Effects of Multi-pass Friction Stir Processing on Microstructures and Mechanical Properties of 1060Al/Q235 Explosive Composite Plate

There always exist steel cuttings, holes and cracks at the interfaces in the explosive composite plate. Multi-pass friction stir processing (M-FSP) is proposed in this research to optimize the interface microstructure and the interface connection for 1060Al/Q235 explosive composite plate. Results show that the microstructures of 1060Al after M-FSP are fine and uniform owing to the strong stirring effect and recrystallization. Micro-defects formed by the explosive welding can be repaired by the M-FSP. However, M-FSP can also form tunnel defects in the aluminum, especially when the passes are one and two. The melting block and the melting lump in the composite plates are easy to become source of crack. The shear strengths and the bending properties for the 1060Al/Q235 explosive composite plate after M-FSP are the best when the passes are three, with the tool rotation speed of 1200rpm and the forward speed of 60mm/min. The optimized interfaces for the explosive composite plate after M-FSP are mainly by the metallurgical bondings, with a certain thickness and are discontinuous. Therefore, the crack extension stress is the largest and the mechanical properties are the best.

disappearance of cold flake [13] . Pourali et al. investigated the different tool rotations and weld speeds for lap weld joint of 1100Al/St37 steel composite plate. They found that the tensile strength of composite with high tool rotation and low weld speed is superior since the interface of this joint formed a properly thick bonding layer compared with other parameters [14] .
However, FSP can also reduce the mechanical properties when the parameters are inappropriate. Gupta et al. also found that tunnel defects are both formed in AZ31 magnesium alloy whether in the truncated conical tool or the cylindrical tool [11] . Kima et al. found that tunnel defects are easily formed in ADC12 aluminum die casting alloy during welding [15] .
In order to improve the mechanical properties of composite plate, obtaining the appropriate interface is the main method since the interface of composite plate is the main carrying structure during service. Thus, there are two main factors that affecting the microstructures and mechanical properties of composite plate after FSP with different passes, which include the interface connections and the tunnel defects.
In the first part, the bonding strength of composite plate is related to the interface connection mechanisms, which include the mechanical connection and the metallurgical bonding. Pourali et al. found that mechanical connections are the main connection methods of aluminum/steel composite plate under the condition of low welding speed [14] . During explosive welding, the energies of explosion flow cause some of the steel and aluminum to soften or even melt, mix and finally form the morphologies of hooks or vortexes at the interface [16][17] . Meanwhile, defects such as steel cuttings and microcracks caused by explosion weaken the strength of composite. Since FSP can refines the second particles size and reduces the cold flake of aluminum die casting alloy [12][13] , the steel cuttings and microcracks could also be repaired by strong stirring of aluminum. Therefore, FSP is proposed to repair the defects formed by explosive welding.
The metallurgical bondings mainly include intermetallic compounds (IMCs) and barely have few solid solutions. In the process of welding, the interface layer mainly consisting of IMCs is formed at the interface through diffusion and metallurgical Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 29 January 2020 doi:10.20944/preprints202001.0354.v1 reaction due to the effect of temperature and pressure [18][19] . Since the mutual solubility of aluminum and steel is few, the mechanical properties are poor even if forming the solid solution. Therefore, IMCs are interface connections basis for the composite plate.
Fe3Al, FeAl, Fe2Al5, FeAl2 and FeAl3 (Fe4Al13) are the main IMCs formed in the process of welding. According to the content ratio of aluminum to steel, IMCs can be divided into two categories: IMCs rich in Fe which are rigid and IMCs rich in Al which are toughness. [20][21][22][23][24] . In addition, Bozzi et al. found that when the thickness of IMCs is 8μm, mechanical properties of the composite plate are the best; While the thickness increases to 42μm, mechanical properties of the composite plate are the worst [25] . In other words, the thickness of IMCs for the composite plate is a key factor influencing its mechanical properties. When the thickness of IMCs is lower than a certain value, the mechanical properties of composite plate improve with the IMCs thickness increasing [26][27] . With the increase of FSP passes, the accumulated heat input increases and finally the thickness of IMCs increases. However, too thick IMCs decrease the bonding strength of interface. Thus, it is necessary to use M-FSP for obtaining the appropriate thickness of IMCs.
In the second part, tunnel defects are usually found in the composite plate after the single-pass FSP [11,15,16,28] . Kima investigated different tool plunge downforces of FSW for ADC12 aluminum die casting alloy. They found that tunnel defects are caused by the insufficient heat input [15] . Javad et al. presented a mathematical model for the heat input generation during friction stir welding of 1060 aluminum alloy. They also found that the insufficient heat input can form tunnel defects in the matrix of aluminum during welding [29] . Due to the insufficient heat input, the insufficient flow of material stirred causes the tunnel defects. Therefore, the heat input is increased by increasing the The above researches provide a good data basis in the production of aluminum/steel composite plate in the future.

