Patent Publication Number: US-2023141138-A1

Title: Phase control in additive manufacturing

Description:
BACKGROUND 
     Three-dimensional (3D) printing is an additive manufacturing process used to make three-dimensional solid parts from a digital model. In some examples, additive manufacturing may be used in rapid product prototyping, mold generation, mold master generation, and short run manufacturing. Some additive manufacturing techniques are considered additive processes because they involve the application of successive layers of material. This is unlike other machining processes, which often rely upon the removal of material to create the final part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate various examples of the principles described herein and are part of the specification. The illustrated examples are given merely for illustration, and do not limit the scope of the claims. 
         FIG.  1    is a block diagram of an additive manufacturing system for phase control in a build material, according to an example. 
         FIG.  2    illustrates different types of melt pools, according to an example. 
         FIGS.  3 A and  3 B  illustrate different types of melt pools, according to another example. 
         FIG.  4    is a flow diagram illustrating a method for phase control in a build material, according to an example. 
         FIG.  5    is a flow diagram illustrating a method for phase control in a build material, according to another example. 
     
    
    
     Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. The figures are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings provide examples and/or implementations consistent with the description; however, the description is not limited to the examples and/or implementations provided in the drawings. 
     DETAILED DESCRIPTION 
     The present disclosure is drawn to additive manufacturing systems and methods. More particularly, the systems and methods can be used with powder bed fusion (PBF) where a heat source (e.g., a laser, etc.) is used to consolidate a powdered build material to form a 3D object. In some examples, the heat source may be applied to the build material contained within a powder bed to form a layer of the 3D object. Examples of PBF include laser sintering, laser powder bed fusion (LPBF), and pulsed-laser powder bed fusion (pLPBF). 
     In the case of LPBF, a powdered build material (particulate or powder) is spread on a powder bed support (referred to herein as a build area platform) on a layer-by-layer basis. A laser may be used to selectively melt and sinter the build material together at specific points. This can be repeated layer by layer until a three-dimensional object is formed. Once a layer of the 3D object is completed, the build area platform lowers and more build material is distributed on top of the powder bed for a subsequent layer. 
     This disclosure describes examples of methods and additive manufacturing systems to site-specifically control the phase content in multi-phase metal alloys (e.g., carbon steel, stainless steel, titanium, etc.) by varying the cooling rate of the build material through melt pool engineering during pulsed-laser powder bed fusion processing. In some examples, pLPBF provides the ability to create and customize the mechanical behavior and other functional properties (e.g., corrosion resistance) of multi-phase alloys at an unprecedented level of detail by controlling the spatial distribution, arrangement, and size of the constituent microstructural phases (e.g., body-centered cubic (BCC) ferrite and face-centered cubic (FCC) austenite, or martensite and BCC ferrite). 
     The described examples enable generating 3D objects made of alloys such as steel—arguably one of the most ubiquitous alloys in society—with tunable strength, ductility, and corrosion resistance. With other manufacturing approaches, a manufactured object may exhibit randomized phase ratio and distribution. The described examples enable phase control both in-plane (i.e., within each layer of the build material generated during LPBF) as well as out-of-plane (i.e., along the build direction of the 3D object). As such, these examples open the path to completely new microstructure designs, which transcend those offered by approaches involving layer-wise microstructure control. Furthermore, these examples may be applied to control the phase content in a wide range of alloy systems. This latter feature is expected to facilitate new alloy designs for additive manufacturing. 
     The examples described herein may be used to control the melt pool depth and cooling rate in an LPBF process. In some examples, the phase control enabled by these methods may be achieved by tuning the pulsed-laser parameters to drive the formation of a melt pool pattern in the build material. As used herein the term “melt pool” refers to a volume of build material that is heated by a laser to transition from a solid state to a liquid state. The melt pool then resolidifies as the liquid build material cools. Melt pools may have different characteristics depending on the parameters used by the laser to melt the build material. For example, laser parameters may include the peak power of the laser, the spot size of the laser on the build material, the laser pulse frequency, the laser pulse duration, and the spacing of the laser emissions on the build material. The term “melt pool pattern” refers to an arrangement of melt pools throughout the build material, where melt pools may vary according to the laser parameters. 
