Laser Melting Process 7
Evaluationof the design of Compression spring
Acompressionspringis an elastic object used to store or resist mechanical energy. It ismade of an open-coil helicalspringwhich is usually twisted at a constant diameter and pitch (Bhandari,2010). The type loading on the spring depends upon its application.The following are the parts of the compression helical spring.
Whenthe spring is loaded, it assumes a state of potential energy byabsorbing the mechanical energy induced. This results to deformation,but it regains its original shape and size as it releases bothkinetic and heat energy (Society Of Automotive Engineers, 1962). Totest for mechanical properties of helical spring, a formula is usedto relate the function to its material properties. The propertiesincluded are shear and modulus.
Dimensional terminology for helical compression spring
Where:Fis the force inducing deformation.
thedifference in length between the distance of free length and loadedlength of a spring.
d:the diameter of the wire
D:the diameter of the coil
G: Theshear modulus of the material of which the spring is constructed.
:Canbe calculated as
Where,is the spring deflection and is the number of effective length of the spring.
Shearmodulus can also be calculated directly from spring constant bymodifying the above equation. K =, G =.
Whensprings have same diameters but different , they always return to the same modulus of elasticity since they aremade of the same material.
Springindex:This is the ratio of mean diameter (D) to diameter of wire (d)(Bhandari,2010).
Freelength (Lf):Free length of a helical spring is the average distance between theends of a spring in an un-loaded condition.
Pitch:This is the space of separation between adjacent active coilsmeasured from center.
Esparallelismrefers to length between the ground ends. It is calculated when aspring on a flat surface and determining the difference in freelength around the circumference of the spring.
Bearingsurface: Thearea of contact between the end of the spring and hits restrictionpoint should be least 2700in order to reduce buckling and improve squareness. Bearings of 2700to 3300are supplied to squared and ground compression springs (SocietyOf Automotive Engineers, 1962).
Typesof spring ends:Spring ends are available a variety of designs. They include Plainend and plain end which is ground.
EsSquareness:This is the angular deviation between the axis of compression springandperpendicular the ends in reference to its plane (Friedrich& Mordike, 2004).
Solidheight:It is the length of a spring when all coils are closed. The solidheight of ground coils is the diameter of the coil multiplied by thewire diameter while, for unground springs, it is the number of coilsplus one.
Types of ends for helical compression spring of constant pitch
Springrate: Itis the rate of change of load per unit deflection and is expressed asillustrated bellow
K= = Theequation is only valid when the angle of pitch is less. . than
Torsionalstress:This is the stress a helical spring is exposed to during twisting asa result applied torque.
Whencompression springs are arranged parallel to each other, thecomposite rate is the sum of the rate for individual springs (SocietyOf Automotive Engineers, 1962). But when compressions spring arearranged in series, the rate is calculated from the followingformula.
Bucklingof Compression Spring
Bucklingoccurs in compression springs when length of the spring is four timesgreater than its diameter. Effect of buckling can be minimized byproperly guiding the spring over a rod or tube. The friction betweenthe spring and tube will affect the performance when the aspect ratiois high (POPRAWE,2010).
Choiceof Operating Stress: Static Conditions
Theload-carrying ability of a spring can be compromised by the yieldstrength or stress relaxation resistance of the material (Society OfAutomotive Engineers, 1962). Springs are required to operate for alimited number of cycles, and the velocity of the end coils is toreduce high stress due to surging or impact conditions. Making thespring longer than is its required free length, and then compressingit to solid improves load carrying ability of a spring in static. Theprocess causes the spring to set to the desired final length andinduces favorable residual length (Bhandari, 2010).
Asurge wave is usually established when the spring is loaded orunloaded. Surge waves transmit torsional stress from the part of theloading along the spring length to the point of restraint. The travelvelocity of the surge wave is approximately and is given by
Surgewave velocity Vs.isdependent on the material and spring design but usually ranges from50 to 500m/s. The surge wave limits the rate at which a spring canrelease or absorb energy by reducing impact velocity (Jindal, 2010).The velocity of the spring parallel to the axis is a function ofstress and material constant which gives impact velocity (Society OfAutomotive Engineers, 1962).
