TITLE: AREA SPECIFIC METALLIC FOAMS BY FRICTION STIR PROCESSING

TECHNICAL FIELD

The present invention is about foamed materials. In particular, the present invention is about foamed metal/metal alloy in a specific area. The present invention also discloses a method to synthesize foamed material in a specific area. The method involves, dispersion of a silicon- based polymer into metal by Friction Stir Processing (FSP) followed by pyrolyzing to obtain foamed metal/metal alloy.

BACKGROUND

Metallic foams are emerging engineering materials offering high strength with very low specific weight, high gas permeability, and high energy absorption ability. Due to the unique characteristics, it attracts humongous applications in commercial and research sector, sample being in experimental animal prosthetics to grow bone on it, in automotive vehicles to increase sound damping, reduce weight, increase energy absorption in case of crashes, and in military applications.

The applications of metallic foams are fastened to its hollow cellular structure, which is filled with gas. Two types of metallic foams are commonly in use, closed cell and open cell foams. In open cell structure, the metal framework is connected to allow passage of the air through the foam. In closed cell structure, gas is trapped within the cell which directs its applications to impact absorption, damping and noise control. Though, these materials exhibit potential applications, the usage is hampered due to the challenges associated with the preparation of uniform and consistent foam materials. The conventional methods produce inconsistent distribution of cell or pores in the final product. There are various limitations in conventional methods for the preparation of metallic forms. The entire bulk of the material is foamed due to which selectively foaming of a large part is impossible. The methods require metal to be heated beyond liquids temperature. Due to gravity, the gradient in the pore size and the foam density is difficult to avoid. Controlling the pore size and the density is nearly impossible in the precursor methods. The energy consumption is high as they require complete melting of metal. Blowing agents (e.g. TiH ₂ ), need to be handled with care and are expensive. The melt or foaming route requires adding of elements that will increase its viscosity. These elements, such as calcium along with hydrogen evolved due to dissociation of the TiH ₂ , tend to cause brittleness in matrix. The hydrogen gas evolved during the process can be a fire hazard, needing careful control of all the process parameters. The control of process parameters is rather difficult and expensive; hence the process is carried out only by a few companies. High strength alloys cannot be used in these processes, as the chemistry of these alloys will significantly change during the process. Macro-sized pores of few millimeters in diameter are formed in general.

The existing methods does not help in the selective foaming of material, through this approach a single hybrid sheet with high stiffness and energy absorption can be produced. The pore size and density gradient issues can be reduced to a large extent.

In patent US7402277, a method to form metallic foam on substrate by cold spraying of cold spraying a mixture of metal particles and a foaming agent onto the substrate to form a substrate coated with an unexpanded metallic layer. And heat the substrate with unexpanded metallic layer till the decomposition temperature and thereafter cooling of this heated substrate at ambient temperature to form expanded metal foam. But this is a tedious and expensive method which involves metal powders and foaming agent.

In patent US20110239890, a method to create metal foams to envelop targets is disclosed, which involve the formulation of powdered thermite/metal matrices adding a thickening agent can be blended into the powdered thermite/metal matrix, when the thermite is ignited. A heat sink is required to maintain the temperature inside to be in control during thermite reaction. The thermite/powder matrix also include a binder that holds the constituents together. This again is a tedious, expensive method and result in inconsistent pores.

In patent application WO2010029864, a method of manufacturing a foam metal form a precursor involving imposing foaming agent (TiH ₂ ) between two base materials with friction stir processing (FSP) is discussed. FSP at one side of base material, disperse foaming agent (TiH2) and create bonding of one base material with other to obtain precursor. The metal foam is foamed by heating of precursor at temperature near the melting point of the one and the other of the base material. The usage of TiH ₂ as blowing agent which is toxic, expensive and decompose at lower temperature than metal or metal alloys is not recommended.

In patent US5622542, a process for producing a particle-stabilized metal foam is disclosed. The precursor is formed by heating a matrix metal above its liquidus temperature to form a liquid matrix metal and add with metal stabilizer particles under a covering gas. The stabilizing particles under covering gas gets dispersed in liquid matrix metal and obtain partially-stabilized metal foam with not no uniform pore formation.

