key: cord-0425699-xjr3hy8y
authors: R, Indhu; Tak, Manish; L, Vijayaraghavan; S, Soundarapandian
title: Microstructural and mechanical properties of complex phase steel to aluminium alloy welded dissimilar joint
date: 2020-12-31
journal: Procedia Manufacturing
DOI: 10.1016/j.promfg.2020.05.047
sha: 734c6e33808f109f6252379fd8eccba0d5bacb1f
doc_id: 425699
cord_uid: xjr3hy8y

Abstract Lightweight dissimilar materials that exhibit increased performance and functionality are of great interest in the field of automobiles to reduce fuel consumption and improve fuel economy. The properties of different lightweight materials are jointly utilised to achieve the product performance. In this study, dissimilar materials such as complex phase steel and aluminium alloy are joined using a high power fiber coupled diode laser. Laser processing parameters (laser power and scanning speed) are optimized such that the thickness of the intermetallics is minimum, so as to obtain maximum mechanical strength. The intermetallics formed at the weld interface are mostly aluminium rich, which are brittle in nature and deteriorate the strength of the joint. The microstructural characterization results showed that the depth of penetration of the weld increased with increase in laser power or decrease in scanning speed. The maximum depth achieved was 1200 µm at a laser condition of power 3 kW and 8 mm/s scanning speed. However, due to increased depth of penetration the intermetallic thickness also increased to 17 µm. Maximum mechanical resistance of 254 N/mm was achieved at a laser power of 4 kW and scanning speed of 12 mm/s, when the intermetallics thickness was in the range of 11 µm.

In the recent years, the regulations of carbon dioxide emission and fuel economy are becoming more stringent. Light weight vehicles has been a key strategy in minimizing the fuel consumption. The automobile sectors are trying new ways to decrease the weight of the vehicle without compromising the performance of vehicle and safety of the passengers. The use of multi-material vehicle structures is an efficient countermeasure against this problem [1] . Higher strength, high crash energy absorption and enhanced formability of advanced high strength steels (AHSS) have attracted the attention of these materials in the automotive industry. AHSS are newer generation of steels that meet the requirements of safety, efficiency, emissions and durability at low cost. They provide extremely high strength, while maintaining the high formability requirements for manufacturing [2] . The excellent properties of the AHSS was achieved by their alloying elements (carbon, manganese, silicon, nickel, chromium and molybdenum), micro-alloying elements (niobium, titanium, vanadium, boron) and metallurgical strengthening mechanisms such as grain refinement, solid solution hardening, precipitation hardening and dislocation hardening [3] . Aluminium (Al) on the other hand is considered as the best metal for lightweight structures and components in aerospace and automotive industries [4] . Thus joining of AHSS to Al can effectively contribute to reduction of the weight of the vehicle and fuel usage. Combining new materials will require a systematic approach for material selection, as these materials react in different ways which might need new manufacturing systems. There are significant challenges when materials of

In the recent years, the regulations of carbon dioxide emission and fuel economy are becoming more stringent. Light weight vehicles has been a key strategy in minimizing the fuel consumption. The automobile sectors are trying new ways to decrease the weight of the vehicle without compromising the performance of vehicle and safety of the passengers. The use of multi-material vehicle structures is an efficient countermeasure against this problem [1] . Higher strength, high crash energy absorption and enhanced formability of advanced high strength steels (AHSS) have attracted the attention of these materials in the automotive industry. AHSS are newer generation of steels that meet the requirements of safety, efficiency, emissions and durability at low cost. They provide extremely high strength, while maintaining the high formability requirements for manufacturing [2] . The excellent properties of the AHSS was achieved by their alloying elements (carbon, manganese, silicon, nickel, chromium and molybdenum), micro-alloying elements (niobium, titanium, vanadium, boron) and metallurgical strengthening mechanisms such as grain refinement, solid solution hardening, precipitation hardening and dislocation hardening [3] . Aluminium (Al) on the other hand is considered as the best metal for lightweight structures and components in aerospace and automotive industries [4] . Thus joining of AHSS to Al can effectively contribute to reduction of the weight of the vehicle and fuel usage. Combining new materials will require a systematic approach for material selection, as these materials react in different ways which might need new manufacturing systems. There are significant challenges when materials of 