Impactvelocity is given by the formula: V 10.1S m/sec
Resonanceis the occurrence of a simple ratio between the period of revolutionof two bodies about a single primary (Jindal,2010).When the frequency of the cyclic loading of a spring is near naturalspring frequency or a multiple of its frequency, resonance occurs.Resonance has an effect of increasing individual coil deflection andstress levels above amounts predicted by static or equilibriumanalysis (Bhandari,2010).Resonance causes spring bounce in loads considerably lower thancalculated minimum spring deflection. The natural frequency of acompression spring is inversely proportional to its impact velocity(POPRAWE,2010).
Fatigueis the results of defect in metals which occur as a result fromreoccurrence of stress (SocietyOf Automotive Engineers, 1962).The failure occurs in three distinct stages which includecommencement of cracking, spreading of cracks and finally shatteringfailure. Fatigue failure results when the level of used stress levelbeing expressively below stresses essential to cause standard failure(Jindal,2010).Estimation of fatigue life is obtained from S-N curve. Where S standsfor continuous range of stress while N represents the number ofrepetitive stresses to failure. Tests on a series of samples is amandatory in order to progress the curve for conclusion recurringfailure. A graph of resulting lives is drawn against the matchingstress in the given range (Bhandari,2010).
PowderBed Laser Melting Process
Powderbed melting process is based on selective laser melting. The processis a fabrication technique used to produce geometrically complicatedcomponents. Metal powders of very fine-size grains are melted using ahigh power laser beam (Bhandari, 2010). A 3D part is built from themelt metal based on a CAD model (Society of Automotive Engineers,2007). Components produced by the process of laser melting are muchdifferent in strength, durability and performance than those producedin any other conventional way (Society of Automotive Engineers,2007). They exhibit higher or at least comparable mechanicalproperties than conventionally casted parts.
Toimprove the performance of a compression spring, the EOSINT 280technology is adapted. The melting machine consist of a solid statelaser beam powered by 400 watt of power and protected argon gasatmosphere. In surface-finishing a method of blasting is done aftermanufacturing process. The powder is optimized for processing(Jindal, 2010).
Thepowder is contained in a dispenser bed. A prescribed dose of thepowder is elevated as illustrated bellow. Powder is spread over thesurface of the build plate in the form of a thin-layer by a recoatermechanism which consist of a hardscrapper (Society of AutomotiveEngineers, 2007).
Atypical powder bed laser melting process is shown above.
Finelygrounded metal powder is finely spread in a dispenser bed. Thefocused energy of the beam fuses the powder particles together on athin layer by layer basis (Jindal, 2010). This process is called theselect layer melting method. The most preferable thickness of powderlayer is roughly between 9 μm and 100μm. .Prevention of oxidationof the metal during the process of fusion is necessary (Society ofAutomotive Engineers, 2007). The process must be carried out in anitrogen, argon or both filled atmosphere.
Inorder to obtain a finished part of a layer-by-layer style, theprocess is repeated to a buildup.Several parameters are put in place to ensure the final product is ofgood quality (Collins, Busby & Staab, 2010). They includebuilding environment parameters, powder properties, and recoatingparameter and laser and scanning parameters (POPRAWE,2010).
Inthe process of melting proceeds, the molten area diameter isnoticeably bigger than the diameter of the lase beam (Society ofAutomotive Engineers, 2007). In order to compensate for thedemensional error, it is necessary to shift the width of the beam byaproximately half. This is done from the outline to the inside toensure the corespondance to the original data fom the CAD. (Polskie,1963). The value of offset beam determines the quality of the melt.The particle of the exposed to the beam may be over-melted or notliquefied completely. The laser beam moves several times from line toline to ensure that liquifaction is complete (Friedrich &Mordike, 2004).Powder thickness layer as a parameter can lead tointerruption of the process when its not within the required range.Extreem great values between the single layers may be of suspectionwhen the melting depth is not deep enough (Jindal, 2010).Mechanicaltension developed can advance to detarchment. Tearing-off from thestructure can also happen during the recoating proces.
Themost technically complex metal geometries can be built layer by layerusing direct metal laser melting. The layers can be built fromlayers as thin as two microns (Polskie,1963).The process is done directly from 3D CAD data (Friedrich& Mordike, 2004).A dense, fine, homogenous structure can be created when the metalpowder is melted wholly. Due to unique geometry of freedom in design,direct metal laser melting can be used to build extremely complexgeometry of metal components. This complexity of design provesdifficult with the convectional machining techniques (Collins, Busby& Staab, 2010). The products of direct laser melting partakeexceptional mechanical properties. They are equivalent to wroughtmaterials with crystal resolution, and extraordinary surfacesuperiority (POPRAWE,2010).