Considering the potential applications of the foamed materials, in particular, area specific foamed materials of high quality, it is necessary to develop simple, cost effective, environmentally benign method which can provide high quality area specific foamed materials.

SUMMARY OF INVENTION

Accordingly, the present invention provides foamed metal and a method foaming of metal using polymers by Friction Stir Processing (FSP) and pyrolysis. The method helps in providing closed pore metallic foams in any selected area of a metal.

The present disclosure provides method of foaming a metal, said method comprising acts of; a) providing one or more groove in the metal, b) dispersing a polymer which release gas at a temperature between the solidus and liquidus of the metal, in to groove and closing the groove with metal strip, c) friction stir processing the metal groove with subsequent passing, and d) pyrolyzing the friction stirred metal and enabling the gases to be trapped to obtain foamed metal.

The present disclosure provides a method of foaming a metal at specific area, said method comprising acts of; a) providing one or more groove in the metal at specific area,

b) dispersing a polymer which release gas at a temperature between the solidus and liquidus of the metal, in to groove and closing the groove with metal strip, c) friction stir processing the metal groove with subsequent passing, and d) pyrolyzing the friction stirred metal and enabling the gases to be trapped to obtain foamed metal in specific area.

BRIEF DESCRIPTION OF FIGURES The features of the present invention can be understood in detail with the aid of appended Figures. It is to be noted however, that the appended Figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.

Figure 1 : shows the schematic of Friction Stir Process. Figure 2: shows SEM macrographs of the samples foamed at various temperatures.

Figure 3: shows the average pore size as a function of temperature for A12024 foam, with 95% confidence level.

Figure 4: shows the X-ray CT image showing the distribution of pores Figure 5: shows the schematic of the workpiece used for FSP Figure 6: shows the process to disperse polymer into metal by FSP.

Figure 7: shows cross section of the plate with grove sealed with strip.

Figure 8: shows transverse section macrographs of A12024 -polymer composite after FSP. Figure 9: shows the effect of the pyrolysis temperature on a) absolute and b) relative density of Al 2024 foam.

Figure 10: shows the open pores caused due to rupture of gas pores.

Figure 11 : shows effect of pyrolysis time on a) absolute and b) relative density of A12024 foam.

Figure 12: shows SEM image of A12024 foam a) at low magnification b) high magnification image showing polymer next to polymer.

Figure 13: shows 2D and 3D X-ray tomography image of the A12024 foam.

Figure 14: shows the distribution of pore size.

Figure 15: shows EPMA images confirm pyrolysis product within the pore diameter.

Figure 16: shows stress- curve of A12024 base metal and foam.

Figure 17: shows a) as-processed A16061 -polymer composite and b) shows composite foamed at 640° C for 1 hr.

Figure 18: shows the effect of the pyrolysis temperature on (a) absolute and (b) relative density of Al 6061.

Figure 19: shows the effect of pyrolysis time on a) absolute and b) relative density of A16061 foam.

Figure 20: shows SEM images of sample heat treated to 640°C for 1 hr. a) unprocessed region b) nugget region.

Figure 21 : shows the effect of frequency on a) complex modulus and b) loss tangent

Figure 22: shows the effect of stress amplitude on a) complex modulus and b) loss tangent of various samples c) fracture and debonding of the low strength polymer particles within the matrix due to increase in stress.

Figure 23: shows the effect of temperature on a) complex modulus, and b) loss tangent of various samples. DESCRIPTION OF INVENTION

The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the Figures, description and claims.lt may further be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.

Definition: Metal means single metal or combination of more than one metal- a metal alloy.

The present invention is in relation to a method of foaming a metal, said method comprising acts of; a) providing one or more groove in the metal, b) dispersing a polymer which release gas at a temperature between the solidus and liquidus of the metal, in to groove and closing the groove with metal strip, c) friction stir processing the metal groove with subsequent passing, and d) pyrolyzing the friction stirred metal and enabling the gases to be trapped to obtain foamed metal.