Lightweight dissimilar materials that exhibit increased performance and functionality are of great interest in the field of automobiles to reduce fuel consumption and improve fuel economy. The properties of different lightweight materials are jointly utilised to achieve the product performance. In this study, dissimilar materials such as complex phase steel and aluminium alloy are joined using a high power fiber coupled diode laser. Laser processing parameters (laser power and scanning speed) are optimized such that the thickness of the intermetallics is minimum, so as to obtain maximum mechanical strength. The intermetallics formed at the weld interface are mostly aluminium rich, which are brittle in nature and deteriorate the strength of the joint. The microstructural characterization results showed that the depth of penetration of the weld increased with increase in laser power or decrease in scanning speed. The maximum depth achieved was 1200 µm at a laser condition of power 3 kW and 8 mm/s scanning speed. However, due to increased depth of penetration the intermetallic thickness also increased to 17 µm. Maximum mechanical resistance of 254 N/mm was achieved at a laser power of 4 kW and scanning speed of 12 mm/s, when the intermetallics thickness was in the range of 11 µm.

In the recent years, the regulations of carbon dioxide emission and fuel economy are becoming more stringent. Light weight vehicles has been a key strategy in minimizing the fuel consumption. The automobile sectors are trying new ways to decrease the weight of the vehicle without compromising the performance of vehicle and safety of the passengers. The use of multi-material vehicle structures is an efficient countermeasure against this problem [1] . Higher strength, high crash energy absorption and enhanced formability of advanced high strength steels (AHSS) have attracted the attention of these materials in the automotive industry. AHSS are newer generation of steels that meet the requirements of safety, efficiency, emissions and durability at low cost. They provide extremely high strength, while maintaining the high formability requirements for manufacturing [2] . The excellent properties of the AHSS was achieved by their alloying elements (carbon, manganese, silicon, nickel, chromium and molybdenum), micro-alloying elements (niobium, titanium, vanadium, boron) and metallurgical strengthening mechanisms such as grain refinement, solid solution hardening, precipitation hardening and dislocation hardening [3] . Aluminium (Al) on the other hand is considered as the best metal for lightweight structures and components in aerospace and automotive industries [4] . Thus joining of AHSS to Al can effectively contribute to reduction of the weight of the vehicle and fuel usage. Combining new materials will require a systematic approach for material selection, as these materials react in different ways which might need new manufacturing systems. There are significant challenges when materials of 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to different chemical, mechanical, thermal or electrical properties are joined together [5] . In fusion welding process, the most relevant difficulty in joining steel to Al is the formation of intermetallic compounds (IMCs). The IMCs exhibit extremely low plastic deformation, which is the major cause for brittleness of the joint [6] . Thus reducing the IMCs thickness can help in achieving a sound joint with good mechanical properties. Another difficulty is the low wettability of liquid Al on solid steel. The wettability can be improved by coating a layer of zinc on surface steel. Most of the AHSS available are generally galvanized, which is beneficial for fusion welding techniques. According of Al-Fe system, the Al-rich phases formed at room temperature are FeAl2, Fe2Al5 and FeAl3. These Al-rich IMCs are brittle in nature. The Fe-rich phases are FeAl and Fe3Al, which are relatively ductile than Al-rich IMCs [7] . The challenge in fusion welding of steel to Al is the ability to avoid or minimize the formation of IMCs below 10 µm [8] . The formation of IMCs is mainly controlled by atomic diffusion, which is a time dependent phenomenon and greatly enhanced by temperature. Therefore it is necessary to keep the process temperature as low as possible. Laser welding process has several advantages in terms of design flexibility and fabrication rate. Laser welding is a low heat input process, which could effectively minimize the heat affected zone (HAZ). There are different laser welding techniques regarding welding of zinc coated steel to Al alloys: keyhole welding, conduction welding and reactive welding. In keyhole welding method, the laser was focused on steel side. A keyhole was formed which caused fusion between steel and Al alloy. Deep penetration welds can be achieved in keyhole welding [9] . But the thickness of the IMC attained was higher. Various research work were to control the metallurgical reaction between Fe-Al. A two pass laser welding was tried in conduction welding mode. Initially, the welding was carried out with a defocused laser beam which partially vaporized the zinc coating. The welding was done in the second pass with a focused beam. A sound joint with an IMC thickness below 10 µm was produced [10] . A backing block technique was tried out to control the flow of molten metal in the weld region and suppress the thickness of IMC [11] . The zinc layer on steel is beneficial as it increases the wettability of liquid Al on solid steel. But at the same time, the presence of zinc can be detrimental in keyhole mode. The vaporization of zinc can result in the formation of porosities in the weld zone. An attempt was made to reduce the porosities by maintaining an intersheet gap of 100 µm by proper fixturing method [7] . Laser roll welding technique was tried to establish good contact between steel and Al, which led to rapid heat transfer by conduction between the weld surfaces under excessive roll pressure (100-175 MPa). The IMC thickness was controlled below 10 µm and failure occurred along the steel surface [12] . Thus it was evident from the literatures that a sound weld joint could be achieved between Fe-Al with an IMC thickness below 10 µm under optimal laser process parameters.