Toenhance the functionality of components, AlSi10Mg, a typical castingalloy with good casting properties is used (Friedrich & Mordike,2004). It is most preferable in casting thin walls, parts and complexgeometry (Jindal, 2010). It offers good strength, hardness anddynamic properties. These are the qualities which constitute the mainrequirements of compression spring subjected to high loads (Polskie,1963).
Difficultyin Powder Bed Laser Melting Process
Difficultyin processing is the main factor affecting the use of aluminum indirect laser melting process. Aluminum is difficult to cast or weld(Jindal, 2010). Aluminum and its alloys are much hard to weld evenwith laser. The casting in most cases result in high degree ofporosity (Friedrich & Mordike, 2004). This is no exceptional todirect laser melting. The mechanical properties of the final productsare greatly affected by porosity (Lü, Fuh & Wong, 2001).
Thereare two main factors that lead to porosity. They include surfaceoxidation and moisture pick-up.
Aluminumand its alloys rapidly form an oxide layer when exposed to air. Theresult of this behavior proves that aluminum alloy powder favorsporosity (Lü, Fuh & Wong, 2001). The defects are noticeable inlaser melted aluminum alloy. This effect is particularly encouragedin aluminum-alloy Laser Melting because the powder feedstock has ahigher surface-to-volume ratio (Collins, Busby & Staab, 2010).
Diffusion is hindered by the oxidation on the surface of the particles (Friedrich & Mordike, 2004). This refrains compaction and formation of solid mass between the un-melted particles. As a result of this regions of weakness develop in the component. Oxide films can form at a relatively low oxygen concentration due to their adherent (Lü, Fuh & Wong, 2001).
When the molten metal is stirred, the oxide forms entrapped gasses. These gasses end up in pores within the component (Friedrich & Mordike, 2004).
Oxide films form between the laser hatches at every layer during melting. Formation of oxide films on both solid and liquid metal surface occurs in the process. Pores are formed where the two oxide films meet (Battelle Columbus Labs Oh., 1981).
Poreformation in molten aluminum alloy is directly caused by moisturepick-up. Molten liquid dissolves hydrogen but as it solidifies,hydrogen tends to degas as the solidification front passes by(Collins, Busby & Staab, 2010). The presence of anyhydrogen-bearing species in the powder leads to porosity as they willbe released in the molten pool.
Thefigure above illustrates moisture pick-up.
Powdermetallurgy (PM) Ti–6Al–4V alloy is a product of hot isostaticpressing (HIPing) in the process of gas atomized powder.Investigation of high temperature reaction of alloy Ti–6Al–4Vinvolvesnumerous HIPing conditions and heat treatments (Jindal, 2010).Different heattreatments result in significant changes in the microstructure of thepowder. The occurrence of these changes do not bring any differencesin tensile properties of the alloy. Porosity is induced by the effectof temperature after heating to 930ocand allowing it to cool (Collins, Busby & Staab, 2010). Thisshows no considerable effects on tensile impact and fracturetoughness of powder compact (Kazimi, 1984).
Ti–6Al–4Vpowder are spherical in shape. They are produced using a freecrucible system. Atomized gas melting process is adopted by inductiontechnique.
HIPingand Heat Treatment
Lowcarbon content steel containers are used to degass the powder afterbeing heated. In order to determine behavior characteristics of thepowder metal, hot isostatic pressing is carried out on the sample(Lü, Fuh & Wong, 2001). This process involves adoption ofArchimedes method. It is essential to investigate the relativedensities of powder to be hot pressed isostatically (Jindal, 2010).More than one experiment is carried out on heat treatment to provideconsistent results (Friedrich & Mordike, 2004). The purpose ofthe experiments is to investigate microstructure and mechanicalproperties under varies heating conditions (Collins, Busby &Staab, 2010). The metal powder is compacted to full density beforethe experiment can be carried out. Different cooling rates areinduced after annealing at 930 °C per every hour (Kazimi,1984).
Observationmachines like optical and electron microscope are used to observepowder metal when heat-treated or HIPed (Lü, Fuh & Wong, 2001).Before the samples can be observed they are prepared by polishing andcoating with Kroll’s reagent (2 pct. Hf + 8 pc HN03+90pct. + H20). Use of X-ray micro-computerized tomographyenables detail examination of fracture mechanism (Collins, Busby &Staab, 2010). The corresponding areas on the surface occurring onboth halves were give special attention.