Experimental materials and methods
The initial material for the sample used in this experiment was 1060Al/Q235 explosive composite plate and its chemical compositions (wt.%) were measured by ICP direct reading spectrometer, as shown in Table 1. It conformed to 1060Al and Q235 standard specification. The thickness of aluminum/steel explosive composite plate was 6mm, and the thickness ratio of aluminum to steel was 1:1. was used to observe structures of the interface.
The shear specimens were designed on the basis of the national standard GB/T2651-2008. The bending specimens were designed on the basis of the national standard GB/T232-2010. The sizes of these specimens are shown in Fig.1. The test was carried out using an electronic universal testing machine CSS-44100, at an initial strain rate with a loading speed of 1.2×10 -4 /s. The microhardness tests were carried out on HX-1000TM/LCD semi-automatic micro-indentation hardness testing system according to the national standard GB/T 229-2007. boundary distinctly, and the RS is in a shape of a gradual change. This is because the temperature and flow fields on the AS are different from those on the RS [30][31][32][33] .  Meanwhile, holes are clearly seen in Fig.2. For the single-pass FSP ( Fig.2(a)), the area of large hole is 9.8×10 3 μm 2 , and the smaller one is 3.4×10 3 μm 2 . However, just one single hole is found in Fig.2 Fig.2(c). However, when the passes of M-FSP are four, the hole appears again in the AS area ( Fig.2(d)). Since the distances of these holes from the interface are nearly the same, these holes are the tunnel defects resulted after FSP. The areas of tunnel defects in repair zone with different passes are shown in Table 2. Therefore, with the increase of passes, the tunnel defects decrease to some extent. It can be concluded that M-FSP can also repair tunnel defects created by single-pass M-FSP. Fig.3 shows microstructures of 1060Al/Q235 explosive composite plate after FSP with different passes. It can be observed that microstructures of base metal (BM) for the 1060Al are α-Al ( Fig.3(a)). The microstructures of BM for Q235 are ferrites and pearlites ( Fig.3(b)). Compared with BM for 1060Al, the grains of repair zone for 1060Al are refined by the stirring effects of M-FSP (Fig.3(c)). Meanwhile, with the influence of temperature and stir, the microstructures of thermo-mechanical affected zone (TMAZ) are stretched ( Fig.3(d)). Moreover, Fig.3 Fig.4 shows comparisons between the unrepaired interface and the repaired interface. The steel cuttings are clearly seen at the interface of composite plate without FSP ( Fig.4(a)). The sizes of steel cuttings are about 45μm (Fig.4(b)). Such a large steel cutting has a severely disadvantage effect on the strength of the interface. Non-uniform and instantaneous temperatures caused by explosive welding are responsible for these steel cuttings. However, these steel cuttings disappear in the interface with FSP ( Fig.4(c)). Through the temperature and plastic flows of aluminum stirred after FSP, these residual steel cuttings react with aluminum and other materials to form new IMCs at the interface. Fig.4(d) and ( at the interface are a kind of Al-rich phase [21] . This is to say that FSP can repair the defects remained by explosive welding and optimize microstructures and properties of composite plate. certain thickness has a superior effect on the properties of composite plate [22] . Since explosive welding is an instantaneous welding process, the composite plate could not obtain enough thickness IMCs layer. Therefore, M-FSP is used to repair the bonding interface and obtain a proper thickness IMCs layer. With the increase of passes, the proportions of metallurgical bondings increase in the interface (( Fig.5 (d)). The increase of passes brings the increase of heat input, which will aggravate the diffusion and metallurgical reaction at the interface. The IMCs layer becomes thicker due to this.

Interface characterization
Ultimately, the thick IMCs layer leads to the increase of metallurgical bondings. straight with just single-pass FSP ( Fig.6(a)). In other words, FSP can not repair the interface of composite plate with the single-pass FSP effectively. Compared with single-pass FSP, there is a little more thickness of IMCs in the interface with two passes.
However, the effect of repair with two passes is still insufficient in contrast to the whole interface. That is to say, the effect of repair is not obvious (Fig.6(b)). The thickness of IMCs clearly increases with three passes. There is a superior effect of repair with the discontinuous mechanical connections (Fig.6(c)). The interface is so thick that the bonding strength of the interface is low. Too many passes make the heat input too large, which will make the interface too thick. Due to the existence of too thick interface, the original discontinuous mechanical connections have been connected together. The properties of composite plate must be affected with the too thick interface and continuous connections (Fig.6(d)). It is clear that too many passes will cause the deterioration of properties for the interface.  Fig.7 shows the fracture morphologies and mechanical property curves of shear specimens, and the shear strength is shown in Table 3. The fracture mode of shear specimens exhibits typical brittle fracture ( Fig.7(a)). The curve of single-pass shows the phenomenon of yielding caused by the tunnel defects under load. While the curve of two passes and three passes exhibit the more obviously brittle fracture ( Fig.7(b)).   The SEM and EDS analysis of shear specimens are shown in Fig.8. There are some obvious strippings in the fracture surface ( Fig.8(a), (b) and (c)). It is analyzed that these strippings are formed by the failure of the original mechanical connections at the interface under load. The second phase particles are also obviously seen in the SEM.