The current study focuses of welding complex phase steel (CP), which is a category of AHSS steel with AA 6061 alloy. Among the various AHSS, the least tested steel is the CP steel. The characteristic feature of CP steel is their microstructure that consists of hard martensitic islands in soft ferrite matrix. CP steel also contains bainite in addition to martensite. This provides higher local ductility than other AHSS like dual phase (DP) steel of same strength [13] . There is no sufficient work done on understanding the weldability of CP steel with Al alloy. In the current work, preliminary studies on conduction mode laser welding of CP 800 steel to AA 6061 alloy is discussed. The microstructural and mechanical properties of the welded joints are also analysed. The laser processing parameters are optimized such that the thickness of IMC layer is controlled to obtain maximum tensile strength. Nomenclature 

A high power 6 kW fiber coupled diode laser with a wavelength of 980 nm, beam diameter of 1.5 mm and top hat beam laser profile is used in the experiments (Fig. 1a) . All the experiments are carried out in conduction mode laser welding. Materials used in the experiments include galvanized high strength CP 800 steel (1.5 mm) and AA 6061 alloy (3 mm). The CP steels are generally galvanized to improve the corrosion properties. The chemical composition of the materials are given in Table.1 and Table. 2. Experiments are carried out for various laser power (3 kW, 3.5 kW, and 4 kW) and scanning speed (8, 10 and 12 mm/s) (Table. 3). Lap welding was carried out with CP steel on top and Al in the bottom. The laser head was focused on the steel plate and the heat of conduction was used to melt the Al alloy (Fig. 1b) . The laser head is tilted to an angle of 5° to avoid back scattering of laser light that would damage the laser optics. Argon is used as the shielding gas with a flow rate of 25 l/min and pressure of 0.5 bar.

The laser welded samples are then manually grounded using aluminium oxide emery sheet and later polished with diamond paste (2 µm and 1 µm). The weld zone is then etched with an etchant made up of 5 ml hydrofluoric acid (HF), 3 ml nitric acid (HNO3) and 92 ml ethanol (C2H5OH) for 8 -10 seconds to expose the IMC phases. Microstructural analysis is done using optical microscope (OM) and scanning electron microscope (SEM) to determine the IMC phases. The mechanical resistance of the joint is determined using tensile testing machine. The tensile testing was conducted using a universal testing machine with a constant crosshead speed of 1 mm/min to analyze the mechanical strength of the joint. The shape and size of the tensile specimen are shown in Fig.2 . Shims were used at the end the sample to ensure uniaxial loading to minimize the bending of the specimen during shear loading. 