In an embodiment of present invention, said metal plate is selected from a group comprising Aluminium, Magnesium, Copper, Titanium, Iron and alloys thereof.

In another embodiment of present invention, the polymer is selected from a group comprising Poly (methyl hydro siloxane) (PMHS) and Poly (dimethylsiloxane) (PDMS). In still another embodiment of present invention, the groove in the metal is having a width of 0.5 to 10mm.

In still another embodiment of present invention, the polymer is having a pyrolysis temperature above operating temperature of friction stir processing and below melting temperature of metal, preferably from 250°C to 1400°C.

In still another embodiment of present invention, wherein friction stir processing (FSP) using rotating tool iscarried out with tool rotation speed ranging from 50rpm to 3000 rpm and traverse speed of lmm/min to 250 mm/min.

In still another embodiment of present invention, said metallic foams are having pores of size ranging from 1 μιη to 350 μιη.

In still another embodiment of present invention, the grooves are in specific area of metal to obtain area specific metal foam as per requirement.

The present invention is also in relation to a method of foaming a metal at specific area, said method comprising acts of; a) providing one or more groove in the metal at specific area,

b) dispersing a polymer which release gas at a temperature between the solidus and liquidus of the metal, in to groove and closing the groove with metal strip, c) friction stir processing the metal groove with subsequent passing, and d) pyrolyzing the friction stirred metal and enabling the gases to be trapped to obtain foamed metal in specific area.

The present invention is also in relation to foamed metal obtained by a method comprising acts of; a) providing one or more groove in the metal,

b) dispersing a polymer which release gas at a temperature between the solidus and liquidus of the metal, in to groove and closing the groove with metal strip, c) friction stir processing the metal groove with subsequent passing, and d) pyrolyzing the friction stirred metal and enabling the gases to be trapped to obtain foamed metal.

In another embodiment of present invention, the foaming is at specifically identified area as per requirement.

In another embodiment of present invention, the foamed metal is same as metal in its chemical properties.

In another embodiment of present invention, the grain structure is stable for lhr at 500°C. Metallic foams of high strength with very low specific weight, and high energy absorption ability are prepared by using Friction Stir Processing (FSP) in combination with polymers and pyrolyzing. The foams in various metals are fabricated specifically in a particular area according to the requirement. The method however may be adopted for foaming materials other than metal by adopting suitable conditions for the material.

The method offers various advantages including economical preparation, eliminating the use of metal/ material powders, annealing of metals before roll bonding of metal pieces, further it helps in uniform dispersion of the blowing leading to uniform pore formation. By joining two or more precursors using FSW(2), functionally graded structure with different plateau stress achieved. Using FSW(2) many layers of the precursor can be joined to build up a thick structure and then the structure can be foamed.

In the present disclosure FSP (Friction Stir Processing)is used to incorporate blowing agent, a polymer into the metal matrix and then heated above the melting point of metalto produce foamed metal. The metal can be selected from a group comprising Aluminum, Magnesium, Copper, Titanium, Iron and their alloys having wide range of melting temperature and high strength which can lead to large energy absorption during deformation.

In an embodiment of present invention, Aluminum, Magnesium and their alloys have been adopted for exemplifying the invention. The density of foam is customized as it is dependent on time and pyrolysis temperature.

In an embodiment of present invention, a polymer with unique characteristics is dispersed into selected area of the workpiece by FSP. The polymer is chosen such that it evolves large amount of gases at a temperature between the solidus and liquidus of the metal; cost effective and easy to handle. The expansion of released gas creates a cellular structure. As the pyrolysis temperature is below the liquidus temperature of the matrix, the gradient effect is eliminated. Also, the polymer is with a pyrolysis temperature above operating temperature of FSP and below melting temperature of metal so that the polymer is stable enough that it does not decompose during FSP, but it readily pyrolyzes before melting the matrix to form ceramic, enabling evolved gases to be trapped within the matrix, foaming the matrix to form metallic foams. The amount of polymer mixed in the metal can be customized enabling control of final porosity to obtain micro sized pores.