Testingof mechanical properties
Chemicaltests are conducted under room temperature. Tensile force tester atan initial strain rate of 2×10−4 s−1and 2×10−3 s−1 is set after yielding(Friedrich & Mordike, 2004). To ensure accuracy and consistenceof results, three to six tensile test specimens are used for everytest condition.
Effectsof HIPning Parameters
Agraphical illustration of relative density of powder compactresulting from various HIPing cycles are shown in a range of colorsas below. It is observable that the relative density goes up from theinitial of 68 percent to an extreme of 99% (Friedrich & Mordike,2004). It can be concluded that both HIPing temperature and pressurehave direct impacts on the powder consolidation. As HIPingtemperature increases from about 900 °C, the relative densitydecreases as a result of heating. Powder compact with a relativedensity greater than 99.5 percent as the relative density rangersbetween 99.37 to 99.54 percent (Battelle Columbus Labs Oh., 1981). Toattain the powder compacting with full density, HIPing pressureshould be evaluated to more than 110 MPa. As shown in theillustration bellow, highest relative density can be obtained at atemperature range between 920 and 940 degrees Celsius under apressure of 120 Mega Pascal’s is applied (Kaufmann &Uggowitzer, 2007).
Microstructureevolution during heat treatment
Whenthe powder is fully dense its microstructure can be identified aslath-like alpha phase with circular beta phase when observed underelectro- micro scope (Vollertsen,2013).Alpha and beta can be represented as light and dark phasesrespectively. Beta phase contribute 5 percent of the entire volume ofsample. Powder densification takes place in three stages whichinclude plastic yielding, power-low creep and diffusion bonding(Collins, Busby & Staab, 2010). The process of densificationoccurs at a temperature of 940OCunder pressure of about 120mega Pascal’s (Vollertsen,2013).
Analysisof the plot of ultimate tensile strength against elongation ofTi-6Al-4V shows the safe area in the graph (Battelle Columbus LabsOh., 1981). The results when compared to wrought and cast powdermetal, show that PmTi-6Al-4V alloy is comparably better than thelatter. The tensile strength of Pm alloy ranges between 920 and 1080mega Pascal’s (Kazimi, 1984). The tensile values from the plot arespread over a broader range in both X and Y axes. Pm Ti-6Al-4V is agood fabrication alloy than the wrought because it has low heatresponse (Collins, Busby & Staab, 2010).
Impactfracture and HCF Properties
Toevaluate the effect of fracture toughness and fatigue on HIPed andheat treated samples, tests are carried out at room temperature(Vollertsen,2013).It is observed that there is significant difference in fatiguestrength of both heat-treated and as-HIPed sample. They are bothequivalent to as-forged sample similar bidal microstructure(Vollertsen,2013).Fatigue strength is found to be relatively larger than that forannealed at 9300Cfor one hour than as-HIPed. It is concluded that powder compact is asexcellent option to heat-treated or as-forged sample (Vollertsen,2013).
Fromthe analysis of the S-N curve from the figure below, it is seen thatbetter fatigue properties are shown by HIPed-sample (Kazimi, 1984).For instance the fatigue strength of HIPed sample is 580 megaPascal’s when fatigue life is 106cycles (Collins, Busby & Staab, 2010).
Itcan also clearly be observed that the slope of S–Ncurve of as-HIPed samples is steeper than the slop of heat-treatedsamples. The later demonstration enhanced fatigue properties in thesmaller fatigue lives region.
Themechanical characteristics of Powder metal titanium alloy are readilyaffected by powder compact permeability. The observation ofsponginess and the HIP after heat treatment in powder compact is doneby using X- ray’s tomography (Kaufmann& Uggowitzer, 2007). HIP is dependent on its miniature, theamount of gasses held within and temperature of cooling after heatingand time of raising its temperature (Vollertsen,2013).HIP in powder compact can also be caused by the emanation of powdercontainer. Most recommended guidelines are presented to lessen orhalt the permeability, especially for hot isostatic pressing (Kazimi,1984).