     The present specification describes examples of a method for phase control in a build material. The example method includes determining parameters for a pulsed laser to generate a melt pool pattern in a three-dimensional (3D) object to produce different phases in the 3D object that vary according to the melt pool pattern. The example method also includes controlling the pulsed laser to form the 3D object in an additive manufacturing process based on the determined parameters and the melt pool pattern. 
     In another example, the present specification describes another example method that includes generating a first melt pool in a first region of a build material with a pulsed laser set to a first set of parameters to control phase properties in the first region. The example method also includes adjusting the pulsed laser to a second set of parameters. The example method further includes generating a second melt pool in a second region of the build material with the pulsed laser set to the second set of parameters to produce phase properties in the second region that differ from the phase properties in the first region. 
     In yet another example, the present specification describes an additive manufacturing system. In some examples, the additive manufacturing system includes a build material distributor, a pulsed laser, a controller, and a non-transitory computer readable medium having stored thereon computer executable instructions to cause the controller to utilize the build material distributor to dispense the build material in a plurality of layers. The instructions also cause the controller to control the pulsed laser to generate a melt pool pattern in the plurality of layers of the build material to form a 3D object having different phases that vary according to the melt pool pattern. 
     As used in the present specification and in the appended claims, the term “controller” may be a processor resource, a processor, an application-specific integrated circuit (ASIC), a semiconductor-based microprocessor, a central processing unit (CPU), and a field-programmable gate array (FPGA), and/or other hardware device that executes instructions. 
     As used in the present specification and in the appended claims, the term “memory” may include a non-transitory computer-readable storage medium, where the computer-readable storage medium may contain, or store computer-usable program code for use by or in connection with an instruction execution system, apparatus, or device. The memory may take many types of memory including volatile memory (e.g., RAM) and non-volatile memory (e.g., ROM). 
     Turning now to the figures,  FIG.  1    is a block diagram of an additive manufacturing system  102  for phase control in a build material  108 , according to an example. It is to be understood that the additive manufacturing system  102  may include additional components and that some of the components described herein may be removed and/or modified. Furthermore, components of the additive manufacturing system  102  depicted in  FIG.  1    may not be drawn to scale and thus, the additive manufacturing system  102  may have a different size and/or configuration other than as shown therein. 
     The additive manufacturing system  102  includes a build area platform  104 , a build material supply  107  containing build material  108 , a build material distributor  110 , and a pulsed laser  111 . In some examples, the build material  108  may be a powdered metal alloy (e.g., carbon steel, stainless steel, titanium, etc.). While the example of a metal alloy is described, other materials may be used for the build material  108 . The build material  108  may include a material that can contain multiple phases when in a solid state. As used herein, the term “phase” refers to a physically homogeneous state of matter, where the phase has a given chemical composition, and a distinct type of atomic bonding and arrangement of elements. For a solid material, the phase may be defined by the crystal structure of the elements forming the solid material. For example, one phase of a solid material may have a body-centered cubic (BCC) crystal structure and a second phase of a solid material may have a face-centered cubic (FCC) crystal structure. 
     In some examples, within an alloy, two or more different phases can be present at the same time. Each phase within an alloy may have distinct physical, mechanical, electrical, and electrochemical properties. For example, in carbon steel, ferrite may be a relatively soft phase and cementite is a hard, brittle phase. When they are present together, the strength of the alloy is much greater than for ferrite and the ductility is much better compared to cementite. 
