Friction Stir Processing as a Technique of Metal Foam Formation

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 6 (2018) pp. 254-258 © Research India Publications. http://www.ripublication.com Friction Stir Processing as a Technique of Metal Foam Formation Sharaf u nisa1*, Zahid Akhtar Khan2, A.N Siddique3 and Sunil Pandey4 1,4 Indian Institute of Technology , Delhi-110016 2,3 Jamia Millia Islamia , Delhi-110025 Abstract— This paper gives an overview of the applications of porous aluminium and how it can be fabricated using the technique of friction stir processing as the starting process. Also the conventional methods of fabrication of porous aluminium have been discussed and how FSP can become a better technology for the fabrication of metal foam. Although the technology is still in its infancy with very less work reported to be done in India. But if developed properly and commercialized, India can produce aluminium metal foam which will be comparatively environment friendly and economical. gas can be first dissolved in the melt and then released during foaming (‘Gasar line’). 2) ‘Indirect foaming’ (a.k.a. ‘Precursor foaming’, 2nd line) involves melting of a solid material in which a gas source has been embedded that upon melting releases gas and drives the foaming process. Such precursors can be made both by powder pressing (‘powder line’) or by melt processing (‘Form grip line’). Direct foaming (line 1 ) Keywords: Porous Aluminium , Friction Stir Processing, Metal Foam Friction stir processing is a relatively new processing technique, based upon the principle of friction stir welding. FSP causes intense plastic deformation, thermal exposure and material mixing. This characteristics of FSP allows it to form metal matrix composites by uniform mixing of a reinforcement powder into the base metal acting as matrix. The surface composite so created is reported to significantly enhance the surface properties. This application of FSP has been used in the present study for the metal foam fabrication. The metal foam fabrication basically involves two steps, first, mixing of a blowing agent (in the form of powder) into the metal matrix and forming a surface composite; second, heat treatment of the fabricated composite to above characteristic temperature. The first step can be successfully accomplished by using FSP. The metal matrix composite obtained after the completion of the first step is called as the precursor which will serve as the starting material for the fabrication of porous aluminum 6063. Hence, it can be called as one of the indirect method of foaming (where the precursor is formed in the first step). Metal foam is defined as the dispersion of gaseous phase in a solid medium. The metal foams have uniformly distributed gas pores in the solid metal with a volume fraction of 40-98% [1].Therefore they are able to withstand higher stresses because of high energy absorption capacity. Many methods have been used so far for the metal foam fabrication. According to John Barnhart,[2] the foaming methods can be classified into two categories i.e direct foaming and indirect foaming. 1) ‘Direct foaming’ methods convert a liquid metal or alloy into foam without an interruption (1st line). For foaming, a gas source is needed and depending on the gas source we define different ‘lines’: gas can be injected into the melt (‘Alcan line’), a chemical blowing agent can be added to the melt where it decomposes and liberates gas (‘Alporas line’) or Gasar line Foam formatio n INTRODUCTION Alcan line Alporas line Indirect foaming (line 2 ) Powder line Form grip line Figure 1 :Various foaming methods A new method of foaming has been recently developed by Yokoshiko Hangaii et al.[3] This method uses Friction Stir Processing for the formation of precursor.FSP is a relatively new processing technique developed by Mishra et al [4].Based upon the basic principles of FSW, FSP is used for micro structural modifications of various metallic materials. One of the important applications of FSP is the formation of metal matrix composites or in other words FSP facilitates the uniform distribution of particles into the metal matrix[5].Since the indirect foaming involves embedding of the gas source into the metal matrix (precursor), which upon decomposing releases the gas, FSP serves the purpose of forming such precursor. Therefore this method of forming of precursor by using FSP can be categorized as one of the indirect foaming methods. Using FSP as a foaming method has various advantages over the conventional foaming methods. Firstly it reduces the cost of Al foam and lowers the environmental impact. Secondly, FSP is a simple and short-duration process with potentially high productivity [6]. Thirdly, it does not require timeconsuming heating processes that consume a large amount of energy [6] to fabricate the precursor. FSP induces intense stirring, by which powder can be easily mixed into an Al plate [7] and segregated microstructures and gas pores can be easily and uniformly mixed [8,9], by simply traversing the rotating tool used in FSP. Lastly, inexpensive and high-recyclability Al alloy die-casting plates can be used as the starting material instead of expensive Al alloy powder. 254 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 6 (2018) pp. 254-258 © Research India Publications. http://www.ripublication.com INTERNATIONAL STATUS Many companies expertise in the fabrication of metal foams and their sale in the commercial market but all of them use the conventional methods of metal foam fabrication. Some of the company's manufacturing metal foam and the method used are listed in table 1. Table 1:List of various companies manufacturing metal foam Name of the company Aluinvent Inc. Alulight Alveotec FOAMTECH Fraunhaufer institute Country Hungary Route of manufacture of Aluminium foam Gas injection route Austria France Korea Germany Powder metallurgy route Foundry route Alporas route Powder metallurgy route Aluminium metal foam fabrication was first started on a commercial scale by Shinko wire company in Japan. They used the Alporas line for metal foam fabrication. Fabrication of metal