• No results found

Process Parameters

IMPACT OF PROCESS PARAMETERS ON DEFECTS, PROCESS TIME, & THERMAL HISTORY

V.1 EBM Software

V.1.3 Process Parameters

The EBM Control software has a complex way of calculating the beam current, beam speed, preheat parameters, postheat parameters, and order of process steps. Features can be modified by the user, but a thorough understanding is necessary. The understanding of process parameters as presented in this section is attributed to work by I. Elfstrom. [285]

147

Variables in this section will be defined by level according to EBM Control parameter levels.

For example, a menu item that is found under Preheat, then Square, then Preheating 1, then Beam Speed would be labeled as following:

preheat.square.preheating1.beamspeed

To understand the structure of parameter levels, it is useful to look at screen captures of the EBM Control Software. The first process step, Raking, occurs when the rake is engaged to distribute a layer of powder after the build table is lowered.

The next process step, Preheat, occurs in two stages defined as β€œPreheat 1” (Preheat I) and

β€œPreheat 2” (Preheat II) that are defined by the parameters in Figure 68 and Figure 69.

Since preheat.square.autocalculation is always enabled, the software will calculate a Preheat 2 if Preheat 1 is enabled (this is always the case, as using both preheats has been found to improve processing). Preheat 1 parameters are set by either the enclosed area of the entire part. Preheat 1 is done over a square area, with the maximum size being based on the size of the start plate (146mm x 146mm for a 150mm start plate). The actual area of Preheat 1 (A1) is calculated by drawing a square containing all parts and adding the value of preheat.square.offsettopart to each dimension x and y. The current used is calculated according to:

𝑖𝑃1= π‘–π‘Ÿπ‘’π‘“1βˆ—π΄π΄π‘ƒ1

π‘Ÿπ‘’π‘“

π΄π‘Ÿπ‘’π‘“ = (π‘π‘Ÿπ‘’β„Žπ‘’π‘Žπ‘‘. π‘ π‘žπ‘’π‘Žπ‘Ÿπ‘’. 𝑠𝑖𝑧𝑒)2

The Preheat 2 area (A2) must be calculated based on a relationship between currents, the reference area, and A1:

148 π‘–π‘Žπ‘£π‘”2 = π‘–π‘Žπ‘£π‘”1βˆ—π΄π΄2βˆ’π΄1

π‘Ÿπ‘’π‘“ π‘¨πŸ= π’Šπ’Šπ’‚π’—π’ˆπŸ

π’‚π’—π’ˆπŸβˆ— 𝑨𝒓𝒆𝒇+ π‘¨πŸ (Eq. 14)

The average current across the entire preheat step can then be calculated according to:

π’Šπ’‚π’—π’ˆπ‘·π’“π’†π’‰π’†π’‚π’• =π’Šπ’‚π’—π’ˆπŸ(π‘¨πŸβˆ’π‘¨π‘¨ 𝟐)+π’Šπ’‚π’—π’ˆπŸπ‘¨πŸ

𝒓𝒆𝒇 (Eq. 15)

Postheating can be engaged (optionally) at the end of the preheat cycle and again after the melt is complete (although is described in general terms as coming after the melting stage).

In both cases, π‘–π‘Žπ‘£π‘”π‘ƒπ‘Ÿπ‘’β„Žπ‘’π‘Žπ‘‘ sets the average current that the process targets. Postheating is calculated based on the inputs on several parameters: preheat.heating.maximumheattime, melt.heating.maximumheattime, and arguably every other parameter used during

processing. The fact that postheat time is calculated based on an energy balance means that any other parameter that impacts the current or time of the process will impact postheat. The parameters with the most direct effect are those associated with preheat.

The Preheat 1 current (𝑖𝑃1) and Preheat 1 area (A1) are set as the current and area for postheating as well. The term β€œheating” is used in EBM Control to denote Postheating. The actual average current (measured) can be calculated if the postheating time is known:

π‘–π‘Žπ‘£π‘”π‘ƒπ‘Ÿπ‘’β„Žπ‘’π‘Žπ‘‘βˆ— π‘‘π‘‘π‘œπ‘‘π‘Žπ‘™ = π‘–π‘Žπ‘£π‘”π‘ƒπ‘Ÿπ‘’β„Žπ‘’π‘Žπ‘‘βˆ— (π‘‘π‘π‘Ÿπ‘œπ‘π‘’π‘ π‘  + π‘‘π‘π‘œπ‘ π‘‘β„Žπ‘’π‘Žπ‘‘1) = (π‘–π‘Žπ‘£π‘”βˆ— π‘‘π‘π‘Ÿπ‘œπ‘π‘’π‘ π‘ + 𝑖𝑃1βˆ— π‘‘π‘π‘œπ‘ π‘‘β„Žπ‘’π‘Žπ‘‘1)