     The build area platform  104  receives the build material  108  from the build material supply  107 . The build area platform  104  may be integrated with the additive manufacturing system  102  or may be a component that is separately insertable into the additive manufacturing system  102 . For example, the build area platform  104  may be a module that is available separately from the additive manufacturing system  102 . The build material platform  104  that is shown is also one example, and could be replaced with another support member, such as a platen, a fabrication/print bed, a glass plate, or another build surface. 
     The build area platform  104  may be moved in a direction  106  as denoted by the arrow oriented along the z-axis, so that build material  108  may be delivered to the build area platform  104  or to a previously formed 3D object layer (i.e., fused build material). In an example, when the build material  108  is to be delivered, the build area platform  104  may be programmed to advance (e.g., downward) enough so that the build material distributor  110  can push the build material  108  onto the build area platform  104  to form a layer of the build material  108  thereon. The build area platform  104  may also be returned to its original position, for example, when a new 3D object  118  is to be built. 
     The build material supply  107  may be a container, bed, or other surface that is to position the build material  108  between the build material distributor  110  and the build area platform  104 . In some examples, the build material supply  107  may include a surface upon which the build material  108  may be supplied, for instance, from a build material source (not shown) located above the build material supply  107 . Examples of the build material source may include a hopper, an auger convey er, or the like. In some examples, the build material supply  107  may include a mechanism (e.g., a delivery piston) to provide, e.g., move, the build material  108  from a storage location to a position to be spread onto the build area platform  104  or onto a previously formed 3D object layer. 
     The build material distributor  110  may be moved in a direction as denoted by the arrow  112 , e.g., along the y-axis, over the build material supply  107  and across the build area platform  104  to spread a layer of the build material  108  over the build area platform  104 . The build material distributor  110  may also be returned to a position adjacent to the build material supply  107  following the spreading of the build material  108 . In some examples, the build material distributor  110  may be a blade (e.g., a doctor blade), a roller, a combination of a roller and a blade, and/or any other device capable of spreading the build material  108  over the build area platform  104 . For instance, the build material distributor  110  may be a counterrotating roller. 
     In some examples, the additive manufacturing system  102  may also include a pulsed laser  111 . The pulsed laser  111  may be used to expose the build area platform  104  (and any build material  108  and/or agent(s) thereon) to thermal energy (e.g., electromagnetic radiation) that ultimately fuses and/or sinters the build material  108 . The pulsed laser  111  may be a device that emits light through a process of optical amplification based on a stimulated emission of electromagnetic radiation. The pulsed laser  111  may emit a beam of optical energy (e.g., concentrated light) in pulses of a given duration and at a given repetition rate. 
     In some examples, the beam of the pulsed laser  111  may be positioned on the powder bed  114  using mirrors that direct the beam to a given location along the y-axis and x-axis. As used herein, the term “powder bed” refers to build material  108  that is deposited on the build area platform  104  and contained within side walls of the additive manufacturing system  102 . A beam from the pulsed laser  111  may have given energy characteristics based on a number of parameters of the pulsed laser  111 . For example, parameters used to adjust the beam characteristics may include the peak power of the laser, the spot size of the laser beam on the build material, the beam focal length (also referred to as focal distance), the laser pulse frequency, and the laser pulse duration. 
     In some examples, the beam focal length of the pulsed laser  111  may be adjusted by focusing the pulsed laser  111 . For example, a first focal length may focus the laser beam emitted by the pulsed laser  111  on a given location of the powder bed surface, which may result in an area on the surface of the powder bed  114  having a high concentration of thermal energy. In another example, a second focal length may result in a diffused laser beam on the surface of the powder bed  114 , which may result in a lower concentration of thermal energy on the powder bed surface. 
     It should be noted that while the example of a mirror to position the laser beam is described, in other examples, the laser beam may be positioned with other mechanisms. For example, the pulsed laser  111  may be mounted an on a track (e.g., a translational carriage) to move across the build area platform  104 , e.g., along the y-axis and x-axis. This allows for printing and heating as the pulsed laser  111  passes over the build area platform  104 . In some examples, the pulsed laser  111  can make multiple passes over the build area platform  104  depending on the amount of exposure utilized in the method(s) disclosed herein. 