The method is illustrated in the flowchart 1, given below.

Flowchart 1

The schematic of the process with workpiece (1), FSW tool(2), tool pin (3), processed area (4) is as integrated in Figure 1. In friction stir processing, a rotating tool with a shoulder, is plunged into the metal (work piece (1)) and traversed. Friction between the tool and the metal (workpiece (1)) leads to heating of the workpiece (1) and aids in plastic deformation of the metal. FSP can also disperse secondary phase metal into a matrix. The dispersion of the particles is done to positively enhance the mechanical properties of the matrix. To achieve this, the metallic plates with grooves or holes are packed with the secondary phase particles and the FSP is carried out. The combination of heating and large plastic deformation leads to micro structural refinement, thus, dispersing polymer (foaming agent) into the metal matrix by FSP and pyrolyzing. The process does not allow the properties of parent metal to change drastically with the foaming. Figure 2 shows the evolution of pores at various temperatures, as the pyrolysis temperature increases pore density and size increase. A computed tomography image sample pyrolyzed at 560°C is shown in 4 along with the distribution of the pore size.

In an embodiment of present invention, for Aluminum or Magnesium metal foams, the polymer is selected with a pyrolysis temperature above operating temperature of FSP and below melting temperature of Aluminum or Magnesium metal so that the polymer is stable enough that it does not decompose during FSP, but it readily pyrolyzes before melting the matrix to form ceramic. The polymer is dispersed into metal matrix in the selected area and the composite is pyrolyzed to release the gases of decomposed polymer. Thus, a selected area of a metal is foamed.

In an embodiment of present invention, the above conditions are satisfied by silicon-based polymer, Poly (methyl hydro siloxane) (PMHS), with a chemical formula (CH ₃ ) ₃ SiO [(CH ₃ )HSiO] _n Si(CH ₃ ) and structure as shown in Formula A. The polymer, when crosslinked with l,4-Diazabicyclo[2.2.2] octane (5% by weight) for 8 hours and pyrolyzed in an inert atmosphere yields SiOC ceramic. The cross-linked solid polymer is ground to a fine powder using mortar and pestle. Thermo Gravimetric Analysis (TGA) is done for this powder under a nitrogen flow of 20 ml/min and it shows that polymer decomposes between 350°C to 550°C. The possible reactions during pyrolysis are given below in Scheme 1 which will evolve gases.

Si-CH ₃ Si + *CH ₃

*CH ₃ CH ₄ (H subtraction from Si-H group)

*H H ₂ (H subtraction from S-H group)

Si-H + H-Si Si-O-Si + H ₂

Scheme 1

Poly (dimethyl siloxane) (PDMS) (Formula B) with different side groups is another silicon- based polymer and that can be adopted in the method.

n= 28 to 53

Formula A

Formula B

EXPERIMENTAL:

To exemplify the method and characterize the properties, aluminum alloys (A12024 and A16061) and magnesium alloys (AZ31, AZ61, AZ91, AZ81, HK31, HM21, ZK60, ZE41 and ZC71) are foamed. The said materials are chosen as they have wide range of melting temperature and they are high strength alloys which will lead to large energy absorption during deformation.

Silicon based linear chain Polymethylhydrosiloxane (PMHS) polymer with chemical formula (CH ₃ ) ₃ SiO [(CH ₃ )HSiO] _n Si(CH ₃ ) is cross-linked with 5% by weight of 1,4- Diazabicyclo[2.2.2]octane for 8 hours as reported by Srinivasan et al, Journal of advanced ceramics 2, 2013 318-324. Polydimethylsiloxane (PDMS) is prepared as in G. Camino et al, Part 1. Kinetic aspects, Polymer 42(6) (2001) 2395-2402.