π’Šπ’‚π’—π’ˆπ‘·π’“π’†π’‰π’†π’‚π’• =(π’Šπ’‚π’—π’ˆ(π’•βˆ—π’•π’‘π’“π’π’„π’†π’”π’”+π’Šπ‘·πŸβˆ—π’•π’‘π’π’”π’•π’‰π’†π’‚π’•πŸ)

𝒑𝒓𝒐𝒄𝒆𝒔𝒔+π’•π’‘π’π’”π’•π’‰π’†π’‚π’•πŸ) (Eq. 16)

This equation can be rearranged to solve for postheating time (the time needed to bring the average current up to π‘–π‘Žπ‘£π‘”π‘ƒπ‘Ÿπ‘’β„Žπ‘’π‘Žπ‘‘):

149 π’•π’‘π’π’”π’•π’‰π’†π’‚π’•πŸ= (π’Šπ’‚π’—π’ˆπ‘·π’“π’†π’‰π’†π’‚π’•βˆ’π’Šπ’‚π’—π’ˆ)

(π’Šπ‘·πŸβˆ’π’Šπ’‚π’—π’ˆπ‘·π’“π’†π’‰π’†π’‚π’•) βˆ— 𝒕𝒑𝒓𝒐𝒄𝒆𝒔𝒔 (Eq. 17)

The variable π‘–π‘Žπ‘£π‘” is the actual average current (not the target) across the preceding process steps (not including raking), and the variable π‘‘π‘π‘Ÿπ‘œπ‘π‘’π‘ π‘  is the time associated with the

preceding process steps. This same equation applies to calculation of the time for the melt postheating step (π‘‘π‘π‘œπ‘ π‘‘β„Žπ‘’π‘Žπ‘‘2), except that the values used for π‘–π‘Žπ‘£π‘” and π‘‘π‘π‘Ÿπ‘œπ‘π‘’π‘ π‘  are

calculated in a different way. In melt postheating, the preceding process steps include all of the preceding steps except for the optional first postheat. Since the preheat postheating is optional, the calculation for the melt postheating is set to always include heating for previous steps. When the optional step is engaged, it is not included in the energy balance (it is unclear if this is by design). It should be noted that contours are included in the energy balance for the preheat postheating calculation, and are done prior to the preheat

postheating.

Figure 68. Parameters for the Preheat step.

150

Figure 69. Parameters for the Preheat 1 (Preheat I) step.

After a layer is preheated, the next step is Melting. If support structures are included

(Wafer Support), the order of bulk melting and support melting is not considered important.

There are many parameters associated Melting, but the most important is

melt.hatch.square (Figure 70). This menu includes the empirical inputs used to adjust the beam speed and beam current. Unlike a laser-based PBF system, neither speed nor power is constant in an EBM system. In fact, but vary almost constantly (unless the system is run in Manual Mode). First, a nominal beam current (𝑖𝑖𝑛𝑝𝑒𝑑) and beam speed (𝑣𝑖𝑛𝑝𝑒𝑑) are input by the user. The Focus Offset controls the focal point of the beam. [79] If

melt.poweranalyse.automaticpowercalculation is not enabled, then the process will run in manual mode using constant current (𝑖𝑖𝑛𝑝𝑒𝑑) and constant speed (𝑣𝑖𝑛𝑝𝑒𝑑). This mode may be useful for researchers looking to control variables.

151

If the Automatic Power Calculation is enabled, then speed and current are modified according to many functions (this is the standard processing technique). The timing

between beam passes changes based on geometry and location relative to the part edge, so the nominal speed and nominal current are modified. The use of the term line scan length (β„“) is used to denote the length of a given beam pass. Line scan length varies with geometry (based on β„“) and is used to change the power input of the beam to account for small and large regions; a small region should not be melted with the same power/speed combination as a larger region that has more time between beam passes. So, the Current Compensation Function is used to modify 𝑖𝑖𝑛𝑝𝑒𝑑 according to:

π’Šπ’Žπ’†π’π’•= π’Šπ‘ͺπ‘ͺ(𝓡) = π’Šπ’Šπ’π’‘π’–π’• βˆ— (𝟏 + 𝑷𝒓𝒐𝒑𝑲 βˆ—π“΅βˆ’π‘Ίπ’„π’‚π’ π‘³π’†π’π’ˆπ’•π’‰ 𝑹𝒆𝒇𝒆𝒓𝒆𝒏𝒄𝒆