Optimizingof HIPing Parameters
Thereset of conditions that favors reactions in this particular experimentto provide a reliable data for analysis are temperature and pressure(Kaufmann& Uggowitzer, 2007). Most accurate results of maximum value ofrelative density of compact powder, which is slightly lower than 100%occurs when the two parameters range between 900 to 940 degreesCelsius and 100 mega Pascal’s respectively (Vollertsen,2013).Above optimal HIPing temperature range, flow stress of the powdermetal decreases significantly on the effect of increase in truestress applied. However HIPing temperature above 900 degrees Celsiushas a negative impact on the relative density of the powder withresult of lowering its value. The occurrence of powder particles withgas bubbles which are formed during manufacturing of metal powder arealways associated with the process (Vollertsen,2013).
HeatingEffect on Mechanical Properties of AlSi10Mg
Theultimate strength, which is the ability of a material to withstandloads leading to elongation. Tensile stress of a material increaseswith the reduction in the level permeability (Battelle Columbus LabsOh., 1981). Maximum tensile strength at full density is achieved whenthe relative density of the sample is 99 percent or approaching avalue close to it (Brecher, 2011). The permeability of heat-treatedsample of powder compact is relatively low when compared to HIPedsample despite the fact that its porosity is increased by nearly 10%after treatment. Heat-treatment of results to bi-modal microstructureresults to higher tensile strength. The bimodal microstructure isonly achieved after annealing at 930 degrees Celsius or above that.It confirms the increment of ultimate strength and elongation afterheat treatment (Brecher, 2011). There is no evident change in thetensile properties in the microstructure of powder compact afterdifferent heat treatments. This leaves no evidence of postdeformation after globuralization of as-HIPed lathe-like structure ascompared to properties of typical cast or wrought AlSi10Mgalloy after heat treatment (Battelle Columbus Labs Oh., 1981). Thishas no clear effect on tensile properties on microstructure ofas-HIPed sample. Tostimulate the optimum mechanical properties of AlSi10Mg, componentsare produced in the process of selective laser melting. Onlyparameters resulting in maximum density were adopted in themanufacture of parts in this test. The optimization of the parametersis done using a. Laser beam powered by 200W source is used as astandard optimization procedure. It consist of a scan speed of1400mm/s and a spacing of 105μm between the scan tracks. Re-meltingprocess can result to high density of almost 98 percent (Brecher,2011).
Thetable below shows mechanical properties for tensile tests. The valuesare acquired in tensile tests done on parts produced from Xy and Zaxes. From analysis of the data in the tablehigh pressure die casting sample provides the optimum propertiesamongst the other casting procedures (Shackel & Prangnell, 2003). For instanceHigh pressure die casting shows an ultimate tensile strength of 365and 350 mega Pascal’s. This is way higher than cast and aged withan UTS of only 317 mega Pascal’s’ (Brecher,2011).
Mechanicalproperties of selective laser melting built parts and cast plus agedparts
391 +- 6
396 +- 8
5.55 +- 0.4
3.47 +- 0.6
Conventional cast and aged
300 – 317
2.5 – 3.5
High pressure die casting F
High pressure die casting T6
330 – 365
95 – 105
Thefigure above shows die casting AlSi10Mg parts, the properties foras-cast (F) as well as for aged. (T6).
AlSi10Mgalloy parts have mechanical properties which are higher or somehowsimilar to the casted AlSi10Mg material. High pressure die castedAlSi10Mg when compared to HPDC AlSi10Mg in aged condition itsconcluded that the tensile strength of the prier is higher than thelatter in the aged than in as-cast condition (Brecher,2011).The elongation of the as-built AlSi10Mg parts oriented in the Z-axisis similar to the HPDC parts, while the elongation for parts built inXY – axes are higher by almost 2% (Shackel& Prangnell, 2003).Despite strength and hardness being higher, the impact energy of theas-built SLM samples still is superior when compared to traditionallycasted AlSi10Mg sample (Kaufmann& Uggowitzer, 2007).
X +- s
Energy of Impact
3.69 +- 0.48
2.5 – 3.0
SelectiveLaser Melted AlSi10Mg parts shows mechanical properties of Hardness,Ultimate tensile Stress, elongation and impact energy which is highercomparable to the traditionally casted AlSi10Mg sample. This is dueto fineness of the microstructure and constant distribution of the Siphase (Kaufmann& Uggowitzer, 2007).
Sampleof SLMs show some non-isotropy in lengthening on discontinuity. Thisis a result of optimal density skimming approach. The scanningapproach result in the formation of Z-oriented tensile samples toform a series of borderline perviousness. The pores initiatesensitivity on some tensile parts which are oriented in the Z- axis commence cracking, when in comparison to tensile samples oriented inXy axis (Shackel& Prangnell, 2003).
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