A. Foaming of A12024Aluminium alloy

Example 1:

The polymer PHMS is ground to fine powder using mortar and pestle. Aluminum alloy A12024 plate with groove is filled with above powder. The grove is closed with A12024 strip (Figure 7, the white portion of the image constitutes the area, 8.89mm filled by the polymer powder). A plain tool is used to seal the groove. Three overlapping passes of FSP are done with tool rotation speed of 800 rpm and traverse speed of 8 mm/min. The plate is cooled to room temperature between passes. The obtained precursor is then heated above 450° C to foam the material.

Example 2:

The polymer PHMS is filled into the aluminum alloy plate A12024 with dimensions as shown in Figure5. The groove is closed with 2 mm strip cut from the same plate. This assembly is sealed by a single pass FSP with a pin-less tool. Further, a threaded pin is used to disperse the polymer into the matrix (Figure6, wherein integration of Workpiece (1), Metal strip (2), Pin- less tool (3), Taper threaded tool (4), Processed plate (5) and Nugget region (6) are shown). The composite obtained after FSP is heat treated to a temperature between solidus and liquidus of the metal. Due to this heating, the metal is softened, and the polymer is pyrolyzed, which releases the gases. The combination of these leads to a porous metallic structure in the processed area.

B. Foaming of A16061 Aluminium alloy

Example 3

The PHMS polymer is used to prepare the A16061 foam. The size and preparation of workpiece are equivalent to the previous example. Three overlapping passes FSP are done on the sealed plate at a tool rotation speed of 800 rpm and traverse speed of 8 mm/min. The plate is cooled to about 25°C between the passes. The processed composite and sample ispyrolyzed at 640° C for lhr, the area fraction of the composite is around 13.4 % (Figure 17) to obtain the foamed alloy.

Further, the composite samples are heated from 580°C to 650°C for an hour to study the effect of temperature on density, the results are in Figurel8. As the scope of this study is to foam the material without melting the alloy, the samplesare heated below the liquidus temperature of the alloy. Further, the samples are heated to 650° C for duration of 10 to 60 minutes and the results are plotted in Figurel9 and 20. The SEM images (Figure20) shows that after the foaming, grain sizes of unprocessed base metal are an order larger than the grains of cell walls due to different heat transfer rates.

Additionally, as the Magnesium alloys have melting range comparable to the Aluminum alloys, the processing temperature during FSP would be in the same range. Hence the current polymers (PHMS or PDMS) can be used to foam magnesium alloys. The melting ranges of some of the Magnesium alloys are given in the Table 3.

Table 3 showing solidus and liquidus temperatures of various magnesium alloys

Characterization of A12024 Foam

The synthesised foam is analysed for uniform dispersion of the polymer by stereo- microscope. The transverse section of the A12024 -polymer composite (transverse section, Figure 8) shows the uniform dispersion of the polymer into the matrix. The cross-sectional area of the processed zone is 55.48 mm and the calculated area fraction is around 16%.

Scanning Electron Microscopy (SEM), X-ray tomography and Electron Probe Micro- Analysis are used to analyses the compressive behaviour of the foam, typically the effect of temperature and time on the density is evaluated. The effect of pyrolysis temperature and the heating rate on density are studied (Figure 9(a) and (b)). The as-processed samples are pyrolyzed at different temperatures starting from 450°C to 600°C for 60 min. The density of the foam gradually decreases with increasing temperature. It is also seen that the rate of heating had little effect on the density. Figure 3 shows the average pore size as a function of pyrolysis temperature, showing that with the increase in pyrolysis temperature, average pore size of the foam also increases The density of the composite is higher at 600° C due to rupture of gas pores due to the low viscosity of the alloy at a higher temperature, the image of the pore is given in Figure 10. The effect of pyrolysis time on the density that is beyond twenty minutes the density of the composite remains almost constant (Figures 11 (a) & (b). The low magnification SEM Image of the foam shows that the pores are evenly distributed (Figure 12 (a & b)). A higher magnification image shows a pore evolved right next to the dispersed polymer. In X-ray tomography image, shows pores of few micrometres in size (Figure 13(a)). The 3D image of the foam is also seen as in Figure 13(b). The distribution of the pore size is calculated by the image analysis software (Image J). The pores size ranges from 50 μιη to 350 μιη (Figurel4).