The CP 800 steel has a mixed microstructure consisting of ferrite, martensite, bainite and retained austenite. Fig. 3a shows the SEM image of CP steel. The SEM micrograph could reveal only the presence of hard martensite islands in the soft ferrite matrix. Detection of other phases is difficult using SEM, while a light optical microscopy image can reveal the presence of different phases in different colours by using a tint etchant. The etchant used was 2% nital with 10 % aqueous sodium meta bisulfite, which helps as a coloring agent. The LOM image shows the presence of different phases in CP steel (Fig. 3b) .

The penetration depth of the weld depends upon the laser parameters such as laser power and scanning speed. Maximum penetration depth of 1200 µm was obtained at a condition of 3 kW & 8 mm/s (Fig. 4a) . At the same power of 3 kW and higher scanning speed (10 mm/s), the joint failed because it did not have sufficient time for fusion. As the scanning speed increases, the time for fusion between the materials decreases which reduces the penetration depth. The penetration depth at different laser conditions are shown in Fig.4 . 

Because of the large difference in thermo-physical properties of steel and Al alloy, the weld/Al interface is expected to be the weakest point of Fe/Al joint. At the interface, the diffusion of liquid Al in solid steel takes place resulting in the formation of brittle Al-rich IMCs. These brittle IMCs are detrimental to the strength of the joint.

The microstructural variations at the interface of the weld is analyzed at different laser processing conditions (laser power and scanning speed). The SEM micrographs of four samples are discussed here ( The SEM micrograph for 3 kW and 8 mm/s shows the presence of IMC phases at the interface of the weld (Fig. 5) . Two different IMC phases were observed, one is continuous thick layer of Fe2Al5 layer and other is needle shaped FeAl3 phases (Fig. 5b) . Energy dispersive spectroscopy was done to analyse the atomic percentage of Fe and Al along the intermetallic region. The atomic wt% obtained was then compared with previous literatures [8, 10] to confirm the IMC phases.

While, at higher laser power of 3.5 kW the penetration depth did not increase. A lower penetration depth of 330 µm was obtained, this is because, in the conduction welding mode at high laser power the amount of heat conduction into steel increased (Fig. 6 ) thereby reducing the penetration depth. Therefore in conduction welding mode of laser it is necessary that we optimize the laser parameters such that the conduction of laser into the material is reduced so as to obtain higher penetration depth. The SEM micrograph for laser condition of 3.5 kW and 8 mm/s is shown in Fig. 7 . The penetration depth was only 330 µm (Fig.  4) , the fusion of liquid Al to solid steel reduced decreasing the formation of IMCs at the weld interface. It can be seen that there is very thin layer of Fe2Al5 phase and almost no FeAl3 phase was formed at some places of the weld-Al interface (Fig.  7b & 7c ). An average thickness of IMC layer measured is in the range of 2-3 µm. While for laser condition of 3 kW and 8 mm/s, the IMC thickness was about 17 µm.

The SEM micrograph for laser processing parameters of 4 kW and 10 mm/s is shown in Fig. 8 . In this case, a porosity developed at the interface due to entrapment of gases during welding (Fig. 8a) . Cracks can be seen emanating from the porosity. The crack later propagated along the weakest region that is the interface of the weld. The thickness of the IMC layer was only in the range of 4 µm (Fig. 8b) . Similarly for the case of 4 kW and 12 mm/s, the IMC thickness was in the range of 11 µm. In some regions of the interface there was even no IMC phases found (Fig. 9b) . Further mechanical testing is done to analyze the strength of the joint. The mechanical resistance of the joint is calculated and correlated with IMC layer thickness. 