     Each of these physical elements may be operatively connected to a controller  120  of the additive manufacturing system  102 . The controller  120  may control the operations of the build area platform  104 , the build material supply  107 , the build material distributor  110 , and the pulsed laser  111 . As an example, the controller  120  may control actuators (not shown) to control various operations of the additive manufacturing system  102  components. The controller  120  may be a computing device, a semiconductor-based microprocessor, a central processing unit (CPU), an application specific integrated circuit (ASIC), and/or another hardware device. Although not shown, the controller  120  may be connected to the additive manufacturing system  102  components via communication lines. 
     The controller  120  manipulates and transforms data, which may be represented as physical (electronic) quantities within the additive manufacturing system&#39;s registers and memories, to control the physical elements to create the 3D object  118 . As such, the controller  120  is depicted as being in communication with a data store  122 . The data store  122  may include data pertaining a 3D object  118  to be printed by the additive manufacturing system  102 . The data store  122  may include data pertaining parameters for the pulsed laser  111  to generate a melt pool pattern in the 3D object  118 . 
     The data store  122  may also include machine readable instructions (stored on a non-transitory computer readable medium) that are to cause the controller  120  to control the amount of build material  108  that is supplied by the build material supply  107 , the movement of the build area platform  104 , the movement of the build material distributor  110 , movement of the pulsed laser  111 , adjustments to the pulsed laser parameters, etc. 
     In some examples, the data store  122  may include computer executable instructions to cause the controller  120  to utilize the build material distributor  110  to dispense the build material  108  in a plurality of layers. For example, the build material distributor  110  may distribute a layer of build material  108  over the build area platform  104 . 
     In some examples, the data store  122  may include computer executable instructions to cause the controller  120  to control the pulsed laser  111  to generate a melt pool pattern in the plurality of layers of the build material  108  to form a 3D object  118  having different phases that vary according to the melt pool pattern. The melt pool pattern may include the type of melt pools generated by the pulsed laser  111 . The melt pool pattern may also include the placement of the melt pools within a given layer of the 3D object  118  and through the build direction of the 3D object  118 . As used herein, “build direction” refers to the vertical direction (e.g., along the z-axis) that the layers of the 3D object  118  accumulate. In  FIG.  1   , direction  106  corresponds to the build direction of the 3D object  118 . In other words, the build area platform  104  moves along the axis of the build direction. 
     In some examples, the controller  120  may determine parameters for the pulsed laser  111  to generate a melt pool pattern in the 3D object  118  to produce different phases in the 3D object  118  that vary according to the melt pool pattern. As described above, some examples of parameters for the pulsed laser  111  may include peak power, pulse frequency (e.g., how often the pulse occurs), pulse duration (e.g., how long the pulse lasts), and focal distance (e.g., the focus of the pulse). 
     In some examples, the parameters for the pulsed laser  111  to produce a melt pool pattern may include the location and characteristics of melt pools on a given layer (referred to as the build plane) of the 3D object  118  within the powder bed  114 . This approach may allow for control of phases in the build plane. For example, a first melt pool may be generated at a first location of a build layer with a first set of parameters, while a second melt pool may be generated at a second location of the build layer with a second set of parameters, and so forth. 
     In some examples, the parameters for the pulsed laser  111  to produce a melt pool pattern may include locations and characteristics of melt pools in different layers of the 3D object  118  within the powder bed  114 . This approach may allow for control of phases in different layers in the build direction. For example, a first melt pool may be generated at a location of a first layer with a first set of parameters, while a second melt pool may be generated at a location of a second layer with a second set of parameters, and so forth. 