𝑺𝒄𝒂𝒏 𝑳𝒆𝒏𝒕𝒉 𝑹𝒆𝒇𝒆𝒓𝒆𝒏𝒄𝒆 ) (Eq. 18)

This equation is bounded by setting a minimum current (some power is needed to melt even the smallest line scan lengths) and a maximum length (this sets a max current,

effectively) in the process parameters. The value of π‘–π‘šπ‘’π‘™π‘‘ is the current that is actually used to melt. Speed is now determined according to the value set by the Speed Function (the value 𝑣𝑖𝑛𝑝𝑒𝑑 is not used in Automatic mode). The Speed Function is an empirical

relationship between speed and current, which means that speed is also a function of line scan length. Values are not typically published but graphs of this relationship do exist. [79]

A higher value of Speed Function means that the speed will be faster for a given current.

This can be summarized as:

𝒗𝑺𝑭 = 𝒗(π’Šπ’Žπ’†π’π’•, 𝓡) (Eq. 19)

Now that speed is set based on π‘–π‘šπ‘’π‘™π‘‘, other factors need to be considered. This speed is then modified based on the z-distance down to powder; for overhangs, the underlying powder can be more insulative than a solid part. The Thickness Function modifies speed to compensate for the lower amount of applied energy needed in overhangs based on the distance down to powder (zpowder). The function modifies the speed based on:

152

π’—π‘»π’‰π’Šπ’„π’Œπ’π’†π’”π’” = π’—π‘Ίπ‘­βˆ— (𝟏 + 𝑺𝒑𝒆𝒆𝒅𝑭𝒂𝒄𝒕𝒐𝒓

𝟏+𝐞𝐱𝐩(𝑬𝒙𝒑𝒐𝒏𝒆𝒏𝒕 π‘­π’‚π’„π’•π’π’“βˆ—(π’›π’‘π’π’˜π’…π’†π’“βˆ’π‘»π’‰π’Šπ’„π’Œπ’π’†π’”π’” 𝑭𝒂𝒄𝒕𝒐𝒓))) (Eq. 20)

Speed must also be modified to account for edge effects. As a snaking scan strategy is typically used, local heat build near edges can occur; the time that the melt pool has to cool near an edge is less than in the center of a part. To reduce the applied energy near edges, the Turning Point Function is used to increase the speed of the beam as it comes out of an edge. To do this, a Gaussian-type function is applied as:

𝑣𝑇𝑃 = π‘£π‘‡β„Žπ‘–π‘π‘˜π‘›π‘’π‘ π‘ βˆ— (1 + (π‘ƒπ‘Ÿπ‘’ 𝑒π‘₯π‘π‘œπ‘›π‘’π‘›π‘‘π‘–π‘Žπ‘™ π‘“π‘Žπ‘π‘‘π‘œπ‘Ÿ) βˆ— 𝑆)

𝑺 = 𝐞𝐱𝐩 (βˆ’ (π’—π‘»π’‰π’Šπ’„π’Œπ’π’†π’”π’”((π‘¬π’™π’‘π’π’π’†π’π’•π’Šπ’‚π’ 𝑭𝒂𝒄𝒕𝒐𝒓 𝑰) βˆ— 𝓡 βˆ’ (π‘¬π’™π’‘π’π’π’†π’π’•π’Šπ’‚π’ 𝑭𝒂𝒄𝒕𝒐𝒓 𝑰𝑰) βˆ— π’—π‘»π’‰π’Šπ’„π’Œπ’π’†π’”π’”))𝟐)

(Eq. 21)

At this point, an optional compensation for line scan length can be enabled (Speed

Compensation Function) but is not used with typical parameters. The Speed Compensation Function operates the same way as Current Compensation but based on speed. That means that the melt speed is now known:

π’—π’Žπ’†π’π’•= π’—π’Žπ’†π’π’•(π’Š, 𝓡, π’›π’‘π’π’˜π’…π’†π’“) = 𝒗𝑻𝑷 (Eq. 22)

The other features associated with process parameters in the Melt step include Postheating and Contours. It should be noted that the order of Contours can be changed to occur either before or after the Hatch Melting step. Postheating may be engaged after the melt step, as previously discussed.

153

Figure 70. The melt.hatch.square menu is used to understand the values used for beam speed and beam current.

Download now "Additive Manufacturing..."

Outline

Related documents