Mechanical properties
The EDS result shows that Si element is contained in these particles, which indicates that the second phase particles are Al-Si compounds ( Fig.8(d)). A small amount of dispersed Al-Si compounds can improve mechanical properties of composite plate. The shear mechanical properties of BM are also carried out, as shown in Fig.9.
The results show that the phenomenon of yield occurs on the aluminum of BM. The maximum failure tensile force is 1777N. However, the failure strength is not the shear strength, while is the yield strength of 1060Al. The failure strength is 118.47MPa, which is conformed to 1060Al standard specification. The fracture position of BM without FSP is in the aluminum side rather than at the interface. This is due to the refining effect of FSP in aluminum, which makes the aluminum get grain refining strength after FSP. Therefore, FSP has an obvious repairing effect on the interface of composite plate, which can improve the shear strength to some extent. The results of bending test are shown in Table 4. The typical bending specimens are shown in Fig.10. Whether repaired or not, the bending strength is superior. Mainly due to the excellent plasticity of 1060Al, there are almost no cracks during the process of bending. However, cracks can be seen in aluminum after FSP with four passes, as shown in Fig.11. The failure angle of bending is 137°. Excessive reduction of the shoulder leads to the thinning of BM with the cracking phenomenon caused by the reduction of the bearing area in bending. Therefore, the reduction of the shoulder should be well controlled in the M-FSP to ensure the properties of bending.

Discussion
The relationships between microstructures and mechanical properties of aluminum/steel composite plate with different passes are analyzed above. The results show that M-FSP can not only repair the interface to some extent, but can also repair the tunnel defects in the aluminum. However, too many passes also have negative effects on the interface and metal stirred. Therefore, the fracture mechanism of interface for 1060Al/Q235 is obtained by studying the relationship between structures and properties of aluminum/steel composite plate after FSP with different passes. The fracture mechanism can be divided into the following three categories: Firstly, when the interface of aluminum/steel composite plate is thin and flat, the crack extension stress of interface is small due to the flat interface. Meanwhile, the aluminum is so soft that it is easy to occur the phenomenon of yield. Several voids will preferentially be formed in the aluminum. Cracks are formed by the accumulation and growth of voids with the gradually increasing load. Eventually, cracks extend to the aluminum, and lead to the failure of composite plate, as shown in Fig.13. This failure mostly occurs in the aluminum/steel composite plate without FSP. It is basically agreed with the yield fracture of BM in the shear test. Secondly, when the interface of aluminum/steel composite plate has a certain thickness and is discontinuous, the crack extension stress of the interface is large.
Several micro-cracks will preferentially be formed at the interface under load. If the composite plate has been inadequately repaired by less passes, there could be some defects at the interface. The phenomenon of stress concentration caused by defects would accelerate the development of micro-cracks. Therefore, those micro-cracks would coalesce each other rapidly, and ultimately lead to the fracture of the whole interface. However, if the composite plate has been adequately repaired by enough passes, the interface would have a certain thickness. The propagation of micro-cracks needs a path. In addition, those discontinuous mechanical connections improve the bonding strength of interface, which make the cracks propagate slowly. Thus, a larger load is needed to make the composite plate break. This failure occurs mostly in the aluminum/steel composite plate after repairing by FSP, which is basically consistent with the case of single pass, two passes and three passes, as shown in Fig.14. Thirdly, when the interface is too thick, although there are discontinuous mechanical connections at the interface, the overly thick interface connects the discontinuous mechanical connections to each other, which results in a bending and continuous interface. The crack extension stress of the interface is also large due to the bending interface. Micro-cracks will preferentially be formed at the interface under load.
Due to the thicker interface, those micro-cracks could easily bypass the mechanical connections. Therefore, the micro-cracks grow rapidly with the increasing load. Finally, the composite plate will break due to the fracture of the interface. This failure occurs more often in the composite plate after FSP with too many passes. The fracture of the aluminum/steel composite plate is basically the same as that of four passes, as shown in Fig15. 3. The melting block and the melting lump in the composite plates are easy to become originals of crack. Therefore, when the interfaces of composite plate are mainly by the metallurgical bondings, with a certain thickness and are discontinuous, its bonding strength is superior.