     In some examples, the controller  120  may cause the pulsed laser  111  to generate different types of melt pools at different locations through variable cooling rates. For example, the pulsed laser  111  may exhibit higher peak power at low pulse repetition rates than a laser run in continuous mode. The pulsing of the pulsed laser  111  may enable the formation of deep melt pools with high cooling rates. The deepest melt pool may be achieved when the laser beam is in focus. The focused laser beam may remelt multiple underlying layers, which solidify at high cooling rates because of the surrounding dense metal matrix. The use of a focused laser beam to remelt underlying layers is referred to as a keyhole mode and a melt pool generated in this mode may be referred to as a keyhole mode melt pool. The high cooling rates of the keyhole mode favor the formation of metastable phases in the build material  108 . It should be noted that other parameters (e.g., peak power, pulse frequency, pulse duration, etc.) may be used to generate keyhole mode melt pools. 
     In another example approach, defocusing the beam of the pulsed laser  111  may yield shallower melt pools. The use of a defocused beam may be referred to as a conduction mode and a melt pool generated in this mode may be referred to as a conduction mode melt pool. It should be noted that other parameters (e.g., peak power, pulse frequency, pulse duration, etc.) may be used to generate conduction mode melt pools. In this case, the melt pools may be cyclically re-melted and annealed during subsequent layer deposition. With conduction mode, the melt pools may exhibit phases that approach thermodynamic equilibrium. 
     Depending on the spatial alignment of the different melt pool types (e.g., keyhole mode melt pool and conduction melt pool), microstructural changes and phase transformations may be induced both within a build layer and along the build direction.  FIG.  2   ,  FIG.  3 A , and  FIG.  3 B  illustrate examples of keyhole mode melt pools and conduction mode melt pools to control the phase within a 3D object. 
     Referring momentarily to  FIG.  2   , multiple layers  222   a - n  of a 3D object  218  are depicted. In this example, a first layer  222   a  is formed on a build area platform  204  by melting build material with a pulsed laser beam  220  in a number of melt pools  224 . In this example, the first layer  222   a  may be formed with a first set of parameters (e.g., peak power, pulse frequency, pulse duration, and focal distance) for the pulsed laser beam  220 . In this example, the pulsed laser beam  220  may be defocused in the first layer  222   a  to achieve shallow melt pools  224  with a first phase  226 . This melt pools  224  with the first phase  226  may be conduction mode melt pools. In this example, the first set of parameters for the pulsed laser beam  220  may be used for four more layers  222   b - e  in the build direction  230  to form a band of melt pools with the first phase  226 . 
     At the sixth layer  222   f,  the parameters of the pulsed laser beam  220  may be changed to generate keyhole mode melt pools  224 . For example, the pulsed laser beam  220  may be focused to create melt pools  224  with a larger penetration depth compared to the melt pools  224  in conduction mode used to form the previous layers  222   a - e.  The melt pools  224  of the sixth layer  222   f  may have a second phase  228  upon cooling. With the focused pulsed laser beam  220 , the melt pools  224  in the sixth layer  222   f  may penetrate through a number of layers below. In this example, the melt pools  224  in the sixth layer  222   f  penetrate through the two layers  222   d - e  below. 
     This melt pool pattern may be repeated. For example, a number of layers may be generated using the first set of parameters for the pulsed laser beam  220  to form a band of layers with the first phase  226 . The parameters of the pulsed laser beam  220  may be changed to form another layer (e.g., layer  222   m ) in a keyhole mode resulting in melt pools  224  with the second phase  228 . In this example, the melt pools  224  in keyhole mode are aligned in the build plane. The larger penetration depth of the melt pools  224  in keyhole mode compared to the melt pools  224  (e.g., in layers  222   a - e ) in conduction mode ensures the retention of a layer of material that includes metastable phases. 
     In the case of a carbon alloyed mild steel, the layerwise phase control of  FIG.  2    may result in harder martensite dominated layers (e.g., layers  222   f  and  222   m ) surrounded by a softer bainitic and ferritic base microstructure (e.g., layers  222   a - e ). Evident in  FIG.  2    is the control over the spacing between the martensite layers, which may vary from 2 to 5 layers (e.g., corresponding to 37 μm and 188 μm, respectively). 