The EPMA image of the foam shows the pyrolysis products within the pores (Figure 15). The compression behaviour of the foamed sample shows plateauing of the stress (Figurel6), due to this, the foam will be able to absorb large amounts of energy. The energy absorbed during the compression is tabulated in Table 1 for four different samples processed under same condition to measure variability if any. It is seen that the compressive strength, densification strain, and energy absorbed have close distribution indicating evenly foamed specimen.

Table 1 : shows compressive strength, densification strain and energy absorption of 4 samples. sample 1 185 370 466 50.34 49.28 sample 2 187 374 471 50.42 52.57 sample 3 215 430 542 41.29 52.93 sample 4 196 392 494 54.2 64.2

Further, it is seen that the A12024 foam synthesized by this method retained heat treatability. Nominal stress-strain curves and change in energy absorbed after heat treatment (490°C for 3 hrs. followed by 180°C for 6 hrs.) is shown in Figurel6 and Table 2. The energy absorbed during compression test of the heat-treated samples increased around 80%.

Table 2: shows the compressive strength, normalized compressive strength and the densification strain and energy absorbed by as -processed, heat treated foams

The damping properties of the foam A12024 is evaluated for various frequency, stress and temperatures. The properties of the foam are compared with other monolithic samples. The damping properties of the foam are found to be superior. The results and the testing conditions are given below.

Testing

The samples are subjected to dynamic cyclic loading to measure anelastic time-dependent behavior. Dynamic Mechanical Analyzer, Gabo Eplexor 500N, NETZSCH GABO Instruments GmbH, Germany is used for the test. The dynamic load is applied in 3-point bend configuration. A constant static load is applied to the specimen, over which a small dynamic load is overlapped. During one cycle, the dynamic load started from zero, reached a maximum and came back to zero in a sinusoidal variation. The transducer measures the displacement of the specimen. Due to the inelastic behavior of the material, there is a phase lag between the stress applied and the obtained strain. The dynamic stress and strain can be expressed as

σ =G ₀ exp(icot) (1)

Where σ ₀ and ε ₀ are applied stress and resultant strain amplitude, ω is the angular frequency, and φ is the phase difference between stress and strain. The phase difference is used to calculate the complex or dynamic modulus of the material (Eq.3)

* CT CT

E =— =—— (cos + / s n φ)

ε ^ε ° (3)

The term tanc|) is known as 'Loss Tangent' (LT) which represents the damping capacity of a material. Lager LT indicates larger energy dissipation as compared to energy stored, hence indicating larger damping capacity.

Tests are carried out by systematically varying the parameters having a large influence on damping behavior of the material. The parameters chosen for the study are the frequency of oscillations, stress amplitude and temperature. For the frequency variation tests, a static stress of 11 MPa and dynamic stress amplitude of 4.5 MPa is applied and frequency is varied from 1 Hz to 80 Hz. Further, the stress is varied from IMPa to 22.5 MPa with a frequency of 10 Hz. The temperature variation test is also carried out, where dynamic stress amplitude of 4.5 MPa and 10 Hz frequency is applied and the temperature is varied from 40°C to 300°C.

Damping capacity of the materials is a function of frequency, stress amplitude, and temperature, hence, the foam synthesized under optimum condition is evaluated by varying one parameter at a time and keeping another parameters constant. As a comparison, as- received base metal in Rolling (RD) and Transverse Direction (TD), Solutionized (without precipitates) base metalFSPed base metal(reduced grain size), A12024-PMHS precursors are also tested under similar conditions.lt is seen that the damping capacity of the foam is superior to base metal.

Effect of frequency on damping properties

Figure 21a shows Complex Modulus (CM) as a function of frequency. It is seen that the frequency has a marginal effect on the CM of the specimens. The CM of the Solutionized sample is six-fold higher than that of the foamed specimen. Due to the porous structure of the foam, the modulus is considerably lower than that of the base metal. It is interesting to note that the CM of the solutionized sample is higher than as -received samples (both rolling & transverse direction). As-received samples (RD and TD) and A12024-PMHS precursors exhibit comparable CM.

Figure 21b shows the effect of frequency on the Loss Tangent (LT) of the samples. LT of the foamed samples monotonically increases with increasing frequency. Although there are some local peaks at 45 and 70 Hz, they do not suggest a change in damping mechanism. The damping behavior of the foamed sample increases more than thrice over a frequency range of 1 to 80 Hz. A12024-PMHS precursors exhibit a higher LT compared to other monolithic samples. This can be attributed to the presence of polymer particles, which increases the interfacial slip and micro -plasticity arising from the microcracks between the polymer particles and the matrix. Further, it is seen that Solutionized sample exhibits the lowest damping ability, closely followed by as-received samples (rolling and transverse direction). Figure 21 c shows the displacement with respect to load before and after the frequency test and Figure 2 Id shows the nanohardness of the objects before and after the frequency test. One of the dominant mechanism for damping is dislocation driven. But as dislocations density increases, hardness increases. Both solutionized as well as the foamed sample show an increase in hardness indicating dislocation contribution to dampening. Damping in foam is much larger than what is observed in solutionized sample, indicating pore structure of the foam plays more dominant role. Effect of stress amplitude on damping properties.

Figure 22a shows a plot of Complex Modulus (CM) as a function of stress amplitude. Unlike the frequency, the stress amplitude has a negative correlation with complex modulus, i.e. with increasing stress amplitude, the CM modulus reduces for all specimens. As observed earlier, the CM of the foamed sample is lowest and that of the solutionized sample is highest. Interestingly, the CM of A12024-PMHS precursor, reduced by a larger extent with increasing stress amplitude increased, compared to other samples, whereas, for rest of the samples reduction of CM is gradual.

The Loss Tangent (LT) plot shows interesting behavior with increasing stress amplitude Figure 22b), where irrespective of the sample the LT increases with increasing stress amplitude and plateaus beyond certain stress values. However, foam and the A12024-PMHS precursor samples show a dip in the damping behavior after reaching a peak at a stress amplitude of around 7~8MPa. However, drop in LT is far lower that of the A12024-PMHS precursor. The sharp drop observed in case of unpyrolyzed sample which is attributed to fracture and debonding of the low strength polymer particles within the matrix thus lowering internal friction (Figure 22c).

Effect of temperature on damping capacity

Figure 23 (a) shows the Complex Modulus (CM) plot as a function of temperature. CM of all the samples is fairly constant up to 250°C, beyond that the modulus of all samples increases except for A12024-PMHS precursor and foam.

The Loss Tangent (LT) as a function of temperature is shown in Figure 23b. Up to 250°C, Rolling Direction (RD) and Transverse Direction (TD) samples show a marginal change in the loss tangent. In case of solutionized samples, a dip is observed between 50-120°C, beyond which LT is comparable to that of RD and TD samples. Around 250°C, a damping peak is observed in all monolithic samples, after which an increase in complex modulus can also be observed in those samples. Further, the damping peak in case of A12024-PMHS precursor is not as prominent as for other samples due to increased base LT. The damping peak is more prominent in case of the FSPed sample compared to other samples. Beyond 250°C LT of these samples reduces. In case of A12024-PMHS precursor, beyond 150°C LT increases monotonically till 300°C. The LT of the foamed sample shows a steep increase beyond 50°C and reaches a plateau at temperatures between 100°C to 250°C. Beyond 250°C, a steep increase in LT of the foamed sample is observed.

The method thus provides area specific foamed metal with micro sized pores, with high strength, very low specific weight, and high energy absorption ability, the method is economical and easy to adopt. The proposed method uses polymers for foaming which are economical, and easy to handle. The amount of polymer mixed in the material can be controlled to a certain degree enabling control of final porosity. The chemical properties of parent metal (ex: metal, metal alloy) do not change drastically and the grain structure is highly refined and is stable for heating even at 500°C for lhr. The dynamic damping capacity of the foam helps in the usage of the foam as a vibration dampener. Further, as a selectively foamed material, the product can also be used to isolate vibration a particular area.

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