Tensile testing of the weld samples is carried out and the stress and strain is plotted to determine the mechanical resistance of the weld (Fig. 10) . The mechanical resistance of the joint is expressed as N/mm i.e. Failure strength divided by the width of the tensile specimen. The failure of the joint occurred along the weakest region i.e. along the IMC region between weld and Al. With further application of the tensile load the crack propagates along the IMC region and the fracture occurs in the form of separation between steel and Al.

It can be observed that maximum tensile strength of 254 N/mm is obtained for a laser parameters of 4 kW & 12 mm/s. Even though the penetration depth for this case is 540 µm, the IMC layer thickness was only 11 µm. Very thin brittle IMCs formed at the weld interface that could have resulted in higher mechanical resistance (254 N/mm) of the weld. While in the case of 3 kW & 8 mm/s, though the penetration was 1200 µm, the average IMC thickness observed is around 17 µm. Thick brittle IMCs at the interface led to a reduced mechanical resistance of 191 N/mm compared to the previous case. Due to the presence of porosity and crack at the weld-Al interface in case of 4 kW & 10 mm/s, the sample failed during tensile testing. A mechanical resistance of 159 N/mm is observed for the case of 3.5 kW & 8 mm/s. For this case, the depth of penetration is around 330 µm and the IMC thickness was only in the range of 2-3 µm. Even though the IMC thickness achieved is very low the joint failed due to low penetration of the weld. The correlation of tensile strength, IMC thickness and depth of penetration is plotted in the graph (Fig. 11 ). A maximum mechanical resistance of 254 N/mm was achieved when the penetration depth was around 550 µm.

When the penetration depth increased further (1200 µm), the IMC thickness also increased. This led to lower mechanical resistance of 191 N/mm. The mechanical resistance of the current work was compared with the previous work done on welding steel to Al alloy (Table. 4 ). Thus a sound weld between CP steel and Al alloy with good mechanical properties was obtained when the thickness of the IMC region was controlled below 11 µm under optimal process parameters. The laser processing parameters were optimized such that the optimum penetration is achieved and at the same time the conduction area in steel is minimum. More conduction area in CP steel would alter the microstructure of the base metal resulting in reduced performance of the material.

The wide application of Al-steel welding is limited because of its inability to produce welds with mechanical properties sufficient to meet the application performance. Currently, the IMC formation and its detrimental effect on mechanical strength of the joint limits the widespread application of laser welded fusion joint. Research works are more focused on controlling the IMC phases and other weld defects such as cracking, porosity and penetration depths of the weld. Progress has to be made on studying the post welding operation such as forming and stamping. Formability of the welded sheets are very essential to apply these sheets for tailor welded blanks. Therefore more research work should be focused on studying the formability property of the welded dissimilar sheets.

1. The experiments conducted using high power diode laser in conduction mode showed successful weldability between CP 800 steel and AA 6061 alloy. 2. A maximum penetration depth of 1200 µm was obtained for a laser parameters of 3 kW and 8 mm/s. When increasing the laser power to 3.5 kW and maintaining speed at 8 mm/s, the penetration depth did not increase. This is owing to the conduction phenomenon, the conduction region increased in the steel side resulting in lower penetration depth. 3. The microstructural characterization revealed the presence of Fe2Al5 and FeAl3 intermetallics at weld-Al interface. Fe2Al5 was found as a continuous thick layer, while FeAl3 was observed to be needle like structures. 4. The welded joint showed a maximum mechanical resistance of 254 N/mm for laser parameters of 4 kW and 12 mm/s. 5. The maximum mechanical resistance was achieved when an optimum intermetallic thickness was maintained below 11 µm. Above this thickness range the mechanical resistance dropped to 191 µm.

Steel Market Development Institute

The authors would like to thank Center for laser processing of materials, International Advanced Research Centre for Powder Metallurgy & New Materials (ARCI), Hyderabad for allowing us to use their facility.