     Referring now to  FIG.  3 A , multiple layers  322   a - n  of a 3D object  318  are depicted. In this example, a first layer  322   a  is formed on a build area platform  304  by melting build material with a pulsed laser beam  320  in a number of melt pools  324 . In this example, the keyhole mode melt pools  324  are aligned out-of-plane. In other words, the keyhole mode melt pools  324  are aligned along the build direction  330  (e.g., along the z-axis). In this case, differential cooling rates are observed vertically giving rise to a lamellar microstructure that includes vertical walls of different phases. 
       FIG.  3 B  illustrates a single melt pool  324 , according to an example. For a given melt pool  324 , the second phase  328  may dominate the central, interior portion of the melt pool  324 , while the first phase  326  may dominate peripheral portions of the melt pool  324 . 
     In an example of  FIG.  3 A , the build material may include molybdenum alloyed austenitic stainless steel. In this case, the phase control results in two phases: ferrite (BCC) along the center of the melt pool  324  and austenite (FCC) on the outer melt pool area and overlapping melt pool regions. 
     It should be noted that in the example of  FIGS.  3 A and  3 B , the melt pool pattern includes the melt pools  324  aligned in the build direction  330  such that a given melt pool  324  penetrates through multiple melt pools  324  in the layers below. The melt pool pattern in this example also includes spacing the melt pools  324  such that alternating vertical walls of the first phase  326  and the second phase  328  may form in the build direction  330 . In this example, the parameters for the pulsed laser beam  320  may remain fixed for forming the multiple melt pools  324 . However, the melt pool pattern (e.g., the spacing of the melt pools  324 ) may be adjusted to control the phase distribution in the 3D object  318 . 
     Returning now to  FIG.  1   , as seen in the in-plane example of  FIG.  2    and the out-of-plane example of  FIG.  3 A , phase control may be achieved by adjusting parameters for the pulsed laser  111  and/or adjusting the melt pool pattern. On the other hand, because the different melt pool types can be aligned arbitrarily within the 3D space of the 3D object  118 , this method can be used to produce microstructures with complex 3D phase architectures. For example, different phases may be created within the 3D object  118 . Some examples of the phase arrangement include a checkerboard structure, a grid structure, cellular structures with oblique struts formed by a given phase, etc. 
     The pulsed laser  111  may be controlled to form the 3D object  118  in an additive manufacturing process based on the determined parameters and the melt pool pattern. In this manner, the phase content in pLPBF alloys may be controlled both in-plane and out-of-plane by tuning the melt pool depth and cooling rate using different combinations of laser parameters (e.g., peak power, spot size, pulse frequency, and pulse duration). Therefore, the parameters for the pulsed laser  111  may be determined to control melt pool depth and melt pool cooling rate. 
     In some examples, the pulsed laser  111  may form a 3D object  118  with a heterogeneous structure. For example, different regions of the 3D object  118  may have mechanical properties based on the parameters used to generate melt pools in the different regions. A first region may have a first phase generated by a first melt pool type and a second region may have a second phase generated by a second melt pool type. In an example, the controller  120  may adjust parameters for the pulsed laser  111  to generate different melt pool types, where each melt pool type results in different phases of the build material  108 , as described in  FIG.  2   . In this case, the parameters may be determined to produce given phases along a build plane (e.g., along the x-y plane) of the 3D object  118 . 
     In another example, keyhole mode melt pools may be used to penetrate through multiple layers resulting in different phase types in different regions of the melt pool. These keyhole mode melt pools may be arranged in a melt pool pattern to form a controlled structure of varying phases. For example, the controller  120  may adjust parameters for the pulsed laser  111  to generate the melt pool pattern resulting in a lamellar structure along the build direction of the 3D object  118 , as described in  FIG.  3 A . In this case, the parameters for the pulsed laser  111  may be determined to produce given phases in a build plane (e.g., along the z-axis) of the 3D object  118 . 
