding strength ofthe incremental area which increases with the decreaseof surface roughness is weaker than that of a bondingarea at the center corresponding to L % in Fig. 6. This isbecause the bonding strength at the center was beyondthe fracture strength of the aluminium projectile.

3.2. Formation of compound layer at the interface

The microstructure by SEM in the joining interfaceat an impact velocity of 308 m s−1 is shown in Fig. 7.

using the following equation because the width of thebonding area before the tension test did not changeafterwards.

S= (L−L %)/L, (1)

Fig. 5. Comparison of ultrasonic image at impact velocity of 264 ms−1 (a) before and (b) after tension test.

Fig. 7. Micrograph in joining interface at impact velocity of 308 ms−1.

H. Date et al. / Journal of Materials Processing Technology 85 (1999) 166–170 169

Table 1Quantitative analyses of elements of phases at impact velocity of 308m s−1

Cr (mass%) Ni (mass%)Al (mass%) Fe (mass%)

8.2 3.6M 2 55.7 32.635.7 8.81 51.5 47.1

8.232.56 3.455.829.0 3.83 52.1 35.2

been known from the results of the previous experimentthat the maximum compound layer was observed at theneighborhood of the center of the bonding region, themaximum compound layer shown in Fig. 8 denotes amaximum value in the cross section cut along the planewhich includes the center of an ultrasonic image of thebonding region. The maximum thickness of the com-pound layer increases with the impact velocity as shownin Fig. 8.

Since temperature rise of the specimen subjected to adynamic plastic deformation is evaluated by the plasticwork which is a function of strain rate, melting regionincreases with the strain rate determined from impactvelocity. Accordingly, it is valid that its thickness in-creases with impact velocity as shown in Fig. 8. If thecompound layer is brittle, it is deduced that the layerbecomes weaker with the thickness of the layer. How-ever, the strength of the compound layer with a maxi-mum thickness formed at impact velocity of range from340 to 360 m s−1 was stronger than the fracturestrength of an aluminium projectile. Therefore, it isconsidered that the layer is not brittle and the thicknessof the layer has hardly any influence on the bondingstrength.

3.3. Estimation of the concentration of elements

When heat is generated momentarily at the contactsurface of two different materials, like impact-welding,and both materials are melted, the phenomenon shouldbe analyzed as a moving boundary problem includingmelting and solidification considering plastic wavepropagation. However, the analysis is not easy and it isdifficult to formulate for unknown mixing mechanismof two melting materials at impact-welding. It is validfor the basic model of the mixing mechanism that theratio of melted volume is equal to that of the concen-tration of elements as follows.

Assuming that heat is momentarily supplied at theinterface of the two different metals and these metals inthe vicinity of the interface are melted successively bythe conducted heat, the ratio of melted volume is givenby the following equation [6]:

M1

M2

='rav1 cav1 lav1

rav2 cav2 lav2

cav2(Tm2−T0)+Hm2

cav1(Tm1−T0)+Hm1

(2)

where M is the melted volume, l the coefficient of heatconduction, Tm the melting temperature, T0 the roomtemperature, Hm the latent heat. The subscript av indi-cates average of integration from room temperature tothe melting temperature. Subscripts 1 and 2 denote thematerials, respectively.

In addition, assuming that equivalent heat was gener-ated in the metals in the neighborhood of the interface(ignoring heat conduction), Eq. (2) is converted intoEq. (3).

The compound layers were observed in the almost-join-ing interface bonded using the gas-gun method. Thefour white curved lines denote profiles of aluminium(Al), steel (Fe), chromium (Cr) and nickel (Ni) distribu-tions on the straight line scanned by EDX and thenumbers indicate the points at which the elements wereanalyzed quantitatively. M2 shows a middle point inthe compound layer. As shown in Fig. 7, all the ele-ments are distributed uniformly in the layer. In addi-tion, it was confirmed that the thickness of the layerincreased with impact velocity and the region of uni-formly distributed elements also expanded with thethickness of the layer. Though the appearance of asimilar distribution of the elements formed in a com-pound layer has been reported in the explosive welding,the results of the quantitative analysis of the elementswere different from those results [5]. The concentrationsat each point and M2 denoted in Fig. 7 are shown inTable 1. It appears that the concentration is indepen-dent of the position in the layer. The concentration inthe layer also barely depends on impact velocity. Theconcentration of elements at the middle points showsthat Ni is 3–4 mass%, Cr is 8–9 mass%, Fe is 31–37mass% and Al is 49–56 mass% at an impact velocityfrom 260 to 360 m s−1.

The maximum thickness of a compound layer plottedagainst impact velocity is given in Fig. 8. Since it has

Fig. 8. Maximum thickness of compound layer plotted against impactvelocity.

H. Date et al. / Journal of Materials Processing Technology 85 (1999) 166–170170

Table 2Theoretical and experimental results of concentration of aluuminum

Result of Eq. 5 (mass%) Experiment (mass%)Result of Eq. 3 (mass%)

46.5 49–56A1+SUS304 70.6

M1

M2

=cav2(Tm2−T0)+Hm2

cav1(Tm1−T0)+Hm1

(3)

The melting heat of SUS304 used a value of 304.1 kJkg−1. The specific heat, density and coefficient of con-ductivity are assumed to be independent of the impactforce and temperature. The results of a numerical anal-ysis are shown in Table 2. The experimental results ofconcentration of aluminium is 49–56 mass% and theexperimental results are close to the numerical onesobtained using Eq. (3). Accordingly, the ratio of meltedvolume is more affected due to the equivalent heatgeneration at both impact ends, rather than the heatconduction of both metals. The heat is generated by aconversion of plastic work due to plastic wave propaga-tion. The temperature rise is accomplished at the timeduring which an elastic precursor wave moves back andforth for one time in a projectile [7]. Therefore it isindicated that a compound layer is almost formedmomentarily during impact.

4. Conclusion

The impact-welding of an aluminium projectile to astainless steal target was carried out and the followingresults were obtained:

(1) Decrease of the surface roughness caused anincrease of the bonding area. However, the incrementalbonding area due to the decrease of the roughness hadhardly any bonding strength.

(2) The compound layer was formed at the bondinginterface and the maximum thickness of the layer whichappeared at the center of the bonding area increasedwith the impact velocity.

(3) The concentration of the elements was indepen-dent of the impact velocity and close to the theoreticalresult in which an equivalent heat was generated atimpact faces of a projectile and a target.

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Weld. Soc. 11 (1993) 16–21.[6] Y. Ishii, T. Onzawa, T. Oinuma, J. Jpn. Weld. Soc. 38 (1969)

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