foam by using FSP as a starting process has not been used yet as the technique is still in its research stage. Some of the major researches which have been reported in this field are explained. Formation of Al based metal foam without using a blowing agent [10] Yoshihiko Hangaii et al worked on the formation of Aluminum foam by the novel FSP technique .They fabricated ADC12 Aluminum alloys with porosities 67-78% from Al alloy die castings without using any blowing agent. The pore structure and tensile properties of the ADC12 foams were investigated and compared with those of commercially available ALPORAS. They developed a simpler and more cost-effective process using the FSP route without using a blowing agent . Al alloy high-pressure die castings were used as the starting material, which have a high gas porosity . This gas porosity has been used to induce foaming as an alternative to a blowing agent. By using the FSP route the cost of Al foam can be reduced and also the environmental impact is lowered. FSP is a simple and short-duration process as it does not require time-consuming heating processes that consume a large amount of energy to fabricate the precursor. All these advantages make it a highly productive process.FSP induces intense stirring, by which powder can be easily mixed into an Al plate. Also the segregated microstructures and gas pores can be easily and uniformly mixed, by simply traversing the FSP tool into the required area. This method uses inexpensive and highrecyclability Al alloy die-casting plates as the starting material instead of expensive Al alloy powder used in the powder metallurgy method. It has been shown that a porosity of approximately 50%–80% can be achieved using Al alloy die castings, which contain a large number of gas pores, without the use of a blowing agent . The pores of the Al foam obtained this method i.e. without a blowing agent are smaller and have higher sphericity than those of Al foam fabricated using a blowing agent. Figure 2 shows a schematic illustration of the fabrication process of the precursor by the FSP route. As the starting material for the fabrication of the Al foams, asreceived Al-Si-Cu Al alloy ADC12 (equivalent to A383.0 Al alloy) die-casting plates of 3 mm thickness, 80 mm width and 210 mm length were used. The source of N2 gas is considered to be gases derived from air existing in the cavity, the runners and the injection system. In contrast, the source of H 2 and other gases such as CH4 is considered to be the reaction gases formed when the melted Al alloy encounters the parting and lubricant agent. Two aluminum plates were stacked with alumina powder (α-Al2O3, ~1 μm) distributed between them as the stabilization agent as shown in Figure 2a. The mass of alumina used was 5% of that of the Al alloy with respect to the dimensions of the area over which the alumina was distributed and the length of the tool probe. FSP was carried out using a 1D-FSW machine (Hitachi Setsubi Engineering Co., Ltd., Ibaraki, Japan) as shown in Figure 2b. The FSP tool used had a cylindrical shape with a screw probe. The diameter of the tool shoulder was taken as 17 mm, the diameter of the tool probe was 6 mm and its length was taken as 5 mm. The tool rotation speed was 1000 rpm, and the speed of the tool as it traversed the plate was 100 mm/min throughout the experiments. A tilt angle of 3 deg was used. Figure 2: Schematic illustration of the fabrication process of the precursor of ADC12 foam from ADC12 Al alloy diecasting plates by FSP route.[10] The multi pass FSP technique (5 lines × 2 times back and forth, i.e., the FSP region was stirred four times) was applied to obtain a larger amount of precursor and to mix the segregated gases and alumina powder thoroughly. The plate was turned over, and alumina was placed on the reverse side of the FSP surface along the path of the FSP tool. Finally, the same FSP procedures were carried out once again to obtain a thicker precursor. Foaming Procedure Precursor samples of 25 mm width, 25 mm length and 9 mm thickness were machined from the region subjected to FSP . The precursor sample was then heated in a preheated electric furnace. The holding temperature (equal to the preheating temperature) and the holding time during the heating process were fixed at 948 K and 12–14 min, 255 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 6 (2018) pp. 254-258 © Research India Publications. http://www.ripublication.com respectively, with reference to a previous study . Then, the obtained ADC12 foams were cut by electro-discharge machining to fabricate cubic tensile specimens. Each specimen of ADC12 foam had a side of 15 mm. Eleven tensile test specimens with porosities ranging from 66.7% to 78.2% were obtained. In addition, an ALPORAS cubic tensile test specimen with a side of 25 mm was fabricated with a porosity of 88.0% from an as-received ALPORAS block. NATIONAL STATUS Fabrication of metal foam has not been reported in India. Also a very meagre research has been reported in this field i.e. the fabrication of metal foam using FSP. One of the research work available is explained as under : Formation of Al based metal foam using a blowing agent [11] This method has been used by Kaushal Jha Et al. for development of porous aluminium (5052). The basic principle used for making metallic foam, is to distribute the blowing agent (TiH2) in the metal and then heat it. FSP has been found to be a good technique, to distribute the blowing agent in the metal. Various steps involved in the development process include, diffusion bonding of Al plates with TiH 2 placed at predetermined grooves, optimization of bonding process parameters, friction stir process and subsequently controlling the heating cycle, to get the desired porous aluminium structure. FSP involves complex material movement and plastic deformation which results in proper mixing of TiH 2 powder in this case. Material undergoes intense plastic deformation at elevated temperature, resulting in generation of equiaxed recrystallised grains. Various steps involved in the development processes were: 1. Making of channels in aluminum plate on a milling machine. 2. Mixing of Al + Titanium hydride powder for filling in the channels. 3. Diffusion bonding of Al plates with TiH2 placed at pre-determined grooves 4. Optimization of bonding process parameters. 5. Fiction stir process of the plate with optimized parameter. 6. Heat the processed plate in a controlled way to get the desired porous aluminum structure. The process has been shown in the flow chart in fig 3. INITIAL EXPERIMENTATION A rectangular rolled Aluminum 6063 plate was taken as the starting material. A groove was made in the plate and the blowing agent powder (TiH2+Al2O3) was filled into the groove. The powder was packed into the groove with the help of adhesive bonding by applying a shoulder run using flat side of the tool. Friction stir processing was carried out on this plate in order to form the metal matrix composite. A sample of 5.5x1 cm was cut from the processed plate. This sample was heat treated in a preheated muffle furnace. The temperature of furnace was raised upto 650oC. The sample was placed in the furnace for 15 minutes and cooled in ambient air. The cross section of the heat treated sample was analyzed. The cross section of the heat treated sample revealed a porous structure, which could be seen with the naked eye, in the stir zone. The pores can be seen only in the region where the stirring of powder had taken place. So it is clear that the pores have been formed because of the decomposition of the blowing agent powder. This can be seen in the figure which shows the cross section of the friction stirred processed sample and the cross section of the same sample after heat treatment. (a) (b) Figure 4 : (a) cross section of the FSPed sample. (b) cross section of the heat treated sample. APPLICATIONS OF METAL FOAMS The unique combinations of physical and mechanical properties such as high stiffness, low specific weight, high gas permeability, high impact absorption capacity leads to the temptation behind the development of metal foams. The projections of the future fuel crisis, the urgency of achieving high fuel efficiency along with higher passenger safety in automobile industries and the needs of creating light weight construction materials have attracted tremendous consideration for ultra light weight metallic foams. The implementation of metallic foams in these industrial sectors depend to a large extent upon their manufacturing cost, the environmental durability and fire retardancy [12]. Figure 3: Formation of metal foam by FSP[11] 256 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 6 (2018) pp. 254-258 © Research India Publications. http://www.ripublication.com materials manufacturin g 10% aerospace 6% other industries 18% Multi functionality of metal foams can be realized if a lightweight structure is capable of reducing noise and absorbing impact energy in an automobile crash. Foams in transport industries are basically needed for weight saving, impact absorbing and thermal insulation. Figure 7 shows a sample car designed by Karmann, Germany with components made of Al foam. It is indicated in the figure that 67% of invehicle injury cost incurs when the collision takes place at the front end of the car and about 22% in-vehicle injury cost constitute during collision from the side. This indicates that the front end collision of a car is most detrimental during an accident and therefore utilization of foamed materials in these parts becomes necessary for the passenger safety. transport 26% research and education 16% process power industries engineering 3% 5% engineering and manufacturin g component 5% manufacturin g 11% Figure 5: Break up of projected industrial sector requirements of metal foams [1] In general, metallic foams possess a range of thermo mechanical properties that suggest their applications in the areas demanding impact/blast amelioration, heat dissipation, acoustic isolation and heat exchange . The major applications of metallic foams is in automotive and aerospace industries 32%. In addition to this to this, 26% requirements will constitute materials manufacturing, engineering manufacturing and component manufacturing. The response from academic and research institutions was 16% in research and development point of view. As the transport industry has to play a major role in the implementation of metallic foams, it is essential to know the reasons why it is beneficial to apply foams application categories for metallic foams in the automotive industries [13]. The figure is self-explanatory and depicts three major properties associated with their respective utility. The bending stiffness of metal foams are directly proportional to the thickness and inversely proportional to density of foam panels. A high porosity in foams makes them potential candidates to absorb large amount of mechanical energy; therefore, they can also be used as impact energy absorber. The properties of damping vibrations and sound absorption could be well exploited. However, a metal foam can be an alternative option compared to other engineering solutions if two or more of these properties are exploited together. Light weight construction Damping and insulation Energy absorption Figure 6: Application Of Metallic Foams In Automotive Industries [13] Figure 7: Karmann car (courtesy: IFAM, Bremen, Germany) [1] CONCLUSIONS Initial studies and experimentation have revealed that fabrication of aluminium metal foam by FSP route can prove to be an effective technique with both the process and cost efficiency. Also it is a green process hence satisfying the need of present manufacturing trends. Adding to the need of further research in this area is the deficiency of Indian manufacturers in producing metal foams. This technique can pave a way for the manufacturing of metal foams in India itself if it gets a proper research support. REFERENCES [1] V. C. Srivastava and K. L. Sahoo " Metallic Foams: Current Status and Future Prospects", IIM Metal news, 2006 vol 9 (4), pg no 10-13. [2] John Banhart, "Light-Metal Foams – History of Innovation and Technological Challenges" Advanced Engineering Materials 2013. 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