     In some examples, the focus of the pulsed laser  111  may be changed during the LPBF process. Changing the laser focus may lead to a transition of the laser melting mode from keyhole mode to conduction mode. The resulting melt pool morphologies are conducive to variable cooling rates throughout the material and thus to the formation of different metastable phases (such as martensite in steel). Thus, site-specific phase control may be achieved by modulating the laser focus in the LPBF process and through adjustments of other parameters (e.g., peak power, pulse frequency, pulse duration, pulse location, pulse layer, etc.). 
       FIG.  4    is a flow diagram illustrating a method  400  for phase control in a build material, according to an example. In some examples, the method  400  may be performed by an additive manufacturing system, such as the additive manufacturing system  102  of  FIG.  1   . 
     At  402 , parameters for a pulsed laser may be determined to generate a melt pool pattern in a 3D object to produce different phases in the 3D object that vary according to the melt pool pattern. The build material may include an alloy containing multiple phases. For example, the build material may be a powdered metal alloy (e.g., carbon steel, stainless steel, titanium, etc.). 
     In some examples, the parameters for the pulsed laser may include peak power, pulse frequency, pulse duration, and focal distance. The parameters may be determined to control melt pool depth and melt pool cooling rate. In some examples, the parameters are determined to produce given phases along a build plane of the 3D object. For example, this may be accomplished as described in  FIG.  2   . In some examples, the parameters may be determined to produce given phases in a build direction of the 3D object. This may be accomplished as described in  FIG.  3 A . 
     At  404 , the pulsed laser may be controlled to form the 3D object in an additive manufacturing process based on the determined parameters and the melt pool pattern. In some examples, the pulsed laser may form the 3D object with a heterogeneous structure, where different regions of the 3D object have mechanical properties based on the parameters used to generate melt pools in the different regions. 
       FIG.  5    is a flow diagram illustrating a method  500  for phase control in a build material, according to another example. In some examples, the method  500  may be performed by an additive manufacturing system, such as the additive manufacturing system  102  of  FIG.  1   . 
     At  502 , a first melt pool may be generated in a first region of a build material with a pulsed laser set to a first set of parameters to control phase properties in the first region. For example, the pulsed laser may be set to a first focal length and a pulse of the pulsed laser may melt the build material to form the first melt pool. In another example, other parameters (e.g., peak power, pulse frequency, pulse duration) may be selected to form the first melt pool. 
     At  504 , the pulsed laser may be adjusted to a second set of parameters. For example, the pulsed laser may be adjusted to a second focal length. Thus, adjusting the pulsed laser to the second set of parameters may include adjusting the focal length of the pulsed laser from the first focal length to the second focal length. In other examples, other parameters (e.g., peak power, pulse frequency, pulse duration) of the pulsed laser may be adjusted for the second set of parameters. 
     At  506 , a second melt pool may be generated in a second region of the build material with the pulsed laser set to the second set of parameters. The phase properties produced in the second region may differ from the phase properties in the first region. The melt pool depth and cooling rate of the first melt pool may differ from the melt pool depth and cooling rate of the second melt pool. For example, the first melt pool may be a keyhole mode melt pool and the second melt pool may be a conduction mode melt pool. 
     In some examples, the first melt pool may include (e.g., may extend through) a plurality of layers in the first region of the build material. The second melt pool may include a single layer in the second region of the build material. In this case, the first set of parameters may be selected to control the phase properties in the plurality of layers in the first region of the build material, and the second set of parameters may be selected to control the phase properties in the single layer in the second region of the build material. 
     In some examples, the first melt pool and the second melt pool may be formed starting on different layers of the build material. In some examples, the first melt pool and the second melt pool may start on the same